A C++ interface to SWI-Prolog
Jan Wielemaker & Peter Ludemann
SWI-Prolog Solutions b.v.
E-mail: jan@swi-prolog.org
Abstract
This document describes a C++ interface to SWI-Prolog. SWI-Prolog could be used with C++ for a very long time, but only by calling the extern "C" functions of the C-interface. The interface described here provides a true C++ layer around the C-interface for much more concise and natural programming from C++. The interface deals with automatic type-conversion to and from native C data-types, transparent mapping of exceptions, making queries to Prolog and registering foreign predicates.

This document describes version 2 of the C++ interface. Version 1 is considered deprecated. Version 2 is implemented by `SWI-cpp2.h` and `SWI-cpp2.cpp`. This is a much more mature C++ interface has been designed and implemented by Peter Ludemann.

# Table of Contents

# 1 A C++ interface to SWI-Prolog

## 1.1 Summary of changes between Versions 1 and 2

Version 1 is in `SWI-cpp.h`; version 2 is in `SWI-cpp2.h`, `SWI-cpp2.cpp`, `SWI-cpp2-plx.h`, and `SWI-cpp2-atommap.h`.

The overall structure of the API has been retained - that is, it is a thin layer of lightweight classes on top of the interface provided by `SWI-Prolog.h`. Based on experience with the API, most of the conversion operators and some of the comparison operators have been removed or deprecated, and replaced by “getter” methods; the overloaded constructors have been replaced by subclasses for the various types. Some changes were also made to ensure that the ``[]`` operator for `PlTerm` and `PlTermv` doesn't cause unexpected implicit conversions.1If there is an implicit conversion operator from `PlTerm` to `term_t` and also to `char*`, then the ``[]`` operator is ambiguous if `f` is overloaded to accept a `term_t` or `char*` in the code `PlTerm t=...; f(t[0])`.

Prolog errors are now converted to C++ exceptions (which contain the exception term rather being a subclass of `PlTerm` as in version 1), where they can be caught and thrown using the usual C++ mechanisms; and the subclasses that create exceptions have been changed to functions. In addition, an exception type `PlFail` has been added, together with `PlCheckFail`, to allow more compact code by “short circuit” return to Prolog on failure.

A convenience class for creating blobs has been added, so that an existing structure can be converted to a blob with only a few lines of code. More specifically:

- `SWI-cpp2.cpp` has been added, containing the implementation of some functions. This is included by default from `SWI-cpp2.h` or can be compiled separately.
- The constructor PlTerm() is restricted to a few unambiguous cases - instead, you should use the appropriate subclass constructors that specify the type (PlTerm_var(), PlTerm_atom(), etc.).
- Wrapper functions have been provided for almost all the PL_*() functions in `SWI-Prolog.h`, and have the same names with the “PL” replaced by “Plx” .2 “Pl” is used throughout the `SWI-cpp2.h` interface, and the “x” is for “eXtended with eXception handling.’ Where appropriate, these check return codes and throw a C++ exception (created from the Prolog error). See section 1.6.4. Many of these wrapper functions are also methods in the `PlAtom` and `PlTerm` classes, with the arguments changed from `atom_t` and `term_t` to `PlAtom` and `PlTerm` and in some cases `char*` and `wchar_t*` changed to `std::string` and `std::wstring`. These wrappers are available if you include `SWI-cpp2.h` (they are in a separate `SWI-cpp2-plx.h` file for ease of maintenance).
- Instead of returning `false` from a foreign predicate to indicate failure, you can throw PlFail(). The convenience function PlCheckFail(rc) can be used to throw PlFail() if `false` is returned from a function in `SWI-Prolog.h`. If the wrapper functions or class methods are used, Prolog errors result in a C++ `PlException` exception.3If a “Plx_” wrapper is used to call a `SWI-Prolog.h` function, a Prolog error will have already resulted in throwing `PlException`; PlCheckFail(rc) is used to additionally throw `PlFail`, similar to returning `false` from the top-level of a foreign predicate - Prolog will check for an error and call throw/1 if appropriate.
- The `PlException` class is now a subclass of `std::exception` and encapsulates a Prolog error. Prolog errors are converted into `throw PlException(...)`. If the user code does not catch the `PlException`, the PREDICATE() macro converts the error to a Prolog error upon return to the Prolog caller.
- The C++ constructors, functions, and methods use the wrapper functions to throw a C++ exception on error (this C++ exception is converted to a Prolog exception when control returns to Prolog).
- The “cast” operators (e.g., `(char*)t`, `(int64_t)t`, `static_cast<char*>(t)`) have been deprecated, replaced by “getters” (e.g., `t.as_string()`, `t.as_int64_t()`).
- The overloaded assignment operator for unification is deprecated, replaced by PlTerm::unify_term(), PlTerm::unify_atom(), etc., and the helper PlCheckFail().
- Many of the equality and inequality operators are deprecated; replaced by the PlTerm::as_string() or PlTerm::get_nchars() methods and the associated `std::string`, comparison operators. The PlTerm::as_string() method allows specifying the encoding to use whereas the ``==`` and similar operators do not allow for this.
- Methods that return `char*` have been replaced by methods that return `std::string` to ensure that lifetime issues don't cause subtle bugs.4If you want to return a `char*` from a function, you should not do `return t.as_string().c_str()` because that will return a pointer to local or stack memory. Instead, you should change your interface to return a `std::string` and apply the `c_str()` method to it. These lifetime errors can sometimes be caught by specifying the Gnu C++ or Clang options `-Wreturn-stack-address` or `-Wreturn-local-addr` - as of 2023-04, Clang seems to do a better analysis.
- Most constructors, methods, and functions that accept `char*` or `wchar_t*` arguments also accept `std::string` or `std::wstring` arguments. Where possible, encoding information can also be specified.
- Type-checking methods have been added: PlTerm::type(), PlTerm::is_variable(), PlTerm::is_atom(), etc.
- `PlString` has been renamed to `PlTerm_string` to make it clear that it's a term that contains a Prolog string.
- More `PL_...(term_t, ...)` methods have been added to `PlTerm`, and `PL_...(atom_t, ...)` methods have been added to `PlAtom`. Where appropriate, the arguments use `PlTerm`, `PlAtom`, etc. instead of `term_t`, `atom_t`, etc.
- Most functions/methods that return an `int` for true/false now return a C++ `bool`.
- The wrapped C types fields (`term_t`, `atom_t`, etc.) have been renamed from `handle`, `ref`, etc. to `C_`.5This is done by subclassing from `Wrapped<term_t>`, `Wrapped<atom_t>`, etc., which define the field `C_`, standard constructors, the methods is_null(), not_null(), reset(), reset(v), reset_wrapped(v), plus the constant `null`. This value can be accessed by the unwrap() and unwrap_as_ptr() methods. There is also a “friend” function PlUnwrapAsPtr().
- A convenience function `PlControl::context_unique_ptr<ContextType>()` has been added, to simplify dynamic memory allocation in non-deterministic predicates.
- A convenience function PlRewindOnFail() has been added, to simplify non-deterministic code that does backtracking by checking unification results.
- `PlStringBuffers` provides a simpler interface for allocating strings on the stack than PL_STRINGS_MARK() and PL_STRINGS_RELEASE(). However, this is mostly not needed because most functions now use `std::string`: see section 1.6.9.1.
- `PlStream` provides a simpler interface for streams than PL_get_stream(), PL_acquire_stream(), and PL_release_stream(). See section 1.6.9.2.
- Wrapper classes for `record_t` have been added. The `PlRecordExternalCopy` class contains the opaque handle, as a convenience.
- Wrapper class for `control_t` has been added and the PREDICATE_NONDET() has been modified to use it.

More details on the rationale and how to port from version 1 to version 1 are given in section 1.8 and section 1.9.

## 1.2 A simple example

Here is the “simple example” in the Foreign Language Interface, rewritten in C++. As before, it is compiled by

      swipl-ld -o calc -goal true calc.cpp calc.pl

    #include <string>
    #include <SWI-cpp2.h>

    int main(int argc, char **argv) {

      PlEngine e(argv[0]);

      // combine all the arguments in a single string
      std::string expression;
      for (int n = 1; n < argc; n++) {
        if (n != 1) {
          expression.append(" ");
        }
        expression.append(argv[n]);
      }

      // Lookup calc/1 and make the arguments and call

      PlPredicate pred("calc", 1, "user");
      PlTerm_string h0(expression);
      PlQuery q(pred, PlTermv(h0), PL_Q_NORMAL);

      return q.next_solution() ? 0 : 1;
    }

## 1.3 Sample code

The file test_cpp.cpp contains examples of Prolog predicates written in C++. This file is used for testing (called from test_cpp.pl). Notable examples:

- add_num/3 - same as `A3 is A1+A2`, converting the sum to an integer if possible.
- name_arity/3 - C++ implementation of functor/3.
- average/3 - computes the average of all the solutions to Goal
- can_unify/2 - tests whether the two arguments can unify with each other, without instantiating anything (similar to unifiable/3).
- eq1/1, eq2/2, eq3/2 - three different ways of implementing =/2.
- write_list/1 - outputs the elements of a list, each on a new line.
- cappend/3 - appends two lists (requires that the two lists are instantiated).
- square_roots/2 - same as `bagof(Sqrt, X^(between(0,4,X), Sqrt is sqrt(X)), A2)`.
- range_cpp/3 - on backtracking, generates all integers starting at A1 and less than A2 (that is, one less than between/3).
- int_info/2 - on backtracking generates all the integral types with their minimum and maximum values.

The file likes.cpp contains a simple program that calls the Prolog predicate likes/2 and happy/1 (these predicates are defined in likes.pl. The usage and how to compile the code are in comments in `likes.cpp`

## 1.4 Introduction

C++ provides a number of features that make it possible to define a more natural and concise interface to dynamically typed languages than plain C does. Using type-conversion (casting) and overloading, native data-types can be easily translated into appropriate Prolog types, automatic destructors can be used to deal with most of the cleanup required and C++ exception handling can be used to map Prolog exceptions and interface conversion errors to C++ exceptions, which are automatically mapped to Prolog exceptions as control is turned back to Prolog.

However, there are subtle differences between Prolog and C++ that can lead to confusion; in particular, the lifetime of terms do not fit well with the C++ notion of constructor/destructor. It might be possible to handle this with “smart pointers” , but that would lead to other complications, so the decision was made to provide a thin layer between the underlying C functions and the C++ classes/methods/functions.

More information on the SWI-Prolog native types is given in Interface Data Types.

It would be tempting to use C++ implicit conversion operators and method overloading to automatically convert between C++ types such as `std::string` and `int64_t` and Prolog foreign language interface types such as `term_t` and `atom_t`. However, types such as `term_t` are unsigned integers, so many of the automatic type conversions can inadvertently do something other than what the programmer intended, resulting in subtle bugs that are difficult to find. Therefore Version 2 of this interface reduces the amount of automatic conversion and introduces some redundancy, to avoid these subtle bugs, by using “getter” methods rather than conversion operators, and using naming conventions for explicitly specifying constructors.

### 1.4.1 Acknowledgements

I would like to thank Anjo Anjewierden for comments on the definition, implementation and documentation of the original C++ interface. Peter Ludemann implemented the current version (2) of the interface (see section 1.1).

## 1.5 The life of a PREDICATE

A foreign predicate is defined using the PREDICATE() macro, plus a few variations on this, such as PREDICATE_NONDET(), NAMED_PREDICATE(), and NAMED_PREDICATE_NONDET(). These define an internal name for the function, register it with the SWI-Prolog runtime (where it will be picked up by the use_foreign_library/1 directive), and define the names `A1`, `A2`, etc. for the arguments.6You can define your own names for the arguments, for example: `auto dir=A1, db=A2;` or `PlTerm options(A3);`. If a non-deterministic predicate is being defined, an additional parameter `handle` is defined (of type `PlControl`).

The foreign predicate returns a value:

- `true` - success
- `false` - failure or an error (see section 1.15 and Prolog exceptions in foreign code).
- “retry” - for non-deterministic predicates, gives a “context” for backtracking / redoing the call for the next solution.

If a predicate fails, it could be simple failure (the equivalent of calling the builtin fail/0 predicate) or an error (the equivalent of calling the throw/1 predicate). When a Prolog exception is raised, it is important that a return be made to the calling environment as soon as possible. In C code, this requires checking every call for failure, which can become cumbersome; with the C++ API, most errors are thrown as exceptions to the enclosing PREDICATE() wrapper, and turned back into Prolog errors.

The C++ API provides Plx_*() functions that are the same as the PL_*() functions except that where appropriate they check for exceptions and thrown a PlException().

Addditionally, the function PlCheckFail() can be used to check for failure and throw a `PlFail` exception that is handled before returning to Prolog with failure.

The following three snippets do essentially the same thing (for implementing the equivalent of =/2); however the first version (with PlTerm::unify_term()) and second version (with Plx_unify()) throw a C++ `PlExceptionFail` exception if there's an error and otherwise return `true` or `false`; the third version (with PlCheckFail()) throws a `PlFail` exception for failure (and `PlExceptionFail` for an error) and otherwise returns `true` - the PREDICATE() wrapper handles all of these appropriately and reports the same result back to Prolog; but you might wish to distinguish the two situations in more complex code.

    PREDICATE(eq, 2)
    { return A1.unify_term(A2);
    }

    PREDICATE(eq, 2)
    { return Plx_unify(A1.unwrap(), A2.unwrap()));
    }

    PREDICATE(eq, 2)
    { PlCheckFail(A1.unify_term(A2));
      return true;
    }

## 1.6 Overview

One useful area for exploiting C++ features is type-conversion. Prolog variables are dynamically typed and all information is passed around using the C-interface type `term_t`. In C++, `term_t` is embedded in the lightweight class `PlTerm`. Other lightweight classes, such as `PlAtom` for `atom_t` are also provided. Constructors and operator definitions provide flexible operations and integration with important C-types (`char*`, `wchar_t*`, `long` and `double`), plus the C++-types (`std::string`, `std::wstring`). (`char*` and `wchar_t*` are deprecated in the C++ API; `std::string` and `std::wstring` are safer and should be used instead.)

Another useful area is in handling errors and cleanup. Prolog errors can be modeled using C++ exceptions; and C++'s destructors can be used to clean up error situations, to prevent memory and other resource leaks.

### 1.6.1 Design philosophy of the classes

See also section 1.6.5 for more on naming conventions and standard methods.

The general philosophy for C++ classes is that a “half-created” object should not be possible - that is, the constructor should either succeed with a completely usable object or it should throw an exception. This API tries to follow that philosophy, but there are some important exceptions and caveats. (For more on how the C++ and Prolog exceptions interrelate, see section 1.15.)

Most of the PL_*() functions have corresponding wrapper methods. For example, PlTerm::get_atom() calls Plx_get_atom(), which calls PL_get_atom(). If the PL_get_atom() has an error, it creates a Prolog error; the Plx_get_atom() wrapper checks for this and converts the error to a C++ exception, which is thrown; upon return to Prolog, the exception is turned back into a Prolog error. Therfore, code typically does not need to check for errors.

Some functions return `false` to indicate either failure or an error, for example PlTerm::unify_term(); for such methods, a check is made for an error and an exception is thrown, so the return value of `false` only means failure. (The whole thing can be wrapped in PlCheckFail(), in which case a `PlFail` exception is thrown, which is converted to failure in Prolog.) For more on this, see section 1.6.4, and for handling failure, see section 1.13.1.

For PL_*() functions that take or return `char*` or `wchar_t*` values, there are also wrapper functions and methods that use `std::string` or `std::wstring`. Because these copy the values, there is usually no need to enclose the calls with `PlStringBuffers` (which wraps PL_STRING_MARK() and PL_STRING_RELEASE()). See also the rationale for string: section 1.8.2.

Many of the classes (`PlAtom`, `PlTerm`, etc.) are thin wrappers around the C interface's types (`atom_t`, `term_t`, etc.). As such, they inherit the concept of “null” from these types (which is abstracted as `PlAtom::null`, `PlTerm::null`, etc., which typically is equivalent to `0`). Normally, you shouldn't need to check whether the object is “fully created” , for the rare situations where a check is needed, the methods is_null() and not_null() are provided.

Most of the classes have constructors that create a “complete” object. For example,

    PlAtom foo("foo");

will ensure that the object `foo` is useable and will throw an exception if the atom can't be created. However, if you choose to create a `PlAtom` object from an `atom_t` value, no checking is done (similarly, no checking is done if you create a `PlTerm` object from a `term_t` value).

In many situations, you will be using a term; for these, there are special constructors. For example:

    PlTerm_atom foo("foo"); // Same as PlTerm(PlAtom("foo"))
    PlTerm_string str("a string");

To help avoid programming errors, some of the classes do not have a default “empty” constructor. For example, if you with to create a `PlAtom` that is uninitialized, you must explicitly use `PlAtom(PlAtom::null)`. This make some code a bit more cumbersome because you can't omit the default constructors in struct initalizers.

Many of the classes have an as_string() method7This might be changed in future to to_string(), to be consistent with `std::to_string()`, which is useful for debugging.

The method names such as as_int32_t() were chosen itnstead of to_int32_t() because they imply that the representation is already an `int32_t`, and not that the value is converted to a `int32_t`. That is, if the value is a float, `int32_t` will fail with an error rather than (for example) truncating the floating point value to fit into a 32-bit integer.

Many of the classes wrap long-lived items, such as atoms, functors, predicates, or modules. For these, it's often a good idea to define them as `static` variables that get created at load time, so that a lookup for each use isn't needed (atoms are unique, so `PlAtom("foo")` requires a lookup for an atom `foo` and creates one if it isn't found).

C code sometimes creates objects “lazily” on first use:

    void my_function(...)
    { static atom_t ATOM_foo = 0;
       ...
      if ( ! foo  )
         foo = PL_new_atom("foo");
       ...
    }

For C++, this can be done in a simpler way, because C++ will call a local “`static`” constructor on first use.

    void my_function(...)
    { static PlAtom ATOM_foo("foo");
    }

The class `PlTerm` (which wraps `term_t`) is the most used. Although a `PlTerm` object can be created from a `term_t` value, it is intended to be used with a constructor that gives it an initial value. The default constructor calls PL_new_term_ref() and throws an exception if this fails. The various constructors are described in section 1.6.6. Note that the default constructor is not public; to create a “variable” term, you should use the subclass constructor PlTerm_var().

### 1.6.2 Summary of files

The following files are provided:

- `SWI-cpp2.h` - Include this file to get the C++ API. It automatically includes `SWI-cpp2-plx.h` and `SWI-cpp2.cpp`, unless the macro `_SWI_CPP2_CPP_SEPARATE` is defined, in which case you must compile `SWI-cpp2.cpp` separately.

- `SWI-cpp2.cpp` - Contains the implementations of some methods and functions. If you wish to compile this separately, you must define the macro `_SWI_CPP2_CPP_SEPARATE` before your include for `SWI-cpp2.h`.

- `SWI-cpp2-plx.h` - Contains the wrapper functions for the most of the functions in `SWI-Prolog.h`. This file is not intended to be used by itself, but is `#include`d by `SWI-cpp2.h`.

- `SWI-cpp2-atommap.h` - Contains a utility class for mapping atom-to-atom or atom-to-term, which is useful for naming long-lived blobs instead of having to pass them around as arguments.

- `test_cpp.cpp`, `test_cpp.pl` - Contains various tests, including some longer sequences of code that can help in understanding how the C++ API is intended to be used. In addition, there are `test_ffi.cpp`, `test_ffi.pl`, which often have the same tests written in C, without the C++ API.

### 1.6.3 Summary of classes

The list below summarises the classes defined in the C++ interface.

PlTerm
  Generic Prolog term that wraps `term_t` (for more details on `term_t`, see Interface Data Types).

  This is a “base class” whose constructor is protected; subclasses specify the actual contents. Additional methods allow checking the Prolog type, unification, comparison, conversion to native C++-data types, etc. See section 1.11.1.

