A C++ interface to SWI-Prolog
Jan Wielemaker
VU University of Amsterdam
The Netherlands
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 herein 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.

# Table of Contents

## 1 Introduction

C++ provides a number of features that make it possible to define a much more natural and concise interface to dynamically typed languages than plain C does. Using programmable type-conversion (casting), native data-types can be translated automatically 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.

### Competing interfaces

Volker Wysk has defined an alternative C++ mapping based on templates and compatible to the STL framework. See http://www.volker-wysk.de/swiprolog-c++/index.html.

### Acknowledgements

I would like to thank Anjo Anjewierden for comments on the definition, implementation and documentation of this package.

## 2 Overview

The most 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. Constructors and operator definitions provide flexible operations and integration with important C-types (`char *`, `wchar_t*`, `long` and `double`).

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

PlTerm
  Generic Prolog term. Provides constructors and operators for conversion to native C-data and type-checking.
PlString
  Subclass of PlTerm with constructors for building Prolog string objects.
PlCodeList
  Subclass of PlTerm with constructors for building Prolog lists of ASCII values.
PlCharList
  Subclass of PlTerm with constructors for building Prolog lists of one-character atoms (as atom_chars/2).
PlCompound
  Subclass of PlTerm with constructors for building compound terms.
PlTail
  SubClass of PlTerm for building and analysing Prolog lists.
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.
PlException
  Subclass of PlTerm representing a Prolog exception. Provides methods for the Prolog communication and mapping to human-readable text representation.
PlTypeError
  Subclass of PlException for representing a Prolog `type_error` exception.
PlDomainError
  Subclass of PlException for representing a Prolog `domain_error` exception.
PlExistenceError
  Subclass of PlException for representing a Prolog `existence_error` exception.
PlPermissionError
  Subclass of PlException for representing a Prolog `permission_error` exception.
PlAtom
  Allow for manipulating atoms in their internal Prolog representation for fast comparison.
PlQuery
  Represents opening and enumerating the solutions to a Prolog query.
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
  The encapsulation of PL_register_foreign() is defined to be able to use C++ global constructors for registering foreign predicates.

The required C(++) function header and registration of a predicate is arranged through a macro called PREDICATE().

## 3 Examples

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

### 3.1 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 " << (char *)A1 << 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. Casting a PlTerm to a `char *` or `wchar_t *` provides the natural 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

### 3.2 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 = (long)A1 + (long)A2;
    }

Casting a PlTerm to a `long` performs a PL_get_long() 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 `=` operator 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) ?

### 3.3 Average of solutions

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 member-function next_solution() yields `TRUE` if there was a solution and `FALSE` otherwise. If the goal yielded a Prolog exception it is mapped into a C++ exception.

    PREDICATE(average, 3)
    { long sum = 0;
      long n = 0;

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

## 4 The class PlTerm

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.

### 4.1 Constructors

PlTerm :: PlTerm()
  Creates a new initialised term (holding a Prolog variable).
PlTerm :: PlTerm(term_t t)
  Converts between the C-interface and the C++ interface by turning the term-reference into an instance of PlTerm. Note that, being a lightweight class, this is a no-op at the machine-level!
PlTerm :: PlTerm(const char *text)
  Creates a term-references holding a Prolog atom representing text.
PlTerm :: PlTerm(const wchar_t *text)
  Creates a term-references holding a Prolog atom representing text.
PlTerm :: PlTerm(const PlAtom &atom)
  Creates a term-references holding a Prolog atom from an atom-handle.
PlTerm :: PlTerm(long n)
  Creates a term-references holding a Prolog integer representing n.
PlTerm :: PlTerm(double f)
  Creates a term-references holding a Prolog float representing f.
PlTerm :: PlTerm(void *ptr)
  Creates a term-references holding a Prolog pointer. 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)
      { myclass *myobj = new myclass();

        return A1 = (void *)myobj;
      }

      PREDICATE(free_my_object, 1)
      { myclass *myobj = (void *)A1;

        delete(myobj);
        return TRUE;
      }

### 4.2 Casting PlTerm to native C-types

PlTerm can be casted 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*).

### 4.3 Unification

int PlTerm::operator =(Type)
  The operator `=` is defined for the Types PlTerm, `long`, `double`, `char *`, `wchar_t*` and PlAtom. It performs Prolog unification and returns `TRUE` if successful and `FALSE` otherwise.

  The boolean return-value leads to somewhat unconventional-looking code as normally, assignment returns the value assigned in C. Unification however is fundamentally different to assignment as it can succeed or fail. Here is a common example.