  For more details about `PlTerm`, see section 1.6.6

PlCompound
  Subclass of `PlTerm` with constructors for building compound terms. If there is a single string argument, then PL_chars_to_term() or PL_wchars_to_term() is used to parse the string and create the term. If the constructor has two arguments, the first is name of a functor and the second is a `PlTermv` with the arguments.
PlTermv
  Vector of Prolog terms. See PL_new_term_refs(). The ``[]`` operator is overloaded to access elements in this vector. `PlTermv` is used to build complex terms and provide argument-lists to Prolog goals.
PlAtom
  Wraps `atom_t` in their internal Prolog representation for fast comparison. (For more details on `atom_t`, see Interface Data Types). For more details of `PlAtom`, see section 1.11.12.4.
PlFunctor
  A wrapper for `functor_t`, which maps to the internal representation of a name/arity pair.
PlPredicate
  A wrapper for `predicate_t`, which maps to the internal representation of a Prolog predicate.
PlModule
  A wrapper for `module_t`, which maps to the internal representation of a Prolog module.
PlQuery
  Represents opening and enumerating the solutions to a Prolog query.
PlException
  If a call to Prolog results in an error, the C++ interface converts the error into a `PlException` object and throws it. If the enclosing code doesn't intercept the exception, the `PlException` object is turned back into a Prolog error when control returns to Prolog from the PREDICATE() macros. This is a subclass of `PlExceptionBase`, which is a subclass of `std::exception`.
PlFrame
  This utility-class can be used to discard unused term-references as well as to do data-backtracking.
PlEngine
  This class is used in embedded applications (applications where the main control is held in C++). It provides creation and destruction of the Prolog environment.
PlRegister
  Encapsulates PL_register_foreign() to allow using C++ global constructors for registering foreign predicates.
PlFail
  Can be thrown to short-circuit processing and return failure to Prolog. Performance-critical code should use `return false` instead if failure is expected. An error can be signaled by calling Plx_raise_exception() or one of the PL_*_error() functions and then throwing `PlFail`; but it's better style to create the error throwing one of the subclasses of `PlException` e.g., `throw PlTypeError("int", t)`. Subclass of `PlExceptionFailBase`.
PlExceptionFail
  In some situations, a Prolog error cannot be turned into a `PlException` object, so a `PlExceptionFail` object is thrown. This is turned into failure by the PREDICATE() macro, resulting in normal Prolog error handling. Subclass of `PlExceptionFailBase`.
PlExceptionBase
  A “do nothing” subclass of `std::exception`, to allow catching `PlException`, `PlExceptionFail` or `PlFail` in a single “catch” clause.
PlExceptionFailBase
  A “do nothing” subclass of `PlExceptionBase`, to allow catching `PlExceptionFail` or `PlFail` in a single “catch” clause, excluding `PlException`.

### 1.6.4 Wrapper functions

The various PL_*() functions in `SWI-Prolog.h` have corresponding Plx_*() functions, defined in `SWI-cpp2-plx.h`, which is always included by `SWI-cpp2.h`. There are three kinds of wrappers, with the appropriate one being chosen according to the semantics of the wrapped function:

- “as-is” - the PL_*() function cannot cause an error. If it has a return value, the caller will want to use it.

- “exception wrapper” - the PL_*() function can return `false`, indicating an error. The Plx_*() function checks for this and throws a `PlException` object containing the error. The wrapper uses `template<typename C_t> C_t PlEx(C_t rc)`, where `C_t` is the return type of the PL_*() function.

- “success, failure, or error” - the PL_*() function can return `true` if it succeeds and `false` if it fails or has a runtime error. If it fails, the wrapper checks for a Prolog error and throws a `PlException` object containing the error. The wrapper uses `template<typename C_t> C_t PlWrap(C_t rc)`, where `C_t` is the return type of the PL_*() function.

A few PL_*() functions do not have a corresponding Plx*() function because they do not fit into one of these categories. For example, PL_next_solution() has multiple return values (`PL_S_EXCEPTION`, `PL_S_LAST`, etc.) if the query was opened with the `PL_Q_EXT_STATUS` flag.

Most of the PL_*() functions whose first argument is of type `term_t`, `atom_t`, etc. have corresponding methods in classes `PlTerm`, `PlAtom`, etc.

Important: You should use the Plx_*() wrappers only in the context of a PREDICATE() call, which will handle any C++ exceptions. Some blob callbacks can also handle an exception (see section 1.6.8). Everywhere else, the result of calling a Plx_*() function is unpredicatable - probably a crash.

### 1.6.5 Naming conventions, utility functions and methods

See also the discussion on design philosophy in section 1.6.1.

The classes all have names starting with “Pl” , using CamelCase; this contrasts with the C functions that start with “PL_” and use underscores.

The wrapper classes (`PlFunctor`, `PlAtom`, `PlTerm`), etc. all contain a field `C_` that contains the wrapped value (`functor_t`, `atom_t`, `term_t` respectively). If this wrapped value is needed, it should be accessed using the unwrap() or unwrap_as_ptr() methods.

In some cases, it's natural to use a pointer to a wrapper class. For those, the function PlUnwrapAsPtr() returns `nullptr` if the pointer is null; otherwise it returns the wrapped value (which itself might be some kind of “null” ).

The wrapper classes, which subclass `WrappedC<...>`, all define the following methods and constants:

- Default constructor (sets the wrapped value to `null`). Some classes do not have a default constructor because it can lead to subtle bugs - instead, they either have a different way of creating the object or can use the “null” value for the class.
- Constructor that takes the wrapped value (e.g., for `PlAtom`, the constructor takes an `atom_t` value).
- `C_` - the wrapped value. This can be used directly when calling C functions, for example, if `t` and `a` are of type `PlTerm` and `PlAtom`: `PlEx(PL_put_atom(t.unwrap(),a.unwrap()))` (although it's better to do `Plx_put_atom(t.unwrap(),a.unwrap())`, which does the check).
- `null` - the null value (typically `0`, but code should not rely on this).
- `is_null()`, `not_null()` - test for the wrapped value being `null`.
- `reset()` - set the wrapped value to `null`
- `reset(new_value)` - set the wrapped value from the wrapped type (e.g., PlTerm::reset(term_t new_value))
- `reset_wrapped(new_value)` - set the wrapped value from the same type (e.g., PlTerm::reset_wrapped(PlTerm new_value))
- The `bool` operator is disabled - you should use not_null() instead.8The reason: a `bool` conversion causes ambiguity with `PlAtom(PlTterm)` and `PlAtom(atom_t)`.

The method unwrap() can be used to access the `C_` field, and can be used wherever a `atom_t` or `term_t` is used. For example, the PL_scan_options() example code can be written as follows. Note the use of `&callback.unwrap()` to pass a pointer to the wrapped `term_t` value.

    PREDICATE(mypred, 2)
    { auto options = A2;
      int        quoted = false;
      size_t     length = 10;
      PlTerm_var callback;

      PlCheckFail(PL_scan_options(options, 0, "mypred_options", mypred_options,
                                  &quoted, &length, &callback.unwrap()));
      callback.record(); // Needed if callback is put in a blob that Prolog doesn't know about.
                         // If it were an atom (OPT_ATOM): register_ref().

      <implement mypred>
    }

For functions in `SWI-Prolog.h` that don't have a C++ equivalent in `SWI-cpp2.h`, PlCheckFail() is a convenience function that checks the return code and throws a `PlFail` exception on failure or `PlException` if there was an exception. The enclosing PREDICATE() code catches `PlFail` exceptions and converts them to the `foreign_t` return code for failure. If the failure from the C function was due to an exception (e.g., unification failed because of an out-of-memory condition), the foreign function caller will detect that situation and convert the failure to an exception.

The “getter” methods for `PlTerm` all throw an exception if the term isn't of the expected Prolog type. The “getter” methods typically start with “as” , e.g. PlTerm::as_string(). There are also other “getter” methods, such as PlTerm::get_float_ex() that wrap PL_*() functions.

“getters” for integers have an additional problem, in that C++ doesn't define the sizes of `int`, `long`, or `size_t`. It seems to be impossible to make an overloaded method that works for all the various combinations of integer types on all compilers, so there are specific methods for `int64_t`, `uint64_t`, `size_t`.

In some cases,it is possible to overload methods; for example, this allows the following code without knowing the exact definition of `size_t`:

    PREDICATE(p, 1)
    { size_t sz;
      A1.integer(&sz);
         ...
    }

It is strongly recommended that you enable conversion checking. For example, with GNU C++, use these options (possibly with `-Werror`): `-Wconversion -Warith-conversion -Wsign-conversion -Wfloat-conversion`.

There is an additional problem with characters - C promotes them to `int` but C++ doesn't. In general, this shouldn't cause any problems, but care must be used with the various getters for integers.

### 1.6.6 PlTerm class

As we have seen from the examples, the `PlTerm` class plays a central role in conversion and operating on Prolog data. This section provides complete documentation of this class.

There are a number of subclasses that exist only to provide a safe way of constructing at term. There is also a subclass (`PlTermScoped`) that helps reclaim terms.

Most of the `PlTerm` constructors are defined as subclasses of `PlTerm`, with a name that reflects the Prolog type of what is being created (e.g., `PlTerm_atom` creates a term from an atom; `PlTerm_string` creates a term from a Prolog string). This is done to ensure that the there is no ambiguity in the constructors - for example, there is no way to distinguish between `term_t`, `atom_t`, and ordinary integers, so there are constructors PlTerm(), PlTerm_atom(), and PlTerm_integer. All of the constructors are “explicit” because implicit creation of `PlTerm` objects can lead to subtle and difficult to debug errors.

If a constructor fails (e.g., out of memory), a `PlException` is thrown. The class and subclass constructors are as follows.

PlTerm :: PlTerm(PlAtom a)
  Creates a term reference containing an atom, using PL_put_atom(). It is the same as PlTerm_atom().
PlTerm :: PlTerm(term_t t)
  Creates a term reference from a C term (by wrapping it). As this is a lightweight class, this is a no-op in the generated code.
PlTerm :: PlTerm(PlRecord r)
  Creates a term reference from a record, using PL_recorded().
PlTerm_atom :: PlTerm_atom(atom_t a)
  Creates a term reference containing an atom.
PlTerm_atom :: PlTerm_atom(PlAtom a)
  Creates a term reference containing an atom.
PlTerm_atom :: PlTerm_atom(const char *text)
  Creates a term reference containing an atom, after creating the atom from the text.
PlTerm_atom :: PlTerm_atom(const wchar_t *text)
  Creates a term reference containing an atom, after creating the atom from the text.
PlTerm_atom :: PlTerm_atom(const std::string& text)
  Creates a term reference containing an atom, after creating the atom from the text.
PlTerm_atom :: PlTerm_atom(const std::wstring& text)
  Creates a term reference containing an atom, after creating the atom from the text.
PlTerm_var :: PlTerm_var()
  Creates a term reference for an uninstantiated variable. Typically this term is then unified with another object.
PlTerm_term_t :: PlTerm_term_t()
  Creates a term reference from a C `term_t`. This is a lightweight class, so no code is generated.
PlTerm_integer :: PlTerm_integer()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `long`.9PL_put_integer() takes a `long` argument.
PlTerm_int64 :: PlTerm_int64()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `int64_t`.
PlTerm_uint64 :: PlTerm_uint64()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `uint64_t`.
PlTerm_size_t :: PlTerm_size_t()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `size_t`.
PlTerm_float :: PlTerm_float()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog float.
PlTerm_pointer :: PlTerm_pointer()
  Subclass of `PlTerm` with constructors for building a term that contains a raw pointer. This is mainly for backwards compatibility; new code should use blobs. A pointer is represented in Prolog as a mangled integer. The mangling is designed to make most pointers fit into a tagged-integer. Any valid pointer can be represented. This mechanism can be used to represent pointers to C++ objects in Prolog. Please note that `MyClass` should define conversion to and from `void *`.

      PREDICATE(make_my_object, 1)
      { auto myobj = new MyClass();
        return A1.unify_pointer(myobj);
      }

      PREDICATE(my_object_contents, 2)
      { auto myobj = static_cast<MyClass*>(A1.as_pointer());
        return A2.unify_string(myobj->contents);
      }

      PREDICATE(free_my_object, 1)
      { auto myobj = static_cast<MyClass*>(A1.as_pointer());
        delete myobj;
        return true;
      }

PlTerm_string :: PlTerm_string()
  Subclass of `PlTerm` with constructors for building a term that contains a Prolog string object. For constructing a term from the text form, see `PlCompound`.
PlTerm_list_codes :: PlTerm_list_codes()
  Subclass of `PlTerm` with constructors for building Prolog lists of character integer values.
PlTerm_chars :: PlTerm_chars()
  Subclass of `PlTerm` with constructors for building Prolog lists of one-character atoms (as atom_chars/2).
PlTerm_tail :: PlTerm_tail()
  SubClass of `PlTerm` for building and analysing Prolog lists.

The methods are:

bool PlTerm::get_atom(PlAtom* a)
  Wrapper of PL_get_atom(), throwing an exception on Prolog error.
bool PlTerm::get_bool(int* value)
  Wrapper of PL_get_bool(), throwing an exception on Prolog error.
chars PlTerm::get_chars(char**s, unsigned int flags)
  Wrapper of PL_get_chars(), throwing an exception on Prolog error.
chars PlTerm::get_list_chars(char**s, unsigned int flags)
  Wrapper of PL_get_list_chars(), throwing an exception on Prolog error.
bool PlTerm::get_list_chars(char **s, unsigned int flags)
  Wrappper of PL_get_list_chars(), throwing an exception on Prolog error.
bool PlTerm::get_atom_nchars(size_t *len, char **a)
  Wrappper of PL_get_atom_nchars(), throwing an exception on Prolog error.
bool PlTerm::get_list_nchars(size_t *len, char **s, unsigned int flags)
  Wrappper of PL_get_list_nchars(), throwing an exception on Prolog error.
bool PlTerm::get_nchars(size_t *len, char **s, unsigned int flags)
  Wrappper of PL_get_nchars(), throwing an exception on Prolog error. Deprecated: see PlTerm::get_nchars(flags) that returns a `std::string`. If you use this, be sure to wrap it with `PlStringBuffers`, and if you use the `BUF_MALLOC` flag, you can use `std::unique_ptr<char, decltype(&PL_free)>` to manage the pointer.
bool PlTerm::get_wchars(size_t *length, pl_wchar_t **s, unsigned flags)
  Wrappper of PL_get_wchars(), throwing an exception on Prolog error. Deprecated: see PlTerm::getwchars(flags) that returns a `std::wstring`. If you use this, be sure to wrap it with `PlStringBuffers`, and if you use the `BUF_MALLOC` flag, you can use `std::unique_ptr<char, decltype(&PL_close)>` to manage the pointer.
bool PlTerm::get_integer(int *i)
  Wrappper of PL_get_integer(), throwing an exception on Prolog error.
bool PlTerm::get_long(long *i)
  Wrappper of PL_get_long(), throwing an exception on Prolog error.
bool PlTerm::get_intptr(intptr_t *i)
  Wrappper of PL_get_intptr(), throwing an exception on Prolog error.
bool PlTerm::get_pointer(void **ptr)
  Wrappper of PL_get_pointer(), throwing an exception on Prolog error.
bool PlTerm::get_float(double *f)
  Wrapper Plx_get_float(), throwing an exception on Prolog error.
bool PlTerm::get_functor(PlFunctor *f)
  Wrappper of PL_get_functor(), throwing an exception on Prolog error.
bool PlTerm::get_name_arity(PlAtom *name, size_t *arity)
  Wrappper of PL_get_name_arity(), throwing an exception on Prolog error.
bool PlTerm::get_compound_name_arity(PlAtom *name, size_t *arity)
  Wrappper of PL_get_compound_name_arity(), throwing an exception on Prolog error.
bool PlTerm::get_module(PlModule *module)
  Wrappper of PL_get_module(), throwing an exception on Prolog error.
bool PlTerm::get_arg(size_t index, PlTerm a)
  Wrappper of PL_get_arg(index,), throwing an exception on Prolog error.
bool PlTerm::get_dict_key(PlAtom key, PlTerm dict, PlTerm value)
  Wrappper of PL_get_dict_key(key.), throwing an exception on Prolog error.
bool PlTerm::get_list(PlTerm h, PlTerm t)
  Wrappper of PL_get_list(), throwing an exception on Prolog error.
bool PlTerm::get_head(PlTerm h)
  Wrappper of PL_get_head(), throwing an exception on Prolog error.
bool PlTerm::get_tail(PlTerm t)
  Wrappper of PL_get_tail(), throwing an exception on Prolog error.
bool PlTerm::get_nil()
  Wrappper of PL_get_nil(), throwing an exception on Prolog error.
bool PlTerm::get_blob(void **blob, size_t *len, PL_blob_t **type)
  Wrappper of PL_get_blob(), throwing an exception on Prolog error.
bool PlTerm::get_file_name(char **name, int flags)
  Wrappper of PL_get_file_name() (does not throw a C++ exception).
bool PlTerm::get_file_nameW(wchar_t **name, int flags)
  Wrappper of PL_get_file_nameW(), (does not throw a C++ exception).
std::string PlTerm::get_file_name(char **name, int flags)
  Wrapper of PL_get_file_name(), ignoring `PL_FILE_NOERRORS` - throws `PlFail` on failure, which is interpreted by the enclosing `PREDICATE` as either failure or an error, depending on the flag bit `PL_FILE_NOERRORS`.
std::wstring PlTerm::get_file_nameW(int flags)
  Same as PlTerm::get_file_name(), but returns a wide-character string.
bool PlTerm::get_attr(term_t a)
  Wrappper of PL_get_attr(), throwing an exception on Prolog error.
void PlTerm::get_atom_ex(PlAtom *a)
  Wrapper of PL_get_atom_ex(), throwing an exception on Prolog error.
void PlTerm::get_integer_ex(int *i)
  Wrapper of PL_get_integer_ex(), throwing an exception on Prolog error.
void PlTerm::get_long_ex(long *i)
  Wrapper of PL_get_long_ex(), throwing an exception on Prolog error.
void PlTerm::get_int64_ex(int64_t *i)
  Wrapper of PL_get_int64_ex(), throwing an exception on Prolog error.
void PlTerm::get_uint64_ex(uint64_t *i)
  Wrapper of PL_get_uint64_ex(), throwing an exception on Prolog error.
void PlTerm::get_intptr_ex(intptr_t *i)
  Wrapper of PL_get_intptr_ex(), throwing an exception on Prolog error.
void PlTerm::get_size_ex(size_t *i)
  Wrapper of PL_get_size_ex(), throwing an exception on Prolog error.
void PlTerm::get_bool_ex(int *i)
  Wrapper of PL_get_bool_ex(), throwing an exception on Prolog error.
void PlTerm::get_float_ex(double *f)
  Wrapper of PL_get_float_ex(), throwing an exception on Prolog error.
void PlTerm::get_char_ex(int *p, int eof)
  Wrapper of PL_get_char_ex(), throwing an exception on Prolog error.
void PlTerm::unify_bool_ex(int val)
  Wrapper of PL_unify_bool_ex(), throwing an exception on Prolog error.
void PlTerm::get_pointer_ex(void **addrp)
  Wrapper of PL_get_pointer_ex(), throwing an exception on Prolog error.
bool PlTerm::unify_list_ex(PlTerm h, PlTerm t)
  Wrappper of PL_unify_list_ex(), throwing an exception on Prolog error.
void PlTerm::unify_nil_ex()
  Wrapper of PL_unify_nil_ex(), throwing an exception on Prolog error.
bool PlTerm::get_list_ex(PlTerm h, PlTerm t)
  Wrappper of PL_get_list_ex(), throwing an exception on Prolog error.
void PlTerm::get_nil_ex()
  Wrapper of PL_get_nil_ex(), throwing an exception on Prolog error.
int PlTerm::type()
  Wrapper of PL_term_type(unwrap()), returning `PL_VARIABLE`, `PL_ATOM`, etc, throwing an exception on Prolog error. bo
ol PlTerm::is_attvar()
  Wrappper of PL_is_attvar(), throwing an exception on Prolog error.
bool PlTerm::is_variable()
  Wrappper of PL_is_variable(), throwing an exception on Prolog error.
bool PlTerm::is_ground()
  Wrappper of PL_is_ground(), throwing an exception on Prolog error.
bool PlTerm::is_atom()
  Wrappper of PL_is_atom(), throwing an exception on Prolog error.
bool PlTerm::is_integer()
  Wrappper of PL_is_integer(), throwing an exception on Prolog error.
bool PlTerm::is_string()
  Wrappper of PL_is_string(), throwing an exception on Prolog error.
bool PlTerm::is_atom_or_string()
  `is_atom()` or `is_string()`.
bool PlTerm::is_float()
  Wrappper of PL_is_float(), throwing an exception on Prolog error.
bool PlTerm::is_rational()
  Wrappper of PL_is_rational(), throwing an exception on Prolog error.
bool PlTerm::is_compound()
  Wrappper of PL_is_compound(), throwing an exception on Prolog error.
bool PlTerm::is_callable()
  Wrappper of PL_is_callable(), throwing an exception on Prolog error.
bool PlTerm::is_list()
  Wrappper of PL_is_list(), throwing an exception on Prolog error.
bool PlTerm::is_dict()
  Wrappper of PL_is_dict(), throwing an exception on Prolog error.
bool PlTerm::is_pair()
  Wrappper of PL_is_pair(), throwing an exception on Prolog error.
bool PlTerm::is_atomic()
  Wrappper of PL_is_atomic(), throwing an exception on Prolog error.
bool PlTerm::is_number()
  Wrappper of PL_is_number(), throwing an exception on Prolog error.
bool PlTerm::is_acyclic()
  Wrappper of PL_is_acyclic(), throwing an exception on Prolog error.
bool PlTerm::is_functor(PlFunctor f)
  Wrappper of PL_is_functor(), throwing an exception on Prolog error.
bool PlTerm::is_blob(PL_blob_t **type)
  Wrappper of PL_is_blob(), throwing an exception on Prolog error.
void PlTerm::must_be_attvar()
  Throw `PlTypeError` if PlTerm::is_attvar() fails.
void PlTerm::must_be_variable()
  Throw `PlTypeError` if PlTerm::is_variable() fails.
void PlTerm::must_be_ground()
  Throw `PlTypeError` if PlTerm::is_ground() fails.
void PlTerm::must_be_atom()
  Throw `PlTypeError` if PlTerm::is_atom() fails.
void PlTerm::must_be_integer()
  Throw `PlTypeError` if PlTerm::is_integer() fails.
void PlTerm::must_be_string()
  Throw `PlTypeError` if PlTerm::is_string() fails.
void PlTerm::must_be_atom_or_string()
  Throw `PlTypeError` if PlTerm::is_atom_or_string() fails.
void PlTerm::must_be_float()
  Throw `PlTypeError` if PlTerm::is_float() fails.
void PlTerm::must_be_rational()
  Throw `PlTypeError` if PlTerm::is_rational() fails.
void PlTerm::must_be_compound()
  Throw `PlTypeError` if PlTerm::is_compound() fails.
void PlTerm::must_be_callable()
  Throw `PlTypeError` if PlTerm::is_callable() fails.
void PlTerm::must_be_list()
  Throw `PlTypeError` if PlTerm::is_list() fails.
void PlTerm::must_be_dict()
  Throw `PlTypeError` if PlTerm::is_dict() fails.
void PlTerm::must_be_pair()
  Throw `PlTypeError` if PlTerm::is_pair() fails.
void PlTerm::must_be_atomic()
  Throw `PlTypeError` if PlTerm::is_atomic() fails.
void PlTerm::must_be_number()
  Throw `PlTypeError` if PlTerm::is_number() fails.
void PlTerm::must_be_acyclic()
  Throw `PlTypeError` if PlTerm::is_acyclic() fails.
void PlTerm::put_variable()
  Wrapper of PL_put_variable(), throwing an exception on Prolog error.
void PlTerm::put_atom(PlAtom a)
  Wrapper of PL_put_atom(), throwing an exception on Prolog error.
void PlTerm::put_bool(int val)
  Wrapper of PL_put_bool(), throwing an exception on Prolog error.
void PlTerm::put_atom_chars(const char *chars)
  Wrapper of PL_put_atom_chars(), throwing an exception on Prolog error.
void PlTerm::put_string_chars(const char *chars)
  Wrapper of PL_put_string_chars(), throwing an exception on Prolog error.
void PlTerm::put_chars(int flags, size_t len, const char *chars)
  Wrapper of PL_put_chars(), throwing an exception on Prolog error.
void PlTerm::put_list_chars(const char *chars)
  Wrapper of PL_put_list_chars(), throwing an exception on Prolog error.
void PlTerm::put_list_codes(const char *chars)
  Wrapper of PL_put_list_codes(), throwing an exception on Prolog error.
void PlTerm::put_atom_nchars(size_t l, const char *chars)
  Wrapper of PL_put_atom_nchars(), throwing an exception on Prolog error.
void PlTerm::put_string_nchars(size_t len, const char *chars)
  Wrapper of PL_put_string_nchars(), throwing an exception on Prolog error.
void PlTerm::put_list_nchars(size_t l, const char *chars)
  Wrapper of PL_put_list_nchars(), throwing an exception on Prolog error.
void PlTerm::put_list_ncodes(size_t l, const char *chars)
  Wrapper of PL_put_list_ncodes(), throwing an exception on Prolog error.
void PlTerm::put_integer(long i)
  Wrapper of PL_put_integer(), throwing an exception on Prolog error.
void PlTerm::put_pointer(void *ptr)
  Wrapper of PL_put_pointer(), throwing an exception on Prolog error.
void PlTerm::put_float(double f)
  Wrapper of PL_put_float(), throwing an exception on Prolog error.
void PlTerm::put_functor(PlFunctor functor)
  Wrapper of PL_put_functor(), throwing an exception on Prolog error.
void PlTerm::put_list()
  Wrapper of PL_put_list(), throwing an exception on Prolog error.
void PlTerm::put_nil()
  Wrapper of PL_put_nil(), throwing an exception on Prolog error.
void PlTerm::put_term(PlTerm t2)
  Wrapper of PL_put_term(), throwing an exception on Prolog error.
void PlTerm::put_blob(void *blob, size_t len, PL_blob_t *type)
  Wrapper of PL_put_blob(), throwing an exception on Prolog error.
PlRecord PlTerm::record()
  Returns a `PlRecord` constructed from the term. Same as PlRecord(*this).
void PlTerm::integer(bool *v)
  Wrapper of PL_cvt_i_bool().
void PlTerm::integer(char *v)
  Wrapper of PL_cvt_i_char().
void PlTerm::integer(int *v)
  Wrapper of PL_cvt_i_int().
void PlTerm::integer(long *v)
  Wrapper of PL_cvt_i_long().
void PlTerm::integer(long long *v)
  Wrapper of PL_cvt_i_llong().
void PlTerm::integer(short *v)
  Wrapper of PL_cvt_i_short().
void PlTerm::integer(signed char *v)
  Wrapper of PL_cvt_i_schar().
void PlTerm::integer(unsigned char *v)
  Wrapper of PL_cvt_i_uchar().
void PlTerm::integer(unsigned int *v)
  Wrapper of PL_cvt_i_uint().
void PlTerm::integer(unsigned long *v)
  Wrapper of PL_cvt_i_ulong().
void PlTerm::integer(unsigned long long *v)
  Wrapper of PL_cvt_i_ullong().
void PlTerm::integer(unsigned short *v)
  Wrapper of PL_cvt_i_ushort().
const std::string PlTerm::as_string(PlEncoding enc=ENC_OUTPUT)
  Calls PlTerm::get_nchars(CVT_ALL|CVT_WRITEQ|CVT_EXCEPTION). This method is provided mainly for debugging. The definition is subject to change in future - if you want precise control, use PlTerm::get_nchars().
const std::wstring PlTerm::as_wstring()
  Calls PlTerm::get_wchars(CVT_ALL|CVT_WRITEQ|CVT_EXCEPTION). This method is provided mainly for debugging. The definition is subject to change in future - if you want precise control, use PlTerm::get_nchars().
long PlTerm::as_long()
  Wrapper of PL_cvt_i_*().
int32_t PlTerm::as_int32_t()
  Wrapper of PL_cvt_i_*().
uint32_t PlTerm::as_uint32_t()
  Wrapper of PL_cvt_i_*().
uint64_t PlTerm::as_uint64_t()
  Wrapper of PL_cvt_i_*().
int64_t PlTerm::as_int64_t()
  Wrapper of PL_cvt_i_*().
size_t PlTerm::as_size_t()
  Wrapper of PL_cvt_i_*().
int PlTerm::as_int()
  Wrapper of PL_cvt_i_*().
unsigned PlTerm::as_uint()
  Wrapper of PL_cvt_i_*().
unsigned long PlTerm::as_ulong()
  Wrapper of PL_cvt_i_*().
bool PlTerm::as_bool()
  Wrapper of PL_cvt_i_*().
void PlTerm::as_nil()
  Wrapper of PL_get_nil_ex(), throwing an exception if the term isn't “nil” .
double PlTerm::as_float()
  Wrapper of PL_get_float_ex(), throwing an exception if the term isn't a float.
double PlTerm::as_double()
  Wrapper of PL_get_float_ex(), throwing an exception if the term isn't a float.
void * PlTerm::as_pointer()
  (Deprecated: should use blob API). Wrapper of PL_get_pointer_ex(), throwing an exception if the term isn't a blob.
const std::string PlTerm::get_nchars(unsigned int flags)
  Calls PL_get_nchars(..., flags) and converts the result to a `std::string`. The flags `BUF_MALLOC`, `BUF_STACK`, and `BUF_ALLOW_STACK` are ignored and replaced by `BUF_DISCARDABLE`.
const std::wstring PlTerm::get_wchars(unsigned int flags)
  Calls PL_get_wchars(..., flags) and converts the result to a `std::wstring`. The flags `BUF_MALLOC`, `BUF_STACK`, and `BUF_ALLOW_STACK` are ignored and replaced by `BUF_DISCARDABLE`.
PlAtom PlTerm::as_atom()
  Wrapper of PL_get_atom_ex(), throwing an exception if the term is not an atom.
bool PlTerm::eq_if_atom(PlAtom a)
  Returns true if the term is an atom and equal to a.
PlTerm::operator[] size_t index(W)
  rapper for PL_get_arg(), throwing an exception if the term isn't a compound or the index is out of range.
size_t PlTerm::arity()
  Gets the arity of the term; throws `PlTypeError` if not a "compound" or atom.
PlAtom PlTerm::name()
  Gets the name of the term; `PlTypeError` if not a "compound" or atom.
bool name_arity(PlAtom *name, size_t *arity)
  Wrapper of PL_get_name_arity(); name and/or arity can be `nullptr`. Returns `false` if the term isn't a compound or atom.
PlTerm PlTerm::copy_term_ref()
  Wrapper of PL_copy_term_ref(). Throws an exception error (e.g., `PlResourceError`).
void PlTerm::free_term_ref()
  Wrapper of PL_free_term_ref(). Is safe to use if the object wraps `PlTerm::null`. Does not reset the wrapped term. This is used implicitly in `PlTermScoped`’s destructor, which does reset the wrapped term.
void PlTerm::free_term_ref_reset()
  Same as PlTerm::free_term_ref() plus PlTerm::reset(). `PlTermScoped`’s destructor, which does reset the wrapped term.
bool nify_term(PlTerm t2)
  Wrapper of PL_unify(). Throws an exception on error and returns `false` if unification fails. If on failure, there isn't an immediate return to Prolog (e.g., by wrapping the call with PlCheckFail()), this method should be called within the context of `PlFrame`, and PlFrame::rewind() should be called.
bool PlTerm::unify_atom(PlAtom a)
  Wrapper of PL_unify_atom(), throwing an exception on error.
bool PlTerm::unify_chars(int flags, size_t len, const char *s)
  Wrapper of PL_unify_chars(), throwing an exception on error.
bool PlTerm::unify_chars(int flags, const std::string& s)
  Wrapper of PL_unify_chars(), throwing an exception on error.
bool PlTerm::unify_atom(const char* v)
  Wrapper of PL_unify_atom_chars(), throwing an exception on error.
bool PlTerm::unify_atom(const wchar_t* v)
  Wrapper of PL_unify_wchars(), throwing an exception on error.
bool PlTerm::unify_atom(const std::string& v)
  Wrapper of PL_unify_atom_nchars(), throwing an exception on error.
bool PlTerm::unify_atom(const std::wstring& v)
  Wrapper of PL_unify_wchars(), throwing an exception on error. cfunctionboolPlTerm::unify_integerbool v Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(char v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(int v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(long v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(long long v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(short v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(signed char v)
  Wrapper of PL_unify_int64(), throwing an exception on error.
bool PlTerm::unify_integer(unsigned char v)
  Wrapper of PL_unify_uint64(), throwing an exception on error.
bool PlTerm::unify_integer(unsigned int v)
  Wrapper of PL_unify_uint64(), throwing an exception on error.
bool PlTerm::unify_integer(unsigned long v)
  Wrapper of PL_unify_uint64(), throwing an exception on error.
bool PlTerm::unify_integer(unsigned long long v)
  Wrapper of PL_unify_uint64(), throwing an exception on error.
bool PlTerm::unify_integer(unsigned short v)
  Wrapper of PL_unify_uint64(), throwing an exception on error.
bool PlTerm::unify_float(double v)
  Wrapper of PL_unify_float(), throwing an exception on error.
bool PlTerm::unify_string(const std::string& v)
  Wrapper of PL_unify_string_nchars(), throwing an exception on error.
bool PlTerm::unify_string(const std::wstring& v)
  Wrapper of PL_unify_wchars(), throwing an exception on error.
bool PlTerm::unify_functor(PlFunctor f)
  Wrapper of PL_unify_functor(), throwing an exception on error.
bool PlTerm::unify_pointer(void *ptr)
  Wrapper of PL_unify_pointer(), throwing an exception on error. An alternative to this is to use a blob that wraps a pointer - see section 1.6.8.7.
bool PlTerm::unify_nil()
  Wrapper of PL_unify_nil(), throwing an exception on error.
bool PlTerm::unify_list(PlTerm h, PlTerm t)
  Wrapper of PL_unify_list(), throwing an exception on error.
bool PlTerm::unify_bool(bool val)
  Wrapper of PL_unify_bool(), throwing an exception on error.
bool PlTerm::unify_blob(const PlBlob* blob)
  Wrapper of PL_unify_blob(), throwing an exception on error.
bool PlTerm::unify_blob(const void *blob, size_t len, const PL_blob_t *type)
  Wrapper of PL_unify_blob(), throwing an exception on error.
int PlTerm::compare(PlTerm t2)
  Wrapper for PL_compare(), returning -1, 0, 1 for the result of standard order comparison of the term with a2.
bool operator ==(PlTerm t2)
  `compare(t2) == 0`.
bool operator !=(PlTerm t2)
  `compare(t2) != 0`.
bool operator <(PlTerm t2)
  `compare(t2) < 0`.
bool operator >(PlTerm t2)
  `compare(t2) > 0`.
bool operator <=(PlTerm t2)
  `compare(t2) <= 0`.
bool operator >=(PlTerm t2)
  `compare(t2) >= 0`.
int write(IOSTREAM *s, int precedence, int flags)
  Wrapper for PL_write_term().
void reset_term_refs()
  Wrapper for PL_reset_term_refs().
bool call(PlModule module)
  Wrapper for PL_call(unwrap()); module defaults to “null” . Throws a C++ exception if there's a Prolog error, otherwise returns the success or failure of the call.

### 1.6.7 PlTermScoped class (experimental)

This class is experimental and subject to change.

Normally all term references in a scope are discarded together or all term references created after a specific one are reclaimed using PlTerm::reset_term_refs(). A `PlTermScoped` object is the same as a `PlTerm` object except that PL_free_term_ref() is called on its wrapped term when the object goes out of scope. This shrinks the current foreign frame if the term is the last one in the frame and otherwise it marks it for reuse.

Here is an example, where `PlTermScoped` is inside a for-loop. If `PlTerm` were used instead, the stack would grow by the number of items in the array; `PlTermScoped` ensures that stack doesn't grow.10Assuming that unify_atom_list() is called from a predicate implementation, if `PlTerm` were used instead of `PlTermCopy`, all the created terms would be discarded when the Prolog stack frame is unwound; the use of `PlTermScoped` reuses the terms in that stack frame. A slightly more effiicient way of preventing the Prolog stack from growing is to use PlTerm::put_term() to reuse a term reference; but that is more difficult to understand and also more error-prone.

    bool
    unify_atom_list(const std::vector<std::string>& array, PlTerm list)
    { PlTermScoped tail(list); // calls PL_copy_term_ref() to copy `list`
      for( auto item : array )
      { PlTermScoped head; // var term
        PlCheckFail(tail.unify_list(head, tail));
        PlCheckFail(head.unify_chars(PL_ATOM, item));
      }
      return tail.unify_nil();
    }

The design of `PlTermScoped` is modeled on `std::unique_ptr`11`unique_ptr` was originally called `scoped_ptr` in the Boost libraries, but the name was changed to contrast with `std::shared_ptr`, which is reference-counted. and uses move semantics to ensure safety.12Move semantics are a relatively new feature in C++ and can be a bit difficult to understand. Roughly speaking, a move is a copies the object and then calls its destructor, so that any further use of the object is an error. If an object defines move methods or constructors, it can optimize this operation, and also can catch certain kinds of errors at compile time.

A `PlTermScoped` object can be created either with or without a wrapped term - the PlTermScoped::reset() method sets (or nulls) the wrapped term. A `PlTermScoped` object cannot be copied or passed as a value to a function; the PlTermScoped::release() method returns the wrapped term and resets the `PlTermScoped` object so that any further use of the `PlTermScoped` object is an error.

As shown in the example above, `PlTermScoped` can be used instead of `PlTerm`, in places where a loop would otherwise cause the stack to grow. There are limitations on the operations that are allowed on a `PlTermScoped` object; in particular, a `PlTermScoped` object cannot be copied and cannot be implicitly converted to a `Plterm`.

The `PlTermScoped` constructors always create a new term ref, by calling either PL_new_term_ref() or PL_copy_term_ref(). If you try to copy or create a `PlTermScoped` object from another `PlTermScoped` object, you will get a compile-time error; you can set the value from a `PlTerm` object, which can be obtained by calling PlTermScoped::release().

The methods derived from the PL_put_*() and PL_cons_*() functions should not be used with a `PlTermScoped` object. If you need to use these, you can use PlTermScoped::get() to get a `PlTerm`, for which a put_*() method can be used.

To copy a `PlTermScoped` object or to pass it as a value in a function call, use the PlTermScoped::release() method or std::move():

      PlTermScoped ts(...);
      PlTerm t;

      // Copy to a PlTerm:
      t = ts.release(); // or: t = std::move(ts);

      // Pass as a value to a function:
      foo(ts.release()); // or: foo(std::move(ts);

      // Copy to a PlTermScoped:
      PlTermScoped ts2;
      ts2.reset(ts.release()); // or: ts2.reset(std::move(ts));

The methods are (in addition to, or overriding the methods in `PlTerm`):

PlTermScoped :: PlTermScoped()
  - same as PlTermScoped(PlTerm::null).
PlTermScoped :: PlTermScoped(PlTerm t)
  - set the value from t.copy_term_ref()
PlTermScoped :: PlTermScoped(term_t t)
  - same as PlTermScoped(PlTerm(t)).
PlTermScoped :: PlTermScoped(PlTermScoped&& m)
  - create a new wrapped object from m and reset m. This is typically used with std::move().
PlTermScoped& PlTermScoped::operator=(PlTermScoped&& m)
  - copy m and reset it. This is typically used with std::move().
~ PlTermScoped()
  - if the wrapped term not null, call PL_free_term_ref() on it.
PlTermScoped :: PlTermScoped(PlTermScoped& m)
  - deleted method.
PlTermScoped& operator=(PlTermScoped& m)
  - deleted method.
void PlTermScoped::reset()
  - same as PlTermScoped::reset(PlTerm::null).
void PlTermScoped::reset(PlTerm src)
  - sets the wrapped term from src. To set the wrapped term from a `PlTermScoped`, use PlTermScoped::release() to convert it to a `PlTerm`.
PlTerm PlTermScoped::get()
  - convert the object to a `PlTerm`. This is typically used when calling a function that expects a `PlTerm` object and which will not call PlTerm::free_term_ref() on it.
PlTerm PlTermScoped::release()
  - typically used in the context `t2.reset(t.release())` to copy a `PlTermScoped`; this can also be written `t2=std::move(t)`.
void PlTermScoped::swap(PlTermScoped& src)
  - swap two `PlTermScoped` objects’wrapped terms.

### 1.6.8 Blobs

Nomenclature warning:

There are two different `release()` functions:

- The release() callback for a blob (see the definition of `PL_blob_t`).
- std::unique_ptr::release(), which passes ownership of a `unique_ptr`.

Disclaimer:

The blob API for C++ is not completely general, but is designed to make common use cases easy to write. For other use cases, the underlying C API can still be used. The use case is:

- The blob is defined as a subclass of `PlBlob`, which provides a number of fields and methods, of which a few can be overridden in the blob (notably: write_fields(), compare_fields(), save(), load(), and the destructor).
- The blob will not be subclassed.
- The blob contains the foreign object or a pointer to it (e.g., a database connection or a pointer to a database connection), plus optionally some other data.
- The blob is created by a predicate that makes the foreign object and stores it (or a pointer to it) within the blob - for example, making a connection to a database or compiling a regular expression into an internal form. This “create” predicate uses `std::unique_ptr` to manage the blob (that is, the blob is created using the new operator and is not created on the stack).
- Optionally, there can be a predicate that deletes the foreign object, such as a file or database connection close.
- The blob can be garbage collected, althought this might require calling the predicate that deletes the foreign object first. There is no provision for handling “weak references” (e.g., a separate lookup table or cache for the foreign objects).
- The blob must have a default constructor that sets all the fields to appropriate initial values.13This is used by the load() callback; the default implementation for a C++ blob is to throw an error.
- The blob's constructor throws an exception and cleans up any resources if it cannot create the blob.14This is not a strong requirement, but the code is simpler if this style is used.
- The foreign object can be deleted when the blob is deleted. That is, the foreign object is created using the `new` operator and passes ownership to the blob. More complex behavior is possible, using PlAtom::register_ref() and PlAtom::unregister_ref().
- The blob's lifetime is controlled by Prolog and its destructor is invoked when the blob is garbage collected. Optionally, the predicate that deletes the foreign object deletes the foreign object and the Prolog garbage collector only frees the blob.

A Prolog blob consists of five parts:

- A `PL_blob_t` structure that defines the callbacks. The PL_BLOB_DEFINITION() macro is typically used to create this, with the callbacks pointing to methods in the C++ blob.
- A structure that contains the blob data. This must have a constructor that references the `PL_blob_t` structure, and optionally a virtual destructor. The `PL_BLOB_SIZE` macro is used to define some required methods.
- A “create” or “open” predicate that unifies one of its arguments with a newly created blob that contains the foreign object. The blob is created using the new operator (not on the stack) and managed with `std::unique_ptr`.
- (Optionally) a “close” predicate that does the opposite of the “create” or “open” predicate.
- Predicates that manipulate the foreign object (e.g., for a file-like object, these could be read, write, seek, etc.).

For the `PL_blob_t` structure, the C++ API provides the PL_BLOB_DEFINITION(blob_class,blob_name) macro, which references a set of template functions that allow easily setting up the callbacks. The C interface allows more flexibility by allowing some of the callbacks to default; however, the C++ API for blobs provides suitable callbacks for all of them, using the PL_BLOB_DEFINITION() macro.

For the data, which is subclassed from `PlBlob`, the programmer defines the various fields, a constructor that initializes them, and a destructor. Optionally, override methods can be defined for one of more of the methods PlBlob::compare_fields(), PlBlob::write_fields(), PlBlob::save(), PlBlob::load(), PlBlob::pre_delete(). More details on these are given later.

There is a mismatch between how Prolog does memory management (and garbage collection) and how C++ does it. In particular, Prolog assumes that cleanup will be done in the release() callback function associated with the blob whereas C++ typically does cleanup in a destructor. The blob interface gets around this mismatch by providing a default release() callback that assumes that the blob was created using `PL_BLOB_NOCOPY` and manages memory using a `std::unique_ptr`.15This release() function has nothing to do with std::unique_ptr::release(). More details on this are in section 1.6.8.1.

The C blob interface has a flag that determines how memory is managed: `PL_BLOB_NOCOPY`. The PL_BLOB_DEFINITION() macro sets this, so Prolog will call the C++ destructor when the blob is garbage collected. (This call is done indirectly, using a callback that is registeered with Prolog.)

The C++ API for blobs only supports blobs with `PL_BLOB_NOCOPY`.16The API can probably also support blobs with `PL_BLOB_UNIQUE`, but there seems to be little point in setting this flag for non-text blobs.

#### 1.6.8.1 A review of C++ features used by the API

Some slightly obscure features of C++ are used with `PlBlob` and `ContextType`, and can easily cause subtle bugs or memory leaks if not used carefully.

When a C++ object is created, its memory is allocated (either on the stack or on the heap using new), and the constructors are called in this order:

- the base class's constructor (possibly specified in the intialization list)
- the constructors for all the fields (possibly specified by an initial value and/or being in the initialization list)
- the object's constructor.

When the object is deleted (either by stack pop or the delete operator), the destructors are called in the reverse order.

There are special forms of the constructor for copying, moving, and assigning. The “copy constructor” has a signature `Type(const Type&` and is used when an object is created by copying, for example by assignment or passing the object on the stack in a function call. The “move constructor” has the signature `Type(Type&&` and is equivalent to the copy constructor for the new object followed by the destructor for the old object. (Assignment is usually allowed to default but can also be specified).

Currently, the copy and move constructors are not used, so it is best to explicitly mark them as not existing:

    Type(const Type&) = delete;
    Type(Type&&) = delete;
    Type& operator =(const Type&) = delete;
    Type& operator =(Type&&) = delete;

A constructor may throw an exception - good programming style is to not leave a “half constructed” object but to throw an exception. Destructors are not allowed to throw exceptions,17because the destructor might be invoked by another exception, and C++ has no mechanism for dealing with a second exception. which complicates the API somewhat.

More details about constructors and destructors can be found in the FAQs for constructors and destructors.

Many classes or types have a constructor that simply assigns a default value (e.g., 0 for `int`) and the destructor does nothing. In particular, the destructor for a pointer does nothing, which can lead to memory leaks. To avoid memory leaks, the smart pointer `std::unique_ptr`18The name “unique” is to distinguish this from a “shared” pointer. A shared pointer can share ownership with multiple pointers and the pointed-to object is deleted only when all pointers to the object have been deleted. A unique pointer allows only a single pointer, so the pointed-to object is deleted when the unique pointer is deleted. can be used, whose destructor deletes its managed object. Note that `std::unique_ptr` does not enforce single ownership; it merely makes single ownership easy to manage and it detects most common mistakes, for example by not having copy constructor or assignment operator.

For example, in the following, the implicit destructor for `p` does nothing, so there will be a memory leak when a `Ex1` object is deleted:

    class Ex1 {
    public:
      Ex1() : p(new int) { }
      int *p;
    };

To avoid a memory leak, the code could be changed to this:

    class Ex1 {
    public:
      Ex1() p(new int) { }
      ~Ex1() { delete p; }
      int *p;
    };

but it is easier to do the following, where the destructor for `std::unique_ptr` will free the memory:

    class Ex1 {
    public:
      Ex1() p(new int) { }
      std::unique_ptr<int> p;
    };

The same concept applies to objects that are created in code - if a C++ object is created using new, the programmer must manage when its destructor is called. In the following, if the call to `data->validate()` fails, there will be a memory leak:

    MyData *foo(int some_value) {
      MyData *data = new MyData(...);
      data->some_field = some_value;
      if (! data->validate() )
        throw std::runtime_error("Failed to validate data");
      return data;
    }

Ths could fixed by adding `delete data` before throwing the `runtime_error`; but this doesn't handle the situation of `data->validate()` throwing an exception (which would require a catch/throw). Instead, it's easiser to use `std::unique_ptr`, which takes care of every return or exception path:

    MyData *foo(int some_value) {
      std::unique_ptr<MyData> data(new MyData(...));
      data->some_field = some_value;
      if (! data->validate() )
        throw std::runtime_error("Failed to validate data");
      return data.release(); // don't delete the new MyData
    }

The destructor for `std::unique_ptr` will delete the data when it goes out of scope (in this case, by return or throw) unless the std::unique_ptr::release() method is called.19The call to `unique_ptr<MYData>::release` doesn't call the destructor; it can be called using std::unique_ptr::get_deleter().

In the code above, the `throw` will cause the `unique_ptr`’s destructor to be called, which will free the data; but the data will not be freed in the `return` statement because of the unique_ptr::release(). Using this style, a pointer to data on the heap can be managed as easily as data on the stack. The current C++ API for blobs takes advantage of this - in particular, there are two methods for unifying a blob:

- PlTerm::unify_blob(const PlBlob* blob) - does no memory management
- PlTerm::unify_blob(std::unique_std<PlBlob>* blob) - if unification fails or raises an error, the memory is automatically freed; otherwise the memory's ownership is transferred to Prolog, which may garbage collect the blob by calling the blob's destructor. Note that this uses a pointer to the pointer, so that PlTerm::unify_blob() can modify it.

`unique_ptr` allows specifying the delete function. For example, the following can be used to manage memory created with PL_malloc():

      std::unique_ptr<void, decltype(&PL_free)> ptr(PL_malloc(...), &PL_free);

or, when memory is allocated within a PL_*() function (in this case, using the Plx_*() wrapper for PL_get_nchars()):

      size_t len;
      char *str = nullptr;
      Plx_get_nchars(t, &len, &str.get(), BUF_MALLOC|CVT_ALL|CVT_WRITEQ|CVT_VARIABLE|REP_UTF8|CVT_EXCEPTION);
      std::unique_ptr<char, decltype(&PL_free)> _str(str, &PL_free);

The current C++ API assumes that the C++ blob is allocated on the heap. If the programmer wishes to use the stack, they can use `std::unique_ptr` to automatically delete the object if an error is thrown - PlTerm::unify_blob(std::unique_ptr<PlBlob>*) prevents the automatic deletion if unification succeeds.

A `unique_ptr` needs a bit of care when it is passed as an argument. The unique_ptr::get() method can be used to get the “raw” pointer; the delete must not be used with this pointer. Or, the unique_ptr::release() method can be used to transfer ownership without calling the object's destructor.

Using unique_ptr::release() is a bit incovenient, so instead the `unique_ptr` can be passed as a pointer (or a reference). This does not create a new scope, so the pointer must be assigned to a local variable. For example, the code for unify_blob() is something like:

    bool PlTerm::unify_blob(std::unique_ptr<PlBlob>* b) const
    { std::unique_ptr<PlBlob> blob(std::move(*b));
      if ( !unify_blob(blob.get()) )
        return false;
      (void)blob.release();
      return true;
    }

The line declaration for `blob` uses the “move constructor” to set the value of a newly scoped variable (`std::move(*b)` is a cast, so `unique_ptr`’s move constructor is used). This has the same effect as calling `b->reset()`, so from this point on, `b` has the value `nullptr`.

Alternatively, the local `unique_ptr` could be set by

    std::unique_ptr<PlBlob> blob(b->release());

or

    std::unique_ptr<PlBlob> blob;
    blob.swap(*b);

If the call to PlTerm::unify_blob() fails or throws an exception, the virtual destructor for `blob` is called. Otherwise, the call to `blob.release()` prevents the destructor from being called - Prolog now owns the blob object and can call its destructor when the garbage collector reclaims it.

#### 1.6.8.2 How to define a blob using C++

TL;DR: Use PL_BLOB_DEFINITION() to define the blob with the flag `PL_BLOB_NOCOPY` and the default `PlBlob` wrappers; define your struct as a subclass of `PlBlob` with no copy constructor, move constructor, or assignment operator; create a blob using `std::unique_ptr<PlBlob>(new ...)`, call PlTerm::unify_blob(). Optionally, define one or more of: compare_fields(), write_fields(), save(), load() methods (these are described after the sample code).

#### 1.6.8.3 The life of a PlBlob

In this section, the blob is of type `MyBlob`, a subclass of `PlBlob`. (Example code is given in section 1.6.8.5) and section 1.6.8.7.

A blob is typically created by calling a predicate that does the following:

- Creates the blob using

      auto ref = std::unique_ptr<PlBlob>(new MyBlob>(...))}

  or

      auto ref = std::make_unique<MyBlob>(...);

- After the fields of the blob are filled in:

      return PlTerm::unify_blob(&ref);

  If unification fails or throws an exception, the object is automatically freed and its destructor is called.

  If make_unique() was used to create the pointer, you need to call PlTerm::unify_blob() as follows, because C++'s type inferencing can't figure out that this is a covariant type:

      std::unique_ptr<PlBlob> refb(ref.release());
      // refb now "owns" the ptr - from here on, ref == nullptr
      return A2.unify_blob(&refb);

  If unification succeeds, Prolog calls:

  - PlBlobV<MyBlob>acquire(), which calls
  - MyBlob::acquire(), which sets the field MyBlob::symbol_, which is usually accessed using the method MyBlob::symbol_term(). If this all succeeds, PlTerm::unify_blob(ref) calls `ref->release()` to pass ownership of the blob to Prolog (when the blob is eventually garbage collected, the blob's destructor will be called).

At this point, the blob is owned by Prolog and may be freed by its atom garbage collector, which will call the blob's destructor (if the blob shouldn't be deleted, it can override the the PlBlob::pre_delete() method to return `false`).

Whenever a predicate is called with the blob as an argument (e.g., as A1), the blob can be accessed by `PlBlobv<MyBlob>::cast_check(A1.as_atom())`.

Within a method, the Prolog blob can be accessed as a term (e.g., for constructing an error term) using the method MyBlob::symbol_term(). This field is initialized by the call to PlTerm::unify_blob(); if MyBlob::symbol_term() is called before a successful call to PlTerm::unify_blob(), MyBlob::symbol_term() returns a `PlTerm_var`.

When the atom garbage collector runs, it frees the blob by first calling the release() callback, which does delete, which calls the destructor MyBlob::~MyBlob(). Note that C++ destructors are not supposed to raise exception; they also should not cause a Prolog error, which could cause deadlock unless the real work is done in another thread.

Often it is desired to release the resources before the garbage collector runs. To do this, the programmer can provide a “close” predicate that is the inverse of the “open” predicate that created the blob. This typically has the same logic as the destructor, except that it can raise a Prolog error.

#### 1.6.8.4 C++ exceptions and blobs

When a blob is used in the context of a PREDICATE() macro, it can raise a C++ exception (`PlFail` or `PlException`) and the PREDICATE() code will convert the exception to the appropriate Prolog failure or error; memory allocation exceptions are also handled.

Blobs have callbacks, which can run outside the context of a PREDICATE(). Their exception handling is as follows:

void PlBlob::acquire()
  , which is called from PlBlobV<MyBlob>::acquire(), can throw a C++ exception. The programmer cannot override this.
int PlBlob::compare_fields(const PlBlob *_b)
  , which is called from PlBlobV<MyBlob>::compare(), should not throw an exception. A Prolog error won't work as it uses “raw pointers” and thus a GC or stack shift triggered by creating the exception will upset the system.
bool PlBlob::write_fields(IOStream *s, int flags)
  , which is called from PlBlobV<MyBlob>::write(), can throw an exception, just like code inside a PREDICATE(). In particular, you can wrap calls to Sfprintf() in PlCheckFail(), although the calling context will check for errors on the stream, so checking the Sfprintf() result isn't necessary.
void PlBlob::PlBlob::save(IOStream *fd)
  can throw a C++ exception, including PlFail().
PlAtom PlBlob::PlBlob::load(IOSTREAM *fd)
  can throw a C++ exception, which is converted to a return value of `PlAtom::null`, which is interpreted by Prolog as failure.
bool PlBlob::PlBlob::pre_delete()
  , which is called from PlBLobV<MyBLOB>::release(), can return `false` (or throw a `PlException` or `PlExceptinFailBase`, which will be interpreted as a return value of `false`), resulting in the blob not being garbage collected, and the destructor not being called. Note that this doesn't work well with final clean-up atom garbage collection, which disregards the return value and also doesn't respect the ordering of blob dependencies (e.g., if an iterator blob refers to a file-like blob, the file-like blob might be deleted before the iterator is deleted).

  This code runs in the `gc` thread. The only PL_*() function that can safely be called are PL_unregister_atom() (which is what PlAtom::unregister_ref() calls).

#### 1.6.8.5 Sample PlBlob code (connection to database)

Here is minimal sample code for creating a blob that owns a connection to a database. It has a single field (`connection`) and defines compare_fields() and write_fields().

A second sample code shows how to wrap a system pointer - section 1.6.8.7

    struct MyConnection
    { std::string name;

      explicit MyConnection();
      explicit MyConnection(const std::string& _name);
      bool open();
      bool close() noexcept;
      void portray(PlStream& strm) const;
    };

    struct MyBlob;

    static PL_blob_t my_blob = PL_BLOB_DEFINITION(MyBlob, "my_blob");

    struct MyBlob : public PlBlob
    { std::unique_ptr<MyConnection> connection;

      explicit MyBlob()
        : PlBlob(&my_blob) { }

      explicit MyBlob(const std::string& connection_name)
        : PlBlob(&my_blob),
          connection(std::make_unique<MyConnection>(connection_name))
      { if ( !connection->open() )
          throw MyBlobError("my_blob_open_error");
      }

      PL_BLOB_SIZE

      ~MyBlob() noexcept
      { if ( !close() )
          Sdprintf("***ERROR: Close MyBlob failed: %s\n", name().c_str()); // Can't use PL_warning()
      }

      inline std::string
      name() const
      { return connection ? connection->name : "";
      }

      bool close() noexcept
      { if ( !connection )
          return true;
        bool rc = connection->close();
        connection.reset(); // Can be omitted, leaving deletion to ~MyBlob()
        return rc;
      }

      PlException MyBlobError(const char* error) const
      { return PlGeneralError(PlCompound(error, PlTermv(symbol_term())));
      }

      int compare_fields(const PlBlob* _b_data) const override
      { auto b_data = static_cast<const MyBlob*>(_b_data); // See note about cast
        return name().compare(b_data->name());
      }

      bool write_fields(IOSTREAM *s, int flags) const override
      { PlStream strm(s);
        strm.printf(",");
        return write_fields_only(strm);
      }

      bool write_fields_only(PlStream& strm) const
      { if ( connection )
          connection->portray(strm);
        else
          strm.printf("closed");
        return true;
      }

      bool portray(PlStream& strm) const
      { strm.printf("MyBlob(");
        write_fields_only(strm);
        strm.printf(")");
        return true;
      }
    };

    // %! create_my_blob(+Name: atom, -MyBlob) is semidet.
    PREDICATE(create_my_blob, 2)
    { // Allocating the blob uses std::unique_ptr<MyBlob> so that it'll be
      // deleted if an error happens - the auto-deletion is disabled by
      // ref.release() inside unify_blob() before returning success.

      auto ref = std::unique_ptr<PlBlob>(new MyBlob(A1.as_atom().as_string()));
      return A2.unify_blob(&ref);
    }

    // %! close_my_blob(+MyBlob) is det.
    // % Close the connection, silently succeeding if is already
    // % closed; throw an exception if something goes wrong.
    PREDICATE(close_my_blob, 1)
    { auto ref = PlBlobV<MyBlob>::cast_ex(A1, my_blob);
      if ( !ref->close() )
        throw ref->MyBlobError("my_blob_close_error");
      return true;
    }

    // %! portray_my_blob(+Stream, +MyBlob) is det.
    // % Hook predicate for
    // %   user:portray(MyBlob) :-
    // %     blob(MyBlob, my_blob), !,
    // %     portray_my_blob(current_output, MyBlob).
    PREDICATE(portray_my_blob, 2)
    { auto ref = PlBlobV<MyBlob>::cast_ex(A2, my_blob);
      PlStream strm(A1, 0);
      return ref->portray(strm);
    }

#### 1.6.8.6 Discussion of the sample PlBlob code

- PL_BLOB_DEFINITION(MyBlob, "my_blob") creates a `PL_blob_t` structure with the wrapper functions and flags set to `PL_BLOB_NOCOPY`. It should be declared outside the `PlBlob` class and should not be marked `const` - otherwise, a runtime error can occur.20The cause of the runtime error is not clear, but possibly has to do with the order of initializing globals, which is unspecified for C++.

- The `MyBlob` struct is a subclass of `PlBlob`. See below for a discussion of the default behaviors.

  - `MyBlob` contains a pointer to a `MyConnection` object and keeps a copy of the connection's name. The `MyConnection` object is handled by a `std::unique_ptr` smart pointer, so that it is automatically freed when the `MyBlob` object is freed.

  - A default constructor is defined - this is needed for the load() and save() methods; it invokes the `PlBlob` constructor.

  - The `MyBlob` class must not provide a copy or move constructor, nor an assignment operator (PlBlob has these as delete, so if you try to use one of these, you will get a compile-time error).

  - `PlBlob`’s constructor sets `blob_t_` to a pointer to the `my_blob` definition. This is used for run-time consistency checking by the various callback functions and for constructing error terms (see PlBlob::symbol_term()).

  - `PlBlob`’s acquire() is called by PlBlobV<MyBlob>::acquire() and fills in the `symbol_` field. `MyBlob` must not override this - it is not a virtual method. The `symbol_` field can be accessed by PlBlob::symbol_term().

  - PlBlob::symbol_term() Creates a term from the blob, for use in error terms. It is always safe to use this; if the symbol hasn't been set (because acquire() hasn't been called), symbol_term() returns a “var” term - this can be checked with PlTerm::is_variable().

  - The MyBlob(connection_name) constructor creates a `MyConnection` object. If this fails, an exception is thrown. The constructor then calls MyConnection::open() and throws an exception if that fails. (The code would be similar if instead the constructor for `MyConnection` also did an open and threw an exception on failure.)

  - The `PL_BLOB_SIZE` is boilerplate that defines a blob_size_() method that is used when the blob is created.

  - The destructor MyBlob() is called when the blob is released by the garbage collector and in turn calls the MyBlob::close(), throwing away the result. If there is an error, a message is printed because there is no other way report the error. For this reason, it is preferred that the program explicitly calls the close_my_blob/1 predicate, which can raise an error. One way of doing this is by using the at_halt/1 hook.

  - The MyBlob::close() method is called by either the destructor or by the close_my_blob/1 predicate. Because it can be called by the garbage collector, which does not provide the usual environment and which may also be in a different thread, the only Prolog function that can be called is PlAtom::unregister_ref(); and the MyBlob::close() method must not throw an exception.21It isn't enough to just catch exceptions; for example, if the code throws `PlUnknownError("...")`, that will try to create a Prolog term, which will crash because the environment for creating terms is not available. Because there is no mechanism for reporting an error, the destructor prints a message on failure (calling PL_warning() would cause a crash).

    PlBlob::close() calls MyConnection::close() and then frees the object. Error handling is left to the caller because of the possibility that this is called in the context of garbage collection. It is not necessary to free the `MyConnection` object here - if it is not freed, the `std::unique_ptr<MyConnection>`’s destructor would free it.

  - PlBlob::MyBlobError() is a convenience method for creating errror terms.

  - PlBlob::compare_fields() makes the blob comparison function more deterministic by comparing the name fields; if the names are the same, the comparison will be done by comparing the addresses of the blobs (which is the default behavior for blobs defined using the C API). PlBlob::compare_fields() is called by PlBlobV<PlBlob>::compare(), which provides the default comparison if PlBlob::compare_fields() returns `0` (``equal” ).

    The _b_data argument is of type `const PlBlob*` - this is cast to `const MyBlob*` using a `static_cast`. This is safe because Prolog guarantees that PlBlobV<PlBlob>::compare() will only be called if both blobs are of the same type.

  - PlBlob::write_fields() outputs the name and the status of the connection, in addition to the default of outputting the blob type and its address. This is for illustrative purposes only; an alternative is to have a my_blob_properties/2 predicate to provide the information.

    The flags argument is the same as given to PlBlobV<PlBlob>::write(), which is a bitwise or of zero or more of the `PL_WRT_*` flags that were passed in to the caling PL_write_term() (defined in `SWI-Prolog.h`). The flags do not have the `PL_WRT_NEWLINE` bit set, so it is safe to call PlTerm::write() and there is no need for writing a trailing newline.

    If anything in PlBlob::write_fields() throws a C++ exception, it will be caught by the calling PlBlobV<PlBlob>::write() and handled appropriately.

  - PlBlob::save() and PlBlob::load() are not defined, so the defaults are used - they throw an error on an attempt to save the blob (e.g., by using qsave_program/[1,2]).22The C API defaults would save the internal form of the blob, which is probably not what you want, so the C++ API throws an error as its default.

- create_my_blob/2 predicate:

  - `std::unique_ptr<PlBlob>()` creates a MyBlob that is deleted when it goes out of scope. If an exception occurs between the creation of the blob or if the call to unify_blob() fails, the pointer will be automatically freed (and the `MyBlob` destructor will be called).

    PlTerm::unify_blob() is called with a pointer to a `std::unique_ptr`, which takes ownership of the object by calling std::unique_ptr<PlBlob>::release() and passes the pointer to Prolog, which then owns it. This also sets ref to `nullptr`, so any attempt to use ref after a call to PlTerm::unify_blob() will be an error.

    If you wish to create a `MyBlob` object instead of a `PlBlob` object, a slightly different form is used:

        auto ref = std::make_unique<MyBlob>(...);
          ...
        std::unique_ptr<PlBlob> refb(ref.release());
        PlCheckFail(A2.unify_blob(&refb));
        return true;

- close_my_blob/1 predicate:

  - The argument is turned into a `MyBlob` pointer using the PlBlobV<MyBlob>::cast_ex() function, which will throw a `type_error` if the argument isn't a blob of the expected type.

  - The MyBlob::close() method is called - if it fails, a Prolog error is thrown.

#### 1.6.8.7 Sample PlBlob code (wrapping a pointer)

    struct MyFileBlob;

    static PL_blob_t my_file_blob = PL_BLOB_DEFINITION(MyFileBlob, "my_file_blob");

    static const PlOptionsFlag<int>
    MyFileBlob_options("MyFileBlob-options",
                       { {"absolute", PL_FILE_ABSOLUTE},
                         {"ospath",   PL_FILE_OSPATH},
                         {"search",   PL_FILE_SEARCH},
                         {"exist",    PL_FILE_EXIST},
                         {"read",     PL_FILE_READ},
                         {"write",    PL_FILE_WRITE},
                         {"execute",  PL_FILE_EXECUTE},
                         {"noerrors", PL_FILE_NOERRORS} });

    struct MyFileBlob : public PlBlob
    { std::FILE* file_;

      std::string mode_;
      int flags_;
      std::string filename_;
      std::vector<char> buffer_; // used by read(), to avoid re-allocation

      explicit MyFileBlob()
        : PlBlob(&my_file_blob) { }

      explicit MyFileBlob(PlTerm filename, PlTerm mode, PlTerm flags)
        : PlBlob(&my_file_blob),
          mode_(mode.as_string())
      { flags_ = MyFileBlob_options.lookup_list(flags);
        filename_ = filename.get_file_name(flags_);
        file_ = fopen(filename_.c_str(), mode_.c_str());
        if ( !file_ ) // TODO: get error code (might not be existence error)
          throw PlExistenceError("my_file_blob_open", PlTerm_string(filename_));
        // for debugging:
        //   PlTerm_string(filename.as_string() + "\" => \"" +
        //                 filename_ + "\", \"" + mode_ +
        //                 ", flags=" + MyFileBlob_options.as_string(flags_) + "\")")
      }

      PL_BLOB_SIZE

      std::string read(size_t count)
      { assert(sizeof buffer_[0] == sizeof (char));
        assert(sizeof (char) == 1);

        buffer_.reserve(count);
        return std::string(buffer_.data(),
                           std::fread(buffer_.data(), sizeof buffer_[0], count, file_));
      }

      bool eof() const
      { return std::feof(file_);
      }

      bool error() const
      { return std::ferror(file_);
      }

      virtual ~MyFileBlob() noexcept
      { if ( !close() )
          // Can't use PL_warning()
          Sdprintf("***ERROR: Close MyFileBlob failed: (%s)\n", filename_.c_str());
      }

      bool close() noexcept
      { if ( !file_ )
          return true;
        int rc = std::fclose(file_);
        file_ = nullptr;
        return rc == 0;
      }

      PlException MyFileBlobError(const std::string error) const
      { return PlGeneralError(PlCompound(error, PlTermv(symbol_term())));
      }

      int compare_fields(const PlBlob* _b_data) const override
      { // dynamic_cast is safer than static_cast, but slower (see documentation)
        // It's used here for testing (the documentation has static_cast)
        auto b_data = dynamic_cast<const MyFileBlob*>(_b_data);
        return filename_.compare(b_data->filename_);
      }

      bool write_fields(IOSTREAM *s, int flags) const override
      { PlStream strm(s);
        strm.printf(",");
        return write_fields_only(strm);
      }

      bool write_fields_only(PlStream& strm) const
      { // For debugging:
        // strm.printf("%s mode=%s flags=%s", filename_.c_str(), mode_.c_str(),
        //             MyFileBlob_options.as_string(flags_).c_str());
        strm.printf("%s", filename_.c_str());
        if ( !file_ )
          strm.printf("-CLOSED");
        return true;
      }

      bool portray(PlStream& strm) const
      { strm.printf("MyFileBlob(");
        write_fields_only(strm);
        strm.printf(")");
        return true;
      }
    };

    PREDICATE(my_file_open, 4)
    { auto ref = std::unique_ptr<PlBlob>(new MyFileBlob(A2, A3, A4));
      return A1.unify_blob(&ref);
    }

    PREDICATE(my_file_close, 1)
    { auto ref = PlBlobV<MyFileBlob>::cast_ex(A1, my_file_blob);
      if ( !ref->close() ) // TODO: get the error code
        throw ref->MyFileBlobError("my_file_blob_close_error");
      return true;
    }

#### 1.6.8.8 Discussion of the sample PlBlob code (wrapping a pointer)

- This code provides a simple wrapper for some of the C “stdio” functions defined in `<cstdio>`. The blob wraps the file pointer returned from fopen() and also keeps a few other values for debugging (the mode, flags, filename from the call to fopen()) plus a buffer for read operations.

- A utility class‘PlOptionsFlag` is defined in fileSWI-cpp2-flags.h, for mapping a list of atoms to a bit-field flag. For example, the list `[search,read]` would map to‘examPL_FILE_SEARCH|PL_FILE_READ‘.

- The `MyFileBlob` struct defines the blob that wraps a `FILE*`. The constructor (which is called by predicate my_file_open/4) converts the flags term (a list of atoms or strings) to a flag that is passed to PL_get_file_name(), to convert the filename to a string containing the abslute file name. This is then passed to fopen(), together with the mode. If the call to fopen() fails, a C++ exception is thrown, to be handled by Prolog. Other errors, such as a wrong argument type to PL_get_file_name() can also cause an exception.

- MyFileBlob::read() ensures that the buffer is big enough and then calls‘fread()‘to return the buffer's contents.

- MyFileBlob::eof() and MyFileBlob::error() call feof() and ferror() respectively. They can be used to check the status of the call to MyFileBlob::read().

- The destructor calls MyFileBlob::close() and outputs a warning if it fails - a destructor is not allowed to throw a C++ exception, so this is the best we can do; it's better if the programmer explicitly closes the file rather than depending on the garbage collector to free the blob.

- MyFileBlob::close() calls fclose(). It then sets the `FILE*` to null, so that close won't be done twice.

- MyFileBlob::compare_fields(), MyFileBlob::write_fields(), MyFileBlob::write_fields_only(), MyFileBlob::portray() are similar to the same methods in `MyBlob` in section 1.6.8.5.

- Predicate my_file_open(File,Filename,Mode,Flags) calls the `MyFileBlob` constructor with Filename, Mode, flags and unifies the blob with File.

- Predicate my_file_close/1 calls MyFileBlob::close(), checks for an error and creates a Prolog error if the close failed.

#### 1.6.8.9 Identifying blobs by atoms

Passing a Prolog blob around can be inconvenient; it is easier if a blob can be identified an atom. An example of this is with streams, which are identified by atoms such as `user_input`.

A utility class `AtomMap` is provided for this situation. See section 1.17.4.

### 1.6.9 Limitations of the interface

The C++ API remains a work in progress.

#### 1.6.9.1 Strings

SWI-Prolog string handling has evolved over time. The functions that create atoms or strings using `char*` or `wchar_t*` are “old school” ; similarly with functions that get the string as `char*` or `wchar_t*`. The PL_get,unify,put_[nw]chars() family is more friendly when it comes to different input, output, encoding and exception handling.

Roughly, the modern API is PL_get_nchars(), PL_unify_chars() and PL_put_chars() on terms. There is only half of the API for atoms as PL_new_atom_mbchars() and PL-atom_mbchars(), which take an encoding, length and char*.

For return values, `char*` is dangerous because it can point to local or stack memory. For this reason, wherever possible, the C++ API returns a `std::string`, which contains a copy of the string. This can be slightly less efficient that returning a `char*`, but it avoids some subtle and pervasive bugs that even address sanitizers can't detect.23If we wish to minimize the overhead of passing strings, this can be done by passing in a pointer to a string rather than returning a string value; but this is more cumbersome and modern compilers can often optimize the code to avoid copying the return value.

Some functions require allocating string space using PL_STRINGS_MARK(). The `PlStringBuffers` class provides a RAII wrapper that ensures the matching PL_STRINGS_RELEASE() is done. The `PlAtom` or `PlTerm` member functions that need the string buffer use `PlStringBuffers`, and then copy the resulting string to a `std::string` value.

The C++ API has functions such as PlTerm::get_nchars() that use `PlStringBuffers` and then copy the result to a `std::string` result, so the programmer often doesn't need to use `PlStringBuffers`.

PlStringBuffers
  A RAII wrapper for allocating a string that is created using `BUF_STACK`. This isn't needed if you use a method such as PlTerm::as_string(), but is needed for calling certain PL_*() or Plx_*() wrapped functions.

  The constructor calls PL_STRINGS_MARK() and the destructor calls PL_STRINGS_RELEASE(). Here is an example of its use, for writing an atom to a stream, using Plx_atom_wchars(), which must be called within a strings buffer:

      PREDICATE(w_atom_cpp, 2)
      { auto stream(A1), term(A2);
        PlStream strm(stream, STIO_OUTPUT);
        PlStringBuffers _string_buffers;
        const pl_wchar_t *sa = Plx_atom_wchars(term.as_atom().unwrap(), nullptr);
        strm.printfX("/%Ws/", sa);
        return true;
      }

#### 1.6.9.2 Stream I/O

`PlStream` can be used to get a stream from a Prolog term, or to lock the stream so that other threads cannot interleave their output. With either usage, `PlStream` is a RAII class that ensure the matchin PL_release_stream() is done, and also handles some subtle problems with C++ exceptions.

The methods are:

PlStream :: PlStream(term_t t, int flags)
  - see PL_get_stream() for documentation of the flags. Throws a C++ exception on error.
PlStream :: PlStream(IOSTREAM *s)
  - calls PL_acquire_stream() to lock the stream. Throws a C++ exception on error.
~ PlStream()
  - calls PlStream::release(). See below for caveats if there are exceptions.
void PlStream::release()
  calls PL_release_stream(), throwing an exception if there has been an I/O error on the stream, and sets the `PlStream` object to an invalid stream (see PlStream::check_stream()).
IOSTREAM* ::operator PlStream(void)
  - when used in a context that requires an `IOSTREAM*`, `PlStream` is implicitly converted to `IOSTREAM*`.
void check_stream()
  checks that the `PlStream` object contains a valid stream and throws an exception if it doesn't. This is used to ensure that PlStream::release() hasn't been called.

Most of the stream I/O functions have corresponding methods in `PlStream`. For example, Sfprintf() corresponds to PlStream::printf(). PlStream::seek() and PlStream::tell() call Sseek64() and Stell64() instead of `long` (they are also deprecated: PlStream::seek64() and PlStream::tell64() are preferred).

The C interface to stream I/O doesn't raise a Prolog error when there's a stream error (typically indicated by a -1 return code). Instead, the error sets a flag on the stream and PL_release_stream() creates the error term. The `PlStream` destructor calls PL_release_stream(); but it's a fatal error in C++ to raise an exception in a destructor if the destructor is invoked by stack-unwinding due to another exception, including the pseudo-exceptions `PlFail` and `PlExceptionFail`.

To get around this, the various stream I/O functions have wrapper methods in the `PlStream` class that check for an error and call PlStream::release() to create the Prolog error, which is thrown as a C++ error.

The destructor calls PlStream::release(), which throws a C++ exception if there is a stream error. This is outside the destructor, so it is safe - the destructor checks if the stream has been released and does nothing in that situation.

The following two code examples do essentially the same thing:

    PREDICATE(name_arity, 1)
    { PlStream strm(Scurrent_output);
      strm.printf("name = %s, arity = %zd\n", A1.name().as_string().c_str(), A1.arity());
      return true;
    }

    PREDICATE(name_arity, 1)
    { PlStream strm(Scurrent_output);
      try
      { strm.printf("name = %s, arity = %zd\n", A1.name().as_string().c_str(), A1.arity());
      } PREDICATE_CATCH({strm.release(); return false;})
      return true;
    }

If you write the code as follows, using Sfprintf() directly, it is possible that a fatal exception will be raised on an I/O error:

    PREDICATE(name_arity, 1)
    { PlStream strm(Scurrent_output);
      Sfprintf(strm, "name = %s, arity = %zd\n", A1.name().as_string().c_str(), A1.arity());
      return true;
      // WARNING: the PlStream destructor might throw a C++
      //          exception on stack unwinding, giving a fatal
      //          fatal runtime exception.
    }

If you don't use these, and want to throw an exception if there's an error, the following code works because `PlStream` (and the underlying PL_acquire_stream()) can be called recursively:

    { PlStream strm(...);
      strm.release();
    }

#### 1.6.9.3 Object handles

Many of the “opaque object handles” , such as `atom_t`, `term_t`, and `functor_t` are integers.24Typically `uintptr_t` values, which the C standard defines as “an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer.’ As such, there is no compile-time detection of passing the wrong handle to a function.

This leads to a problem with classes such as `PlTerm` - C++ overloading cannot be used to distinguish, for example, creating a term from an atom versus creating a term from an integer. There are a number of possible solutions, including:

- A subclass for each kind of initializer;
- A tag for each kind of intializer;
- Change the C code to use a `struct` instead of an integer.

It is impractical to change the C code, both because of the amount of edits that would be required and also because of the possibility that the changes would inhibit some optimizations.

There isn't much difference between subclasses versus tags; but as a matter of design, it's better to specify things as constants than as (theoretically) variables, so the decision was to use subclasses.

### 1.6.10 Linking embedded applications using swipl-ld

The utility program swipl-ld (Win32: swipl-ld.exe) works with both C and C++ programs. See Linking embedded applications using swipl-ld for more details.

Your C++ compiler should support at least C++-17.

To avoid incompatibilities amongst the various C++ compilers’ABIs, the object file from compiling `SWI-cpp2.cpp` is not included in the shared object `libswipl`; instead, it must be compiled along with any foreign predicate files. If the macro `_SWI_CPP2_CPP_SEPARATE` is defined before the include for `SWI-cpp2.h`, then `SWI-cpp2.cpp` is not automatically included and must be compiled separately - either by creating a `.a` file or by adding a `#include <SWI-cpp2.cpp>` to one of your source files.

## 1.7 Examples

Before going into a detailed description of the C++ classes we present a few examples illustrating the “feel” of the interface.

### 1.7.1 Hello(World)

This simple example shows the basic definition of the predicate hello/1 and how a Prolog argument is converted to C-data:

    PREDICATE(hello, 1)
    { cout << "Hello " << A1.as_string() << endl;
      return true;
    }

The arguments to PREDICATE() are the name and arity of the predicate. The macros A<n> provide access to the predicate arguments by position and are of the type `PlTerm`. The C or C++ string for a `PlTerm` can be extracted using as_string(), or as_wstring() methods;25The C-string values can be extracted from `std::string` by using c_str(), but you must be careful to not return a pointer to a local/stack value, so this isn't recommende. and similar access methods provide an easy type-conversion for most Prolog data-types, using the output of write/1 otherwise:

    ?- hello(world).
    Hello world

    Yes
    ?- hello(X)
    Hello _G170

    X = _G170

### 1.7.2 Adding numbers

This example shows arithmetic using the C++ interface, including unification, type-checking, and conversion. The predicate add/3 adds the two first arguments and unifies the last with the result.

    PREDICATE(add, 3)
    { return A3.unify_integer(A1.as_long() + A2.as_long());
    }

You can use your own variable names instead of `A1`, `A2`, etc.:

    PREDICATE(add, 3)  // add(+X, +Y, +Result)
    { PlTerm x(A1);
      PlTerm y(A2);
      PlTerm result(A3);
      return result.unify_integer(x.as_long() + y.as_long());
    }

or more compactly:

    PREDICATE(add, 3)  // add(+X, +Y, +Result)
    { auto x = A1, y = A2, result = A3;
      return result.unify_integer(x.as_long() + y.as_long());
    }

The as_long() method for a `PlTerm` performs a PL_get_long_ex() and throws a C++ exception if the Prolog argument is not a Prolog integer or float that can be converted without loss to a `long`. The unify_integer() method of `PlTerm` is defined to perform unification and returns `true` or `false` depending on the result.

    ?- add(1, 2, X).

    X = 3.
    ?- add(a, 2, X).
    [ERROR: Type error: `integer' expected, found `a']
       Exception: (  7) add(a, 2, _G197) ?

### 1.7.3 Average of solutions - calling a Prolog goal

This example is a bit harder. The predicate average/3 is defined to take the template average(+Var, :Goal, -Average) , where Goal binds Var and will unify Average with average of the (integer) results.

`PlQuery` takes the name of a predicate and the goal-argument vector as arguments. From this information it deduces the arity and locates the predicate. The method PlQuery::next_solution() yields `true` if there was a solution and `false` otherwise. If the goal yields a Prolog exception, it is mapped into a C++ exception. A return to Prolog does an implicit “cut” (PL_cut_query()); this can also be done explicitly by the PlQuery::cut() method.

    PREDICATE(average, 3) /* average(+Templ, :Goal, -Average) */
    { long sum = 0;
      long n = 0;

      PlQuery q("call", PlTermv(A2));
      while( q.next_solution() )
      { sum += A1.as_long();
        n++;
      }
      return A3.unify_float(double(sum) / double(n));
    }

    ?- [user].
    |: p(1).
    |: p(10).
    |: p(20).
    |:
    % user://1 compiled 0.00 sec, 3 clauses
    true.

    ?- average(X, p(X), Average).
    Average = 10.333333333333334.

## 1.8 Rationale for changes from version 1

### 1.8.1 Implicit constructors and conversion operators

The original version of the C++ interface heavily used implicit constructors and conversion operators. This allowed, for example:

    PREDICATE(hello, 1)
    { cout << "Hello " << (char *)A1 << endl; // Deprecated
      return true;
    }

    PREDICATE(add, 3)
    { return A3 = (long)A1 + (long)A2; // Deprecated
    }

Version 2 is a bit more verbose:

    PREDICATE(hello, 1)
    { cout << "Hello " << A1.as_string() << endl;
      return true;
    }

    PREDICATE(add, 3)
    { return A3.unify_int(A1.as_long() + A2.as_long());
    }

There are a few reasons for this:

- The implicit constructors and conversion operators, combined with the C++ conversion rules for integers and floats, could sometimes lead to subtle bugs that were difficult to find -- in one case, a typo resulted in terms being unified with floating point values when the code intended them to be atoms. This was mainly because the underlying C types for terms, atoms, etc. are unsigned integers, leading to confusion between numeric values and Prolog terms and atoms.
- The overloaded assignment operator for unification changed the usual C++ semantics for assignments from returning a reference to the left-hand-side to returning a `bool`. In addition, the result of unification should always be checked (e.g., an “always succeed” unification could fail due to an out-of-memory error); the unify_XXX() methods return a `bool` and they can be wrapped inside a PlCheckFail() to raise an exception on unification failure.
- The C-style of casts is deprecated in C++, so the expression `(char*)A1` becomes the more verbose `static_cast<std::string>(A1)`, which is longer than `A1.as_string()`. Also, the string casts don't allow for specifying encoding.
- The implicit constructors and conversion operators were attractive because they allowed directly calling the foreign language interface functions, for example:

      PlTerm t;
      Pl_put_atom_chars(t, "someName");

  whereas this is now required:

      PlTerm t;
      Pl_put_atom_chars(t.as_term_t(), "someName");

  However, this is mostly avoided by methods and constructors that wrap the foreign language functions:

      PlTerm_atom t("someName");

  or

      auto t = PlTerm_atom("someName");

  Additionally, there are now wrappers for most of the PL_*() functions that check the error return and throw a C++ exception as appropriate.

Over time, it is expected that some of these restrictions will be eased, to allow a more compact coding style that was the intent of the original API. However, too much use of overloaded methods/constructors, implicit conversions and constructors can result in code that's difficult to understand, so a balance needs to be struck between compactness of code and understandability.

For backwards compatibility, much of the version 1 interface is still available (except for the implicit constructors and operators), but marked as “deprecated” ; code that depends on the parts that have been removed can be easily changed to use the new interface.

### 1.8.2 Strings

The version API often used `char*` for both setting and setting string values. This is not a problem for setting (although encodings can be an issue), but can introduce subtle bugs in the lifetimes of pointers if the buffer stack isn't used properly. `PlStringBuffers` makes the buffer stack easier to use, but it would be preferable to avoid its use altogether. C++, unlike C, has a standard string that allows easily keeping a copy rather than dealing with a pointer that might become invalid. (Also, C++ strings can contain null characters.)

C++ has default conversion operators from `char*` to `std::string`, so some of the API support only `std::string`, even though this can cause a small inefficiency. If this proves to be a problem, additional overloaded functions and methods can be provided in future (note that some compilers have optimizations that reduce the overheads of using `std::string`); but for performance-critical code, the C functions can still be used.

There still remains the problems of Unicode and encodings. `std::wstring` is one way of dealing with this. And for interfaces that use `std::string`, an encoding can be specified.26As of 2023-04, this had only been partially implemented. Some of the details for this - such as the default encoding - may change slightly in the future.

## 1.9 Porting from version 1 to version 2

The easiest way of porting from `SWI-cpp.h` to `SWI-cpp2.h` is to change the `#include "SWI-cpp.h"` to `#include "SWI-cpp2.h"` and look at the warning and error messages. Where possible, version 2 keeps old interfaces with a “deprecated” flag if there is a better way of doing things with version 2.

For convenience when calling PL_*() functions, the Plx_*() wrapper functions add error checking. Also, most of the PL_*() functions that work with `term_t`, `atom_t`, etc. have corresponding methods in `PlTerm`, `PlAtom`, etc.

Here is a list of typical changes:

- Replace PlTerm() constructor with PlTerm_var() for uninstantiated variables, PlTerm_atom(a) for atoms, PlTerm_term_t(t) for the raw `term_t`, PlTerm_integer(i), PlTerm_float(v), or PlTerm_pointer(p).

- Examine uses of `char*` or `wchar_t` and replace them by `std::string` or `std::wstring` if appropriate. For example, `cout << "Hello " << (char*)A1 << endl` can be replaced by `cout << "Hello " << A1.as_string() << endl`. In general, `std::string` is safer than `char*` because the latter can potentially point to freed memory.

- Instead of returning `false` from a predicate for failure, you can do `throw PlFail()`. This mechanism is also used by PlCheckFail(rc). Note that throwing an exception is slower than returning `false`, so performance-critical code should avoid PlCheckFail(rc) if failure is expected to happen often.

- You can use the PlEx(rc) to check the return code from a function in `SWI-Prolog` and throw a `PlFail` exception to short-circuit execution and return failure (`false`) to Prolog (or throw a `PlException` if there was a Prolog error.

- `PlAtom::handle` has been replaced by `PlAtom::C_`, which should be accessed by PlAtom::unwrap().

- `PlTerm::ref` has been replaced by `PlTerm::C_`, which should be accessed by PlTerm::unwrap().

- `PlFunctor::functor` has been replaced by `PlFunctor::C_`, which should be accessed by PlFunctor::unwrap().

- The operator ``=`` for unification has been deprecated, replaced by various unify_*() methods (PlTerm::unify_term(t2), PlTerm::unify_atom(a), etc.).

- The various “cast” operators have been deprecated or deleted; you should use the various “getter” methods. For example, `static_cast<char*>(t)` is replaced by `t.as_string().c_str()` (and you should prefer `t.as_striong()`; `static_cast<int32_t>(t)` is replaced by `t.as_int32_t()`, etc.

- It is recommended that you do not use `int` or `long` because of problems porting between Unix and Windows platforms; instead, use `int32_t`, `int64_t`, `uint32_t`, `uint64_t`, etc.

## 1.10 The class PlFail

The `PlFail` class is used for short-circuiting a function when failure or an exception occurs and any errors will be handled in the code generated by the PREDICATE() macro. See also section 1.17.2).

For example, this code, using the C API:

    PREDICATE(unify_zero, 1)
    { if ( !PL_unify_integer(A1.unwrap(), 0) )
        return false; // could be an error or failure
      Sprintf("It's zero!\n");
      return true;
    }

can instead be written this way, using the C++ API:

    PREDICATE(unify_zero, 1)
    { PlCheckFail(A1.unify_integer(0));
      Sprintf("It's zero!\n");
      return true;
    }

Using `throw PlFail()` in performance-critical code can cause a signficant slowdown. A simple benchmark showed a 15x to 20x slowdown using `throw PlFail()` compared to `return false` (comparing the first code sample above with the second and third samples; the speed difference seems to have been because in the second sample, the compiler did a better job of inlining). However, for most code, this difference will be barely noticeable. And if the code usually succeeds, there is no significant difference.

There was no significant performance difference between the C++ version and this C version:

    static foreign_t
    unify_zero(term_t a1)
    { return PL_unify_integer(a1, 0);
    }

### 1.10.1 PlCheckFail(), and PlEx() convenience functions

If one of the C PL_*() functions in `SWI-Prolog.h` returns failure, this can be either a Prolog-style failure (e.g. from PL_unify() or PL_next_solution()) or an error. If the failure is due to an error, it's usually best to immediately return to Prolog - and this can be done with the PlEx() function, which turns a Prolog error into a C++ `PlException`. PlCheckFail() calls PlEx() and additionally throws PlFail() if the failure is for Prolog failure.

PlEx() calls PL_exception() to see if there is a Prolog exception; if so, the Prolog exception is converted to a `PlException` object, which is then thrown. For more details on the C++ exceptions, see section 1.15.

void PlCheckFail(bool rc)
  If rc is `false`, throw `PlFail` to return control to Prolog with failure.
C_t PlWrap(C_t rc, qid_t qid = 0)
  If rc indicates failure or an error, check for an error and throw a `PlException` if there was one; otherwise, return the rc.
void PlEx(C_t rc, qid_t qid = 0)
  If rc is “false” (non-zero), throw `PlFail` to return control to Prolog with failure. This is the same as PlCheckFail() except it can also specify a `qid_t` query ID.

## 1.11 Overview of accessing and changing values

The `SWI-Prolog.h` header provides various functions for accessing, setting, and unifying terms, atoms and other types. Typically, these functions return a `0` (`false`) or `1` (`true`) value for whether they succeeded or not. For failure, there might also be an exception created - this can be tested by calling PL_excpetion(0).

There are three major groups of methods:

- Put (set) a value, corresponding to the PL_put_*() functions.
- Get a value, corresponding to the PL_get_*() and PL_get_*_ex() functions.
- Unify a value, corresponding to the PL_unify_*() and PL_unify_*_ex() functions.

The “put” operations are typically done on an uninstantiated term (see the PlTerm_var() constructor). These are expected to succeed, and typically raise an exception failure (e.g., resource exception) - for details, see the corresponding PL_put_*() functions in Constructing Terms.

For the “get” and “unify” operations, there are three possible failures:

- `false` return code
- unification failure
- exception (value of unexpected type or out of resources)

Each of these is communicated to Prolog by returning `false` from the top level; exceptions also set a “global” exception term (using PL_raise_exception()). The C++ programmer usually doesn't have to worry about this; instead they can `throw PlFail()` for failure or `throw PlException()` (or one of `PlException`’s subclasses) and the C++ API will take care of everything.

### 1.11.1 Converting PlTerm to native C and C++ types

These are deprecated and replaced by the various `as_*()` methods.

`PlTerm` can be converted to the following types:

PlTerm ::operator term_t(void)
  This cast is used for integration with the C-interface primitives.
PlTerm ::operator long(void)
  Yields a `long` if the `PlTerm` is a Prolog integer or float that can be converted without loss to a long. Throws a `type_error` exception otherwise.
PlTerm ::operator int(void)
  Same as for `long`, but might represent fewer bits.
PlTerm ::operator double(void)
  Yields the value as a C double if `PlTerm` represents a Prolog integer or float.
PlTerm ::operator wchar_t *(void)
PlTerm ::operator char *(void)
  Converts the Prolog argument using PL_get_chars() using the flags `CVT_ALL|CVT_WRITE|BUF_RING`, which implies Prolog atoms and strings are converted to the represented text. All other data is handed to write/1. If the text is static in Prolog, a direct pointer to the string is returned. Otherwise the text is saved in a ring of 16 buffers and must be copied to avoid overwriting.
PlTerm ::operator void *(void)
  Extracts pointer value from a term. The term should have been created by PlTerm::PlTerm(void*).

In addition, the Prolog type (`PL_VARIABLE`, `PL_ATOM`, ... `PL_DICT`) can be determined using the type() method. There are also boolean methods that check the type:

int PlTerm::type()
  See PL_term_type()
bool PlTerm::is_variable()
  See PL_is_variable()
bool PlTerm::is_ground()
  See PL_is_ground()
bool PlTerm::is_atom(S)
  ee PL_is_atom()
bool PlTerm::is_integer(S)
  ee PL_is_integer()
bool PlTerm::is_string(S)
  ee PL_is_string()
bool PlTerm::is_atom_or_string(I)
  s true if either PlTerm::is_atom() or PlTerm::is_string() is true.
bool PlTerm::is_float(S)
  ee PL_is_float()
bool PlTerm::is_rational(S)
  ee PL_is_rational()
bool PlTerm::is_compound(S)
  ee PL_is_compound()
bool PlTerm::is_callable(S)
  ee PL_is_callable()
bool PlTerm::is_list(S)
  ee PL_is_list()
bool PlTerm::is_dict(S)
  ee PL_is_dict()
bool PlTerm::is_pair(S)
  ee PL_is_pair()
bool PlTerm::is_atomic(S)
  ee PL_is_atomic()
bool PlTerm::is_number(S)
  ee PL_is_number()
bool PlTerm::is_acyclic(S)
  ee PL_is_acyclic()
bool PlTerm::is_functor(PlFunctor)
  See PL_is_functor()

### 1.11.2 Unification

See also section 1.13.1.

bool PlTerm::unify_term(PlTerm)
bool PlTerm::unify_atom(PlAtom)
bool PlTerm::unify_atom(string)
bool PlTerm::unify_list_codes(string)
bool PlTerm::unify_list_chars(string)
bool PlTerm::unify_integer(int)
bool PlTerm::unify_float(double)
bool PlTerm::unify_string(string)
bool PlTerm::unify_functor(PlFunctor)
bool PlTerm::unify_pointer(void *)
bool PlTerm::unify_nil()
bool PlTerm::unify_blob(PlBlob* blob)
bool PlTerm::unify_blob(std::unique_ptr<PlBlob>* blob)
  Does a call to PL_unify_blob() and, if successful, calls std::unique_ptr<PlBlob>::release() to pass ownership to the Prolog blob; on failure or error, deletes the pointer (ad calls its destructor). After either success and failure, `*blob==nullptr`.
bool PlTerm::unify_blob(void *blob, size_t len, PL_blob_t *type)
bool PlTerm::unify_chars(int flags, size_t len, const char *s)

  A family of unification methods are defined for the various Prolog types and C++ types. Wherever `string` is shown, you can use:

  - `char*`
  - `whar_t*`
  - `std::string`
  - `std::wstring`

Here is an example:

    PREDICATE(hostname, 1)
    { char buf[256];
      if ( gethostname(buf, sizeof buf) == 0 )
        return A1.unify_atom(buf);
      return false;
    }

An alternative way of writing this would use the PlCheckFail() to raise an exception if the unification fails.

    PREDICATE(hostname2, 1)
    { char buf[256];
      PlCheckFail(gethostname(buf, sizeof buf) == 0);
      PlCheckFail(A1.unify_atom(buf));
      return true;
    }

Of course, in a real program, the failure of gethostname(buf)sizeof buf should create an error term than contains information from `errno`.

### 1.11.3 Comparison

int PlTerm::compare(const PlTerm &t2)
bool PlTerm::operator ==(const PlTerm &)
bool PlTerm::operator !=(const PlTerm &)
bool PlTerm::operator <(const PlTerm &)
bool PlTerm::operator >(const PlTerm &)
bool PlTerm::operator <=(const PlTerm &)
bool PlTerm::operator >=(const PlTerm &)
  Compare the instance with t and return the result according to the Prolog defined standard order of terms.
bool PlTerm::operator ==(long num)
bool PlTerm::operator !=(long num)
bool PlTerm::operator <(long num)
bool PlTerm::operator >(long num)
bool PlTerm::operator <=(long num)
bool PlTerm::operator >=(long num)
  Convert `PlTerm` to a `long` and perform standard C-comparison between the two long integers. If `PlTerm` cannot be converted a `type_error` is raised.
bool PlTerm::operator ==(const wchar_t *)
bool PlTerm::operator ==(const char *)
bool PlTerm::operator ==(std::wstring)
bool PlTerm::operator ==(std::string)
  Yields `true` if the `PlTerm` is an atom or string representing the same text as the argument, `false` if the conversion was successful, but the strings are not equal and an `type_error` exception if the conversion failed.

Below are some typical examples. See section 1.11.12.2 for direct manipulation of atoms in their internal representation.

`A1 < 0` | Test A1 to hold a Prolog integer or float that can be transformed lossless to an integer less than zero.
`A1 < PlTerm(0)` | A1 is before the term‘0’in the‘standard order of terms’. This means that if A1 represents an atom, this test yields `true`.
`A1 == PlCompound("a(1)")` | Test A1 to represent the term `a(1)`.
`A1 == "now"` | Test A1 to be an atom or string holding the text “now” .

### 1.11.4 Analysing compound terms

Compound terms can be viewed as an array of terms with a name and arity (length). This view is expressed by overloading the ``[]`` operator.

A `type_error` is raised if the argument is not compound and a `domain_error` if the index is out of range.

In addition, the following functions are defined:

PlTerm PlTerm::operator[](int arg)
  If the `PlTerm` is a compound term and arg is between 1 and the arity of the term, return a new `PlTerm` representing the arg-th argument of the term. If `PlTerm` is not compound, a `type_error` is raised. Id arg is out of range, a `domain_error` is raised. Please note the counting from 1 which is consistent to Prolog's arg/3 predicate, but inconsistent to C's normal view on an array. See also class `PlCompound`. The following example tests x to represent a term with first-argument an atom or string equal to `gnat`.

         ...,
         if ( x[1] == "gnat" )
           ...

const char * PlTerm::name()
  Return a `const char *` holding the name of the functor of the compound term. Raises a `type_error` if the argument is not compound.
size_t PlTerm::arity()
  Returns the arity of the compound term. Raises a `type_error` if the argument is not compound.

### 1.11.5 Miscellaneous

bool is_null()
  `t.is_null()` is the same as `t.unwrap() == PlTerm::null`
bool not_null()
  `t.not_null()` is the same as `t.unwrap() != PlTerm::null`
bool reset()
  `t.reset()` is the same as `t.unwrap() = PlTerm::null`
bool reset(term_t)
  `t.reset(x)` is the same as `t.unwrap() = x`
int PlTerm::type()
  Yields the actual type of the term as PL_term_type(). Return values are `PL_VARIABLE`, `PL_FLOAT`, `PL_INTEGER`, `PL_ATOM`, `PL_STRING` or `PL_TERM`
std::string as_string(PlEncoding enc=EncLocale)
  Returns the string representation of the atom. See PlAtom::as_string() for an explanation of the encodings and caveats about std::string::c_str().
std::string atomic_as_string(PlEncoding enc=EncLocale)
  As PlTerm::as_string(), but throws an exception if the term isn't atomic (see atomic/1).
std::string atom_or_string_as_string(PlEncoding enc=EncLocale)
  As PlTerm::as_string(), but throws an exception if the term isn't an atom or a string.

To avoid very confusing combinations of constructors and therefore possible undesirable effects a number of subclasses of `PlTerm` have been defined that provide constructors for creating special Prolog terms. These subclasses are defined below.

### 1.11.6 The class PlTerm_string

A SWI-Prolog string represents a byte-string on the global stack. Its lifetime is the same as for compound terms and other data living on the global stack. Strings are not only a compound representation of text that is garbage-collected, but as they can contain 0-bytes, they can be used to contain arbitrary C-data structures. However, it is generally preferred to use blobs for storing arbitrary C-data structures (see also `PlTerm_pointer(void *ptr)`).

PlTerm_string :: PlTerm_string(const wchar_t *text)
PlTerm_string :: PlTerm_string(const char *text)
  Create a SWI-Prolog string object from a 0-terminated C-string. The text is copied.
PlTerm_string :: PlTerm_string(const wchar_t *text, size_t len)
PlTerm_string :: PlTerm_string(const char *text, size_t len)
  Create a SWI-Prolog string object from a C-string with specified length. The text may contain 0-characters and is copied.

### 1.11.7 The class PlCodeList

PlCodeList :: PlCodeList(const wchar_t *text)
PlCodeList :: PlCodeList(const char *text)
  Create a Prolog list of ASCII codes from a 0-terminated C-string.

### 1.11.8 The class PlCharList

Character lists are compliant to Prolog's atom_chars/2 predicate.

PlCharList :: PlCharList(const wchar_t *text)
PlCharList :: PlCharList(const char *text)
  Create a Prolog list of one-character atoms from a 0-terminated C-string.

### 1.11.9 The class PlCompound

The `PlCompound` class is a convenience class for creating a term from a string; it is similar to (=..)/2

PlCompound :: PlCompound(const wchar_t *text)
PlCompound :: PlCompound(const char *text)
PlCompound :: PlCompound(const std::wstring& text)
PlCompound :: PlCompound(const std::string& text)
  PlEncoding enc=ENC_INPUT Create a term by parsing (as read/1) the text. If the text is not valid Prolog syntax, a `syntax_error` exception is raised. Otherwise a new term-reference holding the parsed text is created.
PlCompound :: PlCompound(const wchar_t *functor, PlTermv args)
PlCompound :: PlCompound(const char *functor, PlTermv args)
  Create a compound term with the given name from the given vector of arguments. See `PlTermv` for details. The example below creates the Prolog term `hello(world)`.

      PlCompound("hello", PlTermv(PlAtom("world")))

### 1.11.10 The class PlTerm_tail

The class `PlTerm_tail`27This was named `PlTail` in version 1 of the API. is both for analysing and constructing lists. It is called `PlTerm_tail` as enumeration-steps make the term-reference follow the “tail” of the list.

PlTerm_tail :: PlTerm_tail(PlTerm list)
  A `PlTerm_tail` is created by making a new term-reference pointing to the same object. As `PlTerm_tail` is used to enumerate or build a Prolog list, the initial list term-reference keeps pointing to the head of the list.
int PlTerm_tail::append(const PlTerm &element)
  Appends element to the list and make the `PlTerm_tail` reference point to the new variable tail. If A is a variable, and this function is called on it using the argument `"gnat"`, a list of the form `[gnat|B]` is created and the `PlTerm_tail` object now points to the new variable B.

  This function returns `true` if the unification succeeded and `false` otherwise. No exceptions are generated.

  The example below translates the main() argument vector to Prolog and calls the prolog predicate entry/1 with it.

      int
      main(int argc, char **argv)
      { PlEngine e(argv[0]);
        PlTermv av(1);
        PlTerm_tail l(av[0]);

        for(int i=0; i<argc; i++)
          PlCheckFail(l.append(argv[i]));
        PlCheckFail(l.close());

        PlQuery q("entry", av);
        return q.next_solution() ? 0 : 1;
      }

int PlTerm_tail::close()
  Unifies the term with ``[]`` and returns the result of the unification.
int PlTerm_tail::next(PlTerm &)
  Bind t to the next element of the list `PlTerm_tail` and advance `PlTerm_tail`. Returns `true` on success and `false` if `PlTerm_tail` represents the empty list. If `PlTerm_tail` is neither a list nor the empty list, a `type_error` is thrown. The example below prints the elements of a list.

      PREDICATE(write_list, 1)
      { PlTerm_tail tail(A1);
        PlTerm_var e;

        while(tail.next(e))
          cout << e.as_string() << endl;

        return tail.close();
      }

### 1.11.11 The class PlTermv

The class `PlTermv` represents an array of term-references. This type is used to pass the arguments to a foreign defined predicate, construct compound terms (see PlTerm::PlTerm(const char *name)PlTermv arguments ), and to create queries (see `PlQuery`).

The only useful member function is the overloading of ``[]``, providing (0-based) access to the elements. Range checking is performed and raises a `domain_error` exception.

The constructors for this class are below. Note that these can be error-prone because there's no distinction between `term_t` and `size_t`; the form of the constructor is determined by whether the first argument is an integer (`term_t` or `size_t`) or `PlTerm`.

PlTermv :: PlTermv(size_t size)
  Create a new array of term-references, all holding variables.
PlTermv :: PlTermv(size_t size, term_t t0)
  Convert a C-interface defined term-array into an instance. Typyically, t0 was created using Pl_new_term_refs(size).
PlTermv :: PlTermv(PlTerm ...)
  Create a vector from 1 to 5 initialising arguments. For example:

      load_file(const char *file)
      { return PlCall("compile", PlTermv(PlAtom(file)));
      }

  If the vector has to contain more than 5 elements, the following construction should be used:

      { PlTermv av(10);

        av[0].put_term(PlTerm_atom("hello"));
        av[1].put_term(PlTerm_integer(666));
        ...
      }

  Important: be sure that all the arguments are of type `PlTerm` - `PlTermv(i)` is not the same as `PlTermv(PlTerm_integer(i))`, and will result in a runtime error.

### 1.11.12 The class PlAtom - Supporting Prolog constants

Both for quick comparison as for quick building of lists of atoms, it is desirable to provide access to Prolog's atom-table, mapping handles to unique string-constants. If the handles of two atoms are different it is guaranteed they represent different text strings.

Suppose we want to test whether a term represents a certain atom, this interface presents a large number of alternatives:

#### 1.11.12.1 Direct comparision to char *

Example:

    PREDICATE(test, 1)
    { if ( A1 == "read" )
        ...;
    }

This writes easily and is the preferred method is performance is not critical and only a few comparisons have to be made. It validates A1 to be a term-reference representing text (atom, string, integer or float) extracts the represented text and uses strcmp() to match the strings.

#### 1.11.12.2 Direct comparision to PlAtom

Example:

    static PlAtom ATOM_read("read");

    PREDICATE(test, 1)
    { if ( A1 == ATOM_read )
        ...;
    }

This case raises a `type_error` if A1 is not an atom. Otherwise it extacts the atom-handle and compares it to the atom-handle of the global `PlAtom` object. This approach is faster and provides more strict type-checking.

#### 1.11.12.3 Extraction of the atom and comparison to PlAtom

Example:

    static PlAtom ATOM_read("read");

    PREDICATE(test, 1)
    { PlAtom a1(A1);

      if ( a1 == ATOM_read )
        ...;
    }

This approach is basically the same as section 1.11.12.2, but in nested if-then-else the extraction of the atom from the term is done only once.

#### 1.11.12.4 Extraction of the atom and comparison to char *

Example:

    PREDICATE(test, 1)
    { PlAtom a1(A1);

      if ( a1 == "read" )
        ...;
    }

This approach extracts the atom once and for each test extracts the represented string from the atom and compares it. It avoids the need for global atom constructors.

PlAtom :: PlAtom(atom_t handle)
  Create from C-interface atom handle (`atom_t`). Used internally and for integration with the C-interface.
PlAtom :: PlAtom(const char_t *text)
PlAtom :: PlAtom(const wchar *text)
PlAtom :: PlAtom(const std::string& text)
PlAtom :: PlAtom(const std::wstring& text)
  Create an atom from a string. The text is copied if a new atom is created. See PL_new_atom(), PL_new_atom_wchars(), PL_new_atom_nchars(), PL_new_atom_wchars().
PlAtom :: PlAtom(const PlTerm &)
  If t represents an atom, the new instance represents this atom. Otherwise a `type_error` is thrown.
int PlAtom::operator ==(const wchar_t *text)
int PlAtom::operator ==(const char *text)
int PlAtom::operator ==(const std::string& text)
int PlAtom::operator ==(const std::wstring& text)
  Yields `true` if the atom represents text, `false` otherwise. Performs a strcmp() or similar for this.
int PlAtom::operator ==(const PlAtom &)
  Compares the two atom-handles, returning `true` or `false`. Because atoms are unique, there is no need to use strcmp() for this.
int PlAtom::operator !=(const wchar_t *text)
int PlAtom::operator !=(const char *text)
int PlAtom::operator !=(const std::string& text)
int PlAtom::operator !=(const std::wstring& text)
int PlAtom::operator !=(const PlAtom &)
  The inverse of the ``==`` operator.
bool is_valid()
  Verifies that the handle is valid. This can be used after calling a function that returns an atom handle, to check that a new atom was created.
void reset()
  Sets the handle to an invalid valid - a subsequent call to is_null() will return `true`.
const std::string as_string(PlEncoding enc=EncLocale)
  Returns the string representation of the atom.28If you wish to return a `char*` from a function, you should not do `return t.as_string().c_str()` because that will return a pointer into the stack (Gnu C++ or Clang options `-Wreturn-stack-address` or `-Wreturn-local-addr`) can sometimes catch this, as can the runtime address sanitizer when run with `detect_stack_use_after_return=1`. This does not quote or escape any characters that would need to be escaped if the atom were to be input to the Prolog parser. The possible values for `enc` are:
  - `EncLatin1` - throws an exception if cannot be represented in ASCII.
  - `EncUTF8`
  - `EncLocale` - uses the locale to determine the representation.
const std:wstring as_wstring()
  Returns the string representation of the atom. This does not quote or escape any characters that would need to be escaped if the atom were to be input to the Prolog parser.
void register_atom()
  See PL_register_atom().
void unregister_atom()
  See PL_unregister_atom().
void* blob_data(size_t *len, struct PL_blob_t **type)
  See PL_blob_data().

### 1.11.13 Classes for the recorded database: PlRecord and PlRecordExternalCopy

The recorded database is has two wrappers, for supporting the internal records and external records.

Currently, the interface to internal records requires that the programmer explicitly call the dupicate() and erase() methods - in future, it is intended that this will be done automatically by a new `PlRecord` class, so that the internal records behave like “smart pointers” ; in the meantime, the `PlRecord` provides a trivial wrapper around the various recorded database functions.

The class `PlRecord` supports the following methods:

PlRecord(PlTerm)
  Constructor.
PlRecord(PlRecord)
  Copy and move constructors. Currently these do not do any reference counting. The assignment operator is currently not supported.
PlRecord()
  Destructor. Currently this does not call PL_erase().
PlTerm term()
  creates a term from the record, using PL_recorded().
void erase()
  decrements the reference count of the record and deletes it if the count goes to zero, using PL_erase(). It is safe to do this multiple times on the same `PlRecord` object.
PlRecord duplicate()
  increments the reference count of the record, using PL_duplicate_record().

The class `PlRecord` provides direct access to the reference counting aspects of the recorded term (through the duplicate() and erase() methods), but does not connect these with C++'s copy constructor, assignment operator, or destructor. If the recorded term is encapsulated within an object, then the containing object can use the duplicate() and erase() methods in its copy and move constructors and assignment operators (and the erase() method in the destructor).29The copy constructor and assignment use the duplicate() method; the move constructor and assignment use the duplicate() method to assign to the destination and the erase() method on the source; and the destructor uses erase().

Alternatively, the `std::shared_ptr` or `std::unique_ptr` can be used with the supplied `PlrecordDeleter`, which calls the erase() method when the `shared_ptr` reference count goes to zero or when the `std::unique_ptr` goes out of scope.

For example:

    std::shared_ptr<PlRecord> r(new PlRecord(t.record()), PlRecordDeleter());
    assert(t.unify_term(r->term()));

The class `PlRecordExternalCopy` keeps the external record as an uninterpreted string (which may contain nulls). It supports the following methods.

PlRecordExternalCopy :: PlRecordExternalCopy(PlTerm t)
  Creates a string using Pl_record_external(), copies it into the object then deletes the reference using PL_erase_external().
PlRecordExternalCopy :: PlRecordExternalCopy(const std::string& external)
  Saves the external string (which is assumed to have been created using PL_record_external()).
PlRecordExternalCopy :: PlRecordExternalCopy(const char* external, size_t len)
  Saves the external string (which is assumed to have been created using PL_record_external()).
PlTerm term()
  creates a term from the saved external record string, using PL_recorded_external()).
static PlTerm term(const std::string& external)
  Creates a term from the external record string. Equivalent to PlRecordExternalCopy(external).term().
static PlTerm term(const char* external)
  Creates a term from the external record string. Equivalent to PlRecordExternalCopy(external,len).term() except the length is inferred from external’s contents.
const std::string& data()
  Gets the external string that was created by the constructor.

## 1.12 The class PlRegister

This class encapsulates PL_register_foreign(). It is defined as a class rather then a function to exploit the C++ global constructor feature. This class provides a constructor to deal with the PREDICATE() way of defining foreign predicates as well as constructors to deal with more conventional foreign predicate definitions.

PlRegister :: PlRegister(const char *module, const char *name, int arity, foreign_t (f)(term_t t0, int a, control_t ctx))
  Register f as a the implementation of the foreign predicate <name>/<arity>. This interface uses the `PL_FA_VARARGS` calling convention, where the argument list of the predicate is passed using an array of `term_t` objects as returned by PL_new_term_refs(). This interface poses no limits on the arity of the predicate and is faster, especially for a large number of arguments.
PlRegister :: PlRegister(const char *module, const char *name, foreign_t (*f)(PlTerm a0, ...)
  Registers functions for use with the traditional calling conventional, where each positional argument to the predicate is passed as an argument to the function f. This can be used to define functions as predicates similar to what is used in the C-interface:

      static foreign_t
      pl_hello(PlTerm a1)
      { ...
      }

      PlRegister x_hello_1(NULL, "hello", 1, pl_hello);

  This construct is currently supported upto 3 arguments.

## 1.13 The class PlQuery

This class encapsulates the call-backs onto Prolog.

PlQuery :: PlQuery(const char *name, const PlTermv &av, int flags = PL_Q_PASS_EXCEPTION)
  Create a query where name defines the name of the predicate and av the argument vector. The arity is deduced from av. The predicate is located in the Prolog module `user`.
PlQuery :: PlQuery(const char *module, const char *name, const PlTermv &av, int flags = PL_Q_PASS_EXCEPTION)
  Same, but performs the predicate lookup in the indicated module.
int PlQuery::next_solution()
  Provide the next solution to the query. Yields `true` if successful and `false` if there are no (more) solutions. Prolog exceptions are mapped to C++ exceptions. If the PlQuery object was created with the `PL_Q_EXT_STATUS` flag, the extended return codes can also be returned (`TRUE`, `FALSE`, `PL_S_NOT_INNER`, `PL_S_EXCEPTION`, `PL_S_FALSE`, `PL_S_TRUE`, `PL_S_LAST`). Because of this, you shouldn't use PlCheckFail() with PlQuery::next_solution() in this situation.
void PlQuery::cut()
  Discards the query, but does not delete an of the data created by the query. If there is any pending Prolog exception, it is mapped to a C++ exception and thrown. The call to PlQuery::cut() is done implicitly by `PlQuery`’s destructor.

  Below is an example listing the currently defined Prolog modules to the terminal.

      PREDICATE(list_modules, 0)
      { PlTermv av(1);

        PlQuery q("current_module", av);
        while( q.next_solution() )
          cout << av[0].as_string() << endl;

        return true;
      }

In addition to the above, the following functions have been defined.

int PlCall(const char *predicate, const PlTermv &av)
  Creates a `PlQuery` from the arguments generates the first next_solution() and destroys the query. Returns the result of next_solution() or an exception.
int PlCall(const char *module, const char *predicate, const PlTermv &av)
  Same, locating the predicate in the named module.
int PlCall(const wchar_t *goal)
int PlCall(const std::string& goal)
  Translates goal into a term and calls this term as the other PlCall() variations. Especially suitable for simple goals such as making Prolog load a file.
bool PlTerm::call()
  Wrapper for PL_call(), returning `true` or `false` for the success/failure of the call; and throws an exception if there's an error. `t.call()` is essentially the same as `PlCall(t)`.
bool PlTerm::call(PlModule m)
  Same as PlTerm::call() but specifying the module.

### 1.13.1 The class PlFrame - Unification and foreign frames

As documented with PL_unify(), if a unification call fails and control isn't made immediately to Prolog, any changes made by unification must be undone. The functions PL_open_foreign_frame(), PL_rewind_foreign_frame(), PL_discard_foreign_frame(), and PL_close_foreign_frame() are encapsulated in the class `PlFrame`, whose destructor calls PL_close_foreign_frame(). Using this, the example code with PL_unify() can be written:

    PREDICATE(can_unify_ffi, 2)
    { fid_t fid = PL_open_foreign_frame();

      int rval = PL_unify(A1.unwrap(), A2.unwrap());
      PL_discard_foreign_frame(fid);
      return rval;
    }

        /* equivalent to the Prolog code
           T1 = T2 -> do_one_thing ; do_another_thing */
        { PlFrame fr;
          bool t1_t2_unified = A1.unify_term(A2);
          if ( ! t1_t2_unified )
            fr.rewind();
        if ( t1_t2_unified )
          do_one_thing(...);
        else
          do_another_thing(...);
    }

The following is C++ version of the code example for PL_open_foreign_frame(). The calls to PL_close_foreign_frame() and the check for PL_exception(0) in the C code aren't needed in the C++ code:

    static std::vector<std::string> lookup_unifies =
      { "item(one, 1)", "item(two, 2)", "item(three, 3)" };

    PREDICATE(lookup_unify, 1)
    { PlFrame fr;
      for (auto& s : lookup_unifies )
      { PlCompound t(s);
        if ( A1.unify_term(t) )
          return true;
        fr.rewind();
      }
      return false;
    }

or using this convenience wrapper:

        if ( RewindOnFail([t1=A1,t2=A2]()->bool
                          { return t1.unify_term(t2); }) )
          do_one_thing(...);
        else
          do_another_thing(...);

Note that PlTerm::unify_term() checks for an error and throws an exception to Prolog; if you wish to handle exceptions, you must call `PL_unify_term(t1. unwrap(),t2. unwrap())`.

The class `PlFrame` provides an interface to discard unused term-references as well as rewinding unifications (data-backtracking).

Reclaiming unused term-references is automatically performed after a call to a C++-defined predicate has finished and returns control to Prolog. In this scenario `PlFrame` is rarely of any use. This class comes into play if the toplevel program is defined in C++ and calls Prolog multiple times. Setting up arguments to a query requires term-references and using `PlFrame` is the only way to reclaim them.

Another use of of `PlFrame` is when multiple separate unifications are done - if any of them fails, then the earlier unifications must be undone before returning to Prolog.

PlFrame :: PlFrame()
  Creating an instance of this class marks all term-references created afterwards to be valid only in the scope of this instance.
~ PlFrame()
  Reclaims all term-references created after constructing the instance. If either close() or discard() have been called, the destructor does nothing.
void PlFrame::rewind()
  Discards all term-references and global-stack data created as well as undoing all unifications after the instance was created.
void PlFrame::close()
  Reclaims all term-references created after constructing the instance.
void PlFrame::discard()
  Same as PlFrame::rewind() + PlFrame::close().
bool PlRewindOnFail(std::function<bool()> f)
  is a convenience function that does a frame rewind if a function call fails (typically, failure due to unification failure). It takes a std::function<bool>()> as an argument, which is called in the context of a new `PlFrame`.

A typical use for `PlFrame` is the definition of C++ functions that call Prolog and may be called repeatedly from C++. Consider the definition of assertWord(), adding a fact to word/1; the `PlFrame` removes the new term `av[0]` from the stack, which prevents the stack from growing each time assertWord() is called:

    void
    assertWord(const char *word)
    { PlFrame fr;
      PlTermv av(1);

      av[0] = PlCompound("word", PlTermv(word));
      PlQuery q("assert", av);
      PlCheckFail(q.next_solution());
    }

The following example uses `PlFrame` in the context of a foreign predicate. The can_unify/2’s truth-value is the same as for Prolog unification (=/2), but has no side effects. In Prolog one would use double negation to achieve this:

    PREDICATE(can_unify, 2)
    { PlFrame fr;

      int rval = (A1=A2);
      fr.discard(); // or, less efficiently: fr.rewindd();
      return rval;
    }

Here is an example of using PlRewindOnFail(), where `name_to_terms` contains a map from names to terms (which are made global by using the PL_record() function). The frame rewind is needed in the situation where the first unify_term() succeeds and the second one fails.

    static const std::map<const std::string, PlRecord> name_to_term =
        { {"a", PlTerm(...).record(), PlTerm(...).record()},
        ... };

    PREDICATE(name_to_terms, 3)
    { A1.must_be_atom_or_string();
      const auto it = name_to_term.find(A1.as_string());
      return it != name_to_term.cend() &&
        PlRewindOnFail([t1=A2,t2=A3,&it]()
                       { return t1.unify_term(it->second.first.term()) &&
                                t2.unify_term(it->second.second.term()); });
    }

The equivalent code without using PlRewindOnFail() is:

    PREDICATE(name_to_terms, 3)
    { PlTerm key(A1), term1(A2), term2(A3);
      const auto it = name_to_term.find(key.as_string());
      if ( it == name_to_term.cend() )
        return false;
      if ( !term1.unify_term(it->second.first.term()) )
        return false;
      PlFrame fr;
      if ( !term2.unify_term(it->second.second.term()) )
      { fr.discard();
        return false;
      }
      return true;
    }

## 1.14 The PREDICATE and PREDICATE_NONDET macros

The PREDICATE macro is there to make your code look nice, taking care of the interface to the C-defined SWI-Prolog kernel as well as mapping exceptions. Using the macro

    PREDICATE(hello, 1)

is the same as writing:30There are a few more details, such as catching `std::bad_alloc`.:

    static foreign_t pl_hello__1(PlTermv PL_av);

    static foreign_t
    _pl_hello__1(term_t t0, int arity, control_t ctx)
    { (void)arity; (void)ctx;
      try
      { return pl_hello__1(PlTermv(1, t0));
      } catch( PlFail& )
      { return false;
      } catch ( PlException& ex )
      { return ex.plThrow();
      }
    }

    static PlRegister _x_hello__1("hello", 1, _pl_hello__1);

    static foreign_t
    pl_hello__1(PlTermv PL_av)

The first function converts the parameters passed from the Prolog kernel to a `PlTermv` instance and maps exceptions raised in the body to simple failure or Prolog exceptions. The `PlRegister` global constructor registers the predicate. Finally, the function header for the implementation is created.

### 1.14.1 Variations of the PREDICATE macro

The PREDICATE() macros have a number of variations that deal with special cases.

PREDICATE(name, arity)
  Create a predicate with an automatically generated internal name, and register it with Prolog. The various term arguments are accessible as `A1`, `A2`, etc.
PREDICATE0(name)
  This is the same as PREDICATE(name, 0). It avoids a compiler warning that `PL_av` is not used.
NAMED_PREDICATE(plname, cname, arity)
  This version can be used to create predicates whose name is not a valid C++ identifier. Here is a ---hypothetical--- example, which unifies the second argument with a stringified version of the first. The cname is used to create a name for the functions. The concrete name does not matter, but must be unique. Typically it is a descriptive name using the limitations imposed by C++ indentifiers.

          NAMED_PREDICATE("#", hash, 2)
          { return A2.unify_string(A1.as_string());
          }

PREDICATE_NONDET(name, arity)
  Define a non-deterministic Prolog predicate in C++. See also section 1.14.2.
NAMED_PREDICATE_NONDET(plname, cname, arity)
  Define a non-deterministic Prolog predicate in C++, whose name is not a valid C++ identifier. See also section 1.14.2.

### 1.14.2 Non-deterministic predicates

Non-deterministic predicates are defined using PREDICATE_NONDET(plname, cname, arity) or NAMED_PREDICATE_NONDET(plname, cname, arity).

A non-deterministic predicate returns a “context” , which is passed to a subsequent retry. Typically, this context is allocated on the first call to the predicate and freed when the predicate either fails or does its last successful return (the context is `nullptr` on the first call). To simplify this, a template helper function PlControl::context_unique_ptr<ContextType>() provides a “smart pointer” that frees the context on normal return or an exception; when used with PL_retry_address(), the context's std:unique_ptr<ContextType>::release() is used to pass the context to Prolog for the next retry, and to prevent the context from being freed. If the predicate is called with `PL_PRUNE`, the normal `return true` will implicitly free the context.

The skeleton for a typical non-deterministic predicate is as follows. The test for `PL_PRUNED` is done first to avoid an unneeded `PlFrame` and also to ensure that A1, A2, etc. aren't used when they have the value `PlTerm::null`.31This code could be structured as a `switch` statement, but typically the `PL_FIRST_CALL` case falls through to the `PL_REDO` case. There are a number of examples of non-deterministic predicates in the test code `test_cpp.cpp`.

    struct PredContext { ... }; // The "context" for retries

    PREDICATE_NONDET(pred, <arity>)
    { // "ctxt" must be acquired so that the destructor deletes it
      auto ctxt = handle.context_unique_ptr<PredContext>();
      const auto control  = handle.foreign_control();
      if ( control == PL_PRUNED )
        return true;

      // Can use A1, A2, etc. after we know control != PL_PRUNED

      if ( ... ) // deterministic result
      { assert(control == PL_FIRST_CALL);
        if ( ... )
          return true;  // Success (and no more solutions)
        else
          return fase;
      }

      if ( control = PL_FIRST_CALL )
      { ctxt.reset(new PredContext(...));
        ...
      } else
      { assert(control == PL_REDO);
      }

      PlFrame fr;
      for ( ; ctxt->valid(...) ; ctxt->next() )
      { if ( ... unify a result ... )
        { ctxt->next();
          if ( ctxt->valid(...) )
            PL_retry_addresss(ctxt.release()); // Succeed with a choice point
          else
            return true; // deterministic success
        }
        fr.rewind();
      }

      return false;
    }

### 1.14.3 Controlling the Prolog destination module

With no special precautions, the predicates are defined into the module from which load_foreign_library/1 was called, or in the module `user` if there is no Prolog context from which to deduce the module such as while linking the extension statically with the Prolog kernel.

Alternatively, before loading the SWI-Prolog include file, the macro PROLOG_MODULE may be defined to a string containing the name of the destination module. A module name may only contain alpha-numerical characters (letters, digits, _). See the example below:

    #define PROLOG_MODULE "math"
    #include <SWI-Prolog.h>
    #include <math.h>

    PREDICATE(pi, 1)
    { A1 = M_PI;
    }

    ?- math:pi(X).

    X = 3.14159

## 1.15 Exceptions

See also Prolog exceptions in foreign code.

Prolog exceptions are mapped to C++ exceptions using the class `PlException` (a subclass of `PlExceptionBase` to represent the Prolog exception term. All type-conversion functions of the interface raise Prolog-compliant exceptions, providing decent error-handling support at no extra work for the programmer.

For some commonly used exceptions, convenience functions have been created to exploit both their constructors for easy creation of these exceptions. If you wish to trap these, you should use `PlException` or `PlExceptionBase` and then look for the appropriate error name. For example, the following code catches `"type_error"` and passes all other exceptions:

    try
    { do_something(...);
    } catch (const PlException& e)
    { PlTerm e_t = e.term();
      PlAtom ATOM_type_error("type_error");
      // e_t.name() == PlAtom("error") && e_t.arity() == 2
      if ( e_t[1].name() == ATOM_type_error) )
      { ... // expected type and culprit are \exam{e_t[1][1]} and \exam{e_t[1][2]}
      } else throw;
    }

The convenience functions are PlTypeEror() and PlDomainError(), PlDomainError(), PlInstantiationError(), PlExistenceError(), PlUninstantiationError(), PlRepresentationError(), PlPermissionError(), PlResourceError(), PlUnknownError(). There is also a PlGeneralError(inside) that creates `error(inside,_)` terms and is used by the other error convience functions.

To throw an exception, create an instance of `PlException` and use `throw`. This is intercepted by the PREDICATE macro and turned into a Prolog exception. See section 1.17.2.

      char *data = "users";

      throw PlException(PlCompound("no_database", PlTerm(data)));

### 1.15.1 The class PlException

This subclass of `PlExceptionBase` is used to represent exceptions. Currently defined methods are:

PlException :: PlException(const PlTerm &)
  Create an exception from a general Prolog term. This provides the interface for throwing any Prolog terms as an exception.
std::string as_string()
  The exception is translated into a message as produced by print_message/2. The character data is stored in a ring. Example:

        ...;
        try
        { PlCall("consult(load)");
        } catch ( PlException& ex )
        { cerr << ex.as_string() << endl;
        }

int plThrow()
  Used in the PREDICATE() wrapper to pass the exception to Prolog. See PL_raise_exeption().

#### 1.15.1.1 The function PlTypeError

A type error expresses that a term does not satisfy the expected basic Prolog type.

PlTypeError :: PlTypeError(const std::string& expected, const PlTerm &actual)
  Creates an ISO standard Prolog error term expressing the expected type and actual term that does not satisfy this type.

#### 1.15.1.2 The function PlDomainError

A domain error expresses that a term satisfies the basic Prolog type expected, but is unacceptable to the restricted domain expected by some operation. For example, the standard Prolog open/3 call expect an `io_mode` (read, write, append, ...). If an integer is provided, this is a type error, if an atom other than one of the defined io-modes is provided it is a domain error.

PlDomainError :: PlDomainError(const std::string& expected, const PlTerm &actual)
  Creates an ISO standard Prolog error term expressing a the expected domain and the actual term found.

## 1.16 Embedded applications

Most of the above assumes Prolog is‘in charge’of the application and C++ is used to add functionality to Prolog, either for accessing external resources or for performance reasons. In some applications, there is a main-program and we want to use Prolog as a logic server. For these applications, the class `PlEngine` has been defined.

Only a single instance of this class can exist in a process. When used in a multi-threading application, only one thread at a time may have a running query on this engine. Applications should ensure this using proper locking techniques.32For Unix, there is a multi-threaded version of SWI-Prolog. In this version each thread can create and destroy a thread-engine. There is currently no C++ interface defined to access this functionality, though ---of course--- you can use the C-functions.

PlEngine :: PlEngine(int argc, char **argv)
  Initialises the Prolog engine. The application should make sure to pass `argv[0]` from its main function, which is needed in the Unix version to find the running executable. See PL_initialise() for details.
PlEngine :: PlEngine(char *argv0)
  Simple constructure using the main constructor with the specified argument for `argv[0]`.
~ PlEngine()
  Calls PL_cleanup() to destroy all data created by the Prolog engine.

Section 1.11.10 has a simple example using this class.

## 1.17 Considerations

### 1.17.1 The C++ versus the C interface

Not all functionality of the C-interface is provided, but as `PlTerm` and `term_t` are essentially the same thing with type-conversion between the two (using the unwrap() method), this interface can be freely mixed with the functions defined for plain C. For checking return codes from C functions, it is recommended to use PlCheckFail() or PlCheck_PL().

Using this interface rather than the plain C-interface requires a little more resources. More term-references are wasted (but reclaimed on return to Prolog or using `PlFrame`). Use of some intermediate types (`functor_t` etc.) is not supported in the current interface, causing more hash-table lookups. This could be fixed, at the price of slighly complicating the interface.

Global terms and atoms need to be handled slightly differently in C++ than in C - see section 1.17.3

### 1.17.2 Notes on exceptions

Exceptions are normal Prolog terms that are handled specially by the PREDICATE macro when they are used by a C++ `throw`, and converted into Prolog exceptions. The exception term may not be unbound; that is, throw(_) must raise an error. The C++ code and underlying C code do not explicitly check for the term being a variable, and behaviour of raising an exception that is an unbound term is undefined, including the possibility of causing a crash or corrupting data.

The Prolog exception term error(Formal, _) is special. If the 2nd argument of error/2 is undefined, and the term is thrown, the system finds the catcher (if any), and calls the hooks in library(prolog_stack) to add the context and stack trace information when appropriate. That is, `throw PlDomainError(Domain,Culprit)` ends up doing the same thing as calling `PL_domain_error(Domain,Culprit)` which internally calls PL_raise_exception() and returns control back to Prolog.

The VM handling of calling to C finds the `FALSE` return code, checks for the pending exception and propagates the exception into the Prolog environment. As the term references (`term_t`) used to create the exception are lost while returning from the foreign function we need some way to protect them. That is done using a global `term_t` handle that is allocated at the epoch of Prolog. PL_raise_exception() sets this to the term using PL_put_term(). PL_exception(0) returns the global exception `term_t` if it is bound and 0 otherwise.

Special care needs to be taken with data backtracking using PL_discard_foreign_frame() or PL_close_query() because that will invalidate the exception term. So, between raising the exception and returning control back to Prolog we must make sure not to do anything that invalidates the exception term. If you suspect something like that to happen, use the debugger with a breakpoint on __do_undo__LD() defined in `pl-wam.c`.

In order to always preserve Prolog exceptions and return as quickly as possible to Prolog on an exception, some of the C++ classes can throw an exception in their destructor. This is theoretically a dangerous thing to do, and can lead to a crash or program termination if the destructor is invoked as part of handling another exception.

### 1.17.3 Global terms, atoms, and functors

Sometimes it is convenient to put constant terms and atoms as global variables in a file (with a `static` qualifier), so that they are only created (and looked up) cone. This is fine for atoms and functors, which can be created by something like this:

    static PlAtom ATOM_foo("foo");
    static PlFunctor FUNCTOR_ff_2("ff", 2);

C++ makes no guarantees about the order of creating global variables across “translation units” (that is, individual C++ files), but the Prolog runtime ensures that the necessary initialization has been done to allow `PlAtom` and `PlFunctor` objects to be created. However, to be safe, it is best to put such global variables inside functions - C++ will initialize them on their firstuse.

Global Terms need a bit of care. For one thing, terms are ephemeral, so it is wrong to have a `PlTerm` static variable - instead, a `PlRecord` must be used, which will provide a fresh copy of the term using PlRecord::term(). There is no guarantee that the Prolog runtime has initialized everything needed for creating entries in the recorded database (see Recorded database). Therefore, global recorded terms must be wrapped inside a function. C++ will call the constructor upon first use. For example:

    static PlTerm
    term_foo_bar()
    { static PlRecord r(PlCompound("foo", PlTermv(PlTerm_atom("bar"))).record());
      return r.term();
    }

### 1.17.4 Atom map utilities

The include file `SWI-cpp2-atommap.h` contains a templated class `AtomMap` for mapping atoms to atoms or terms. The typical use case is for when it is desired to open a database or stream and, instead of passing around the blob, an atom can be used to identify the blob.

The keys in the map must be standard Prolog atoms and not blobs - the code depends on the fact that an atom has a unique ID.

The `AtomMap` is thread-safe (it contains a mutex). It also takes care of reference counts for both the key and the value. Here is a typical use case:

    static AtomMap<PlAtom, PlAtom> map_atom_my_blob("alias", "my_blob");

    // look up an entry:
       auto value = map_atom_my_blob(A1.as_atom());
       PlCheckFail(value.not_null());

    // insert an entry:
       map_atom_my_blob.insert(A1.as_atom(), A2.as_atom());

    // remove an entry:
       map_atom_my_blob.erase(A1.as_atom());

The constructor and methods are as follows:

template<ValueType, StoredValueType> AtomMap::AtomMap(const std::string& insert_op)
  const std::string& insert_type Construct an `AtomMap`. The ValueType and StoredValueType specify what type you wish for the value. Currently, two value types are supported:
  - `PlAtom` - the StoredValueType should be `PlAtom`.
  - `PlTerm` - the StoredValueType shoud be `PlRecord` (because the term needs to be put on the global stack).
  The insert_op and insert_type values are used in constructing error terms - these correspond to the operation and type arguments to Pl_permission_error().
insert PlAtom key, ValueType value(I)
  nserts a new value; raises a `permission_error` if the value is already in the map, unless the value is identical to the value in the map. The insert() method converts the value to the `StoredValueType`. The insertion code takes care of atom reference counts.
ValueType find(PlAtom key)
  Look up an entry. Success/failure can be determined by using ValueType::is_null() or ValueType::not_null(). The stored value is converted from `StoredValueType` to `ValueType`.
erase PlAtom(r)
  emoves the entry from the map. If there was no entry in the map with that key, this is a no-op. The erasure code takes care of atom reference counts.

### 1.17.5 Static linking and embedding

The mechanisms outlined in this document can be used for static linking with the SWI-Prolog kernel using swipl-ld(1). In general the C++ linker should be used to deal with the C++ runtime libraries and global constructors.

### 1.17.6 Status and compiler versions

The current interface can be entirely defined in the `.h` file using inlined code. This approach has a few advantages: as no C++ code is in the Prolog kernel, different C++ compilers with different name-mangling schemas can cooperate smoothly. However, inlining everything can lead to code bloat, so the larger functions and methods have been put into a `.cpp` file that can be either compiled separately (by the same compiler as used by the foreign predicate) or inlined as if it were part of the `.h` file.

Also, changes to the header file have no consequences to binary compatibility with the SWI-Prolog kernel. This makes it possible to have different versions of the header file with few compatibility consequences.

As of 2023-04, some details remain to be decided, mostly to do with encodings. A few methods have a `PlEncoding` optional parameter (e.g., PlTerm::as_string()), but this hasn't yet been extended to all methods that take or return a string. Also, the details of how the default encoding is set have not yet been decided.

As of 2023-04, the various error convenience classes do not fully match what the equivalent C functions do. That is, `throw PlInstantiationError(A1)` does not result in the same context and traceback information that would happen from `Plx_instantiation_error(A1. unwrap()); throw PlFail()`. See section 1.17.2.

The Plx_*() wrappers may require small adjustments in whether their return values require `[[nodiscard]]` or whether their return values should be treated as an error.

The implementation of `PlException` is likely to change somewhat in the future. Currently, to ensure that the exception term has a sufficient lifetime, it is serialized using PL_record_external(). In future, if this proves unnecessary, the term will be stored as-is. The API will not change if this implementation detail changes.

## 1.18 Conclusions

In this document, we presented a high-level interface to Prolog exploiting automatic type-conversion and exception-handling defined in C++.

Programming using this interface is much more natural and requires only little extra resources in terms of time and memory.

Especially the smooth integration between C++ and Prolog exceptions reduce the coding effort for type checking and reporting in foreign predicates.

# Index

?
add/3
  1.7.2
add_num/3
  1.3
arg/3
  1.11.4
as_string()
as_wstring()
assert
  1.13.1
at_halt/1
  1.6.8.6
atom_chars/2
  1.6.6 1.11.8
atom_or_string_as_string()
atomic/1
  1.11.5
atomic_as_string()
average/3
  1.3 1.7.3
between/3
  1.3
blob_data()
call()
can_unify/2
  1.3 1.13.1
cappend/3
  1.3
check_stream()
close_my_blob/1
  1.6.8.6 1.6.8.6 1.6.8.6
create_my_blob/2
  1.6.8.6
data()
duplicate()
entry/1
  1.11.10
eq1/1
  1.3
eq2/2
  1.3
eq3/2
  1.3
erase()
error/2
  1.17.2
fail/0
  1.5
find()
functor/3
  1.3
happy/1
  1.3
hello/1
  1.7.1
int_info/2
  1.3
is_null()
is_valid()
likes/2
  1.3
load_foreign_library/1
  1.14.3
my_blob_properties/2
  1.6.8.6
my_file_close/1
  1.6.8.8
my_file_open/4
  1.6.8.8
name_arity()
name_arity/3
  1.3
nify_term()
not_null()
open/3
  1.15.1.2
operator!=()
operator<()
operator<=()
operator=()
operator==()
operator>()
operator>=()
print_message/2
  1.15.1
qsave_program/[1,2]
  1.6.8.6
range_cpp/3
  1.3
read/1
  1.11.9
register_atom()
reset()
reset_term_refs()
size_tindex()
square_roots/2
  1.3
term()
throw/1
  1.1 1.5
unifiable/3
  1.3
unregister_atom()
use_foreign_library/1
  1.5
word/1
  1.13.1
write()
write/1
  1.7.1 1.11.1
write_list/1
  1.3
NAMED_PREDICATE()
NAMED_PREDICATE_NONDET()
P
PlAtom!=()
PlAtom()
PlAtom==()
PlAtomkey,ValueTypevalue()
PlBlob::acquire()
PlBlob::compare_fields()
PlBlob::write_fields()
PlBlob::PlBlob::load()
PlBlob::PlBlob::pre_delete()
PlBlob::PlBlob::save()
PlCall()
PlCheckFail()
PlEx()
PlFrame::close()
PlFrame::discard()
PlFrame::rewind()
PlQuery
  1.13
PlQuery::cut()
PlQuery::next_solution()
PlRecord()
PlRecord()
PREDICATE()
PREDICATE0()
PREDICATE_NONDET()
PlRewindOnFail()
PlStream::release()
PlTerm!=()
PlTerm::arity()
PlTerm::as_atom()
PlTerm::as_bool()
PlTerm::as_double()
PlTerm::as_float()
PlTerm::as_int()
PlTerm::as_int32_t()
PlTerm::as_int64_t()
PlTerm::as_long()
PlTerm::as_nil()
PlTerm::as_pointer()
PlTerm::as_size_t()
PlTerm::as_string()
PlTerm::as_uint()
PlTerm::as_uint32_t()
PlTerm::as_uint64_t()
PlTerm::as_ulong()
PlTerm::as_wstring()
PlTerm::call()
PlTerm::compare()
PlTerm::copy_term_ref()
PlTerm::eq_if_atom()
PlTerm::free_term_ref()
PlTerm::free_term_ref_reset()
PlTerm::get_arg()
PlTerm::get_atom()
PlTerm::get_atom_ex()
PlTerm::get_atom_nchars()
PlTerm::get_attr()
PlTerm::get_blob()
PlTerm::get_bool()
PlTerm::get_bool_ex()
PlTerm::get_char_ex()
PlTerm::get_chars()
PlTerm::get_compound_name_arity()
PlTerm::get_dict_key()
PlTerm::get_file_name()
PlTerm::get_float()
PlTerm::get_float_ex()
PlTerm::get_functor()
PlTerm::get_head()
PlTerm::get_int64_ex()
PlTerm::get_integer()
PlTerm::get_integer_ex()
PlTerm::get_intptr()
PlTerm::get_intptr_ex()
PlTerm::get_list()
PlTerm::get_list_chars()
PlTerm::get_list_ex()
PlTerm::get_list_nchars()
PlTerm::get_long()
PlTerm::get_long_ex()
PlTerm::get_module()
PlTerm::get_name_arity()
PlTerm::get_nchars()
PlTerm::get_nil()
PlTerm::get_nil_ex()
PlTerm::get_pointer()
PlTerm::get_pointer_ex()
PlTerm::get_size_ex()
PlTerm::get_tail()
PlTerm::get_uint64_ex()
PlTerm::get_wchars()
PlTerm::integer()
PlTerm::is_acyclic()
PlTerm::is_atom()
PlTerm::is_atom_or_string()
PlTerm::is_atomic()
PlTerm::is_attvar()
PlTerm::is_blob()
PlTerm::is_callable()
PlTerm::is_compound()
PlTerm::is_dict()
PlTerm::is_float()
PlTerm::is_functor()
PlTerm::is_ground()
PlTerm::is_integer()
PlTerm::is_list()
PlTerm::is_number()
PlTerm::is_pair()
PlTerm::is_rational()
PlTerm::is_string()
PlTerm::is_variable()
PlTerm::must_be_acyclic()
PlTerm::must_be_atom()
PlTerm::must_be_atom_or_string()
PlTerm::must_be_atomic()
PlTerm::must_be_attvar()
PlTerm::must_be_callable()
PlTerm::must_be_compound()
PlTerm::must_be_dict()
PlTerm::must_be_float()
PlTerm::must_be_ground()
PlTerm::must_be_integer()
PlTerm::must_be_list()
PlTerm::must_be_number()
PlTerm::must_be_pair()
PlTerm::must_be_rational()
PlTerm::must_be_string()
PlTerm::must_be_variable()
PlTerm::name()
PlTerm::put_atom()
PlTerm::put_atom_chars()
PlTerm::put_atom_nchars()
PlTerm::put_blob()
PlTerm::put_bool()
PlTerm::put_chars()
PlTerm::put_float()
PlTerm::put_functor()
PlTerm::put_integer()
PlTerm::put_list()
PlTerm::put_list_chars()
PlTerm::put_list_codes()
PlTerm::put_list_nchars()
PlTerm::put_list_ncodes()
PlTerm::put_nil()
PlTerm::put_pointer()
PlTerm::put_string_chars()
PlTerm::put_string_nchars()
PlTerm::put_term()
PlTerm::put_variable()
PlTerm::record()
PlTerm::type()
PlTerm::unify_atom()
PlTerm::unify_blob()
PlTerm::unify_bool()
PlTerm::unify_bool_ex()
PlTerm::unify_chars()
PlTerm::unify_float()
PlTerm::unify_functor()
PlTerm::unify_integer()
PlTerm::unify_list()
PlTerm::unify_list_chars()
PlTerm::unify_list_codes()
PlTerm::unify_list_ex()
PlTerm::unify_nil()
PlTerm::unify_nil_ex()
PlTerm::unify_pointer()
PlTerm::unify_string()
PlTerm::unify_term()
PlTerm<()
PlTerm<=()
PlTerm==()
PlTerm>()
PlTerm>=()
PlTerm[]()
PlTerm_tail::append()
PlTerm_tail::close()
PlTerm_tail::next()
PlTermScoped::get()
PlTermScoped::release()
PlTermScoped::reset()
PlTermScoped::swap()
PlTermScoped=()
PlTerm::get_file_nameW()
PlWrap()
T
plThrow()
V
template<ValueType,StoredValueType>AtomMap::AtomMap()