      PREDICATE(hostname, 1)
      { char buf[32];

        if ( gethostname(buf, sizeof(buf)) == 0 )
          return A1 = buf;

        return FALSE;
      }

### 4.4 Comparison

int PlTerm::operator ==(const PlTerm &t)
int PlTerm::operator !=(const PlTerm &t)
int PlTerm::operator <(const PlTerm &t)
int PlTerm::operator >(const PlTerm &t)
int PlTerm::operator <=(const PlTerm &t)
int PlTerm::operator >=(const PlTerm &t)
  Compare the instance with t and return the result according to the Prolog defined standard order of terms.
int PlTerm::operator ==(long num)
int PlTerm::operator !=(long num)
int PlTerm::operator <(long num)
int PlTerm::operator >(long num)
int PlTerm::operator <=(long num)
int 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.
int PlTerm::operator ==(const wchar_t *)
int PlTerm::operator ==(const char *)
  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 6 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''.

### 4.5 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.
int PlTerm::arity()
  Returns the arity of the compound term. Raises a `type_error` if the argument is not compound.

### 4.6 Miscellaneous

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`

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.

### 4.7 The class PlString

A SWI-Prolog string represents a byte-string on the global stack. It's 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.

PlString :: PlString(const wchar_t *text)
PlString :: PlString(const char *text)
  Create a SWI-Prolog string object from a 0-terminated C-string. The text is copied.
PlString :: PlString(const wchar_t *text, size_t len)
PlString :: PlString(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.

### 4.8 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.

### 4.9 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.

### 4.10 The class PlCompound

PlCompound :: PlCompound(const wchar_t *text)
PlCompound :: PlCompound(const char *text)
  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("world"))

### 4.11 The class PlTail

The class PlTail is both for analysing and constructing lists. It is called PlTail as enumeration-steps make the term-reference follow the `tail' of the list.

PlTail :: PlTail(PlTerm list)
  A PlTail is created by making a new term-reference pointing to the same object. As PlTail is used to enumerate or build a Prolog list, the initial list term-reference keeps pointing to the head of the list.
int PlTail::append(const PlTerm &element)
  Appends element to the list and make the PlTail 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 PlTail 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);
        PlTail l(av[0]);

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

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

int PlTail::close()
  Unifies the term with `[]` and returns the result of the unification.
int PlTail::next(PlTerm &t)
  Bind t to the next element of the list PlTail and advance PlTail. Returns `TRUE` on success and `FALSE` if PlTail represents the empty list. If PlTail 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)
      { PlTail tail(A1);
        PlTerm e;

        while(tail.next(e))
          cout << (char *)e << endl;

        return TRUE;
      }

## 5 The class PlTermv

The class PlTermv represents an array of term-references. This type is used to pass the arguments to a foreignly 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.

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

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

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

      { PlTermv av(10);

        av[0] = "hello";
        ...

## 6 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:

### 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.

### 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.

### 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 6, but in nested if-then-else the extraction of the atom from the term is done only once.

### 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. Used internally and for integration with the C-interface.
PlAtom :: PlAtom(const wchar_t *text)
PlAtom :: PlAtom(const char *text)
  Create an atom from a string. The text is copied if a new atom is created.
PlAtom :: PlAtom(const PlTerm &t)
  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)
  Yields `TRUE` if the atom represents text, `FALSE` otherwise. Performs a strcmp() for this.
int PlAtom::operator ==(const PlAtom &a)
  Compares the two atom-handles, returning `TRUE` or `FALSE`.

## 7 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.

## 8 The class PlQuery

This class encapsulates the call-backs onto Prolog.

PlQuery :: PlQuery(const char *name, const PlTermv &av)
  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)
  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.

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 << (char *)av[0] << 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 char *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.

### 8.1 The class PlFrame

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.

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.
void PlFrame::rewind()
  Discards all term-references and global-stack data created as well as undoing all unifications after the instance was created.

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:

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

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

This example shows the most sensible use of PlFrame if it is used in the context of a foreign predicate. The predicate's thruth-value is the same as for the 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.rewind();
      return rval;
    }

## 9 The PREDICATE macro

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:

    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 ( PlTerm &ex )
      { return ex.raise();
      }
    }

    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 Prolog exceptions. The PlRegister global constructor registers the predicate. Finally, the function header for the implementation is created.

### 9.1 Variations of the PREDICATE macro

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

PREDICATE0(name)
  This is the same as PREDICATE(name, 0). It avoids a compiler warning about 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)
          { A2 = (wchar_t*)A1;
          }

NAMED_PREDICATE_NONDET(plname, cname, arity)
  Define a non-deterministic Prolog predicate in C++. See `SWI-cpp.h`. FIXME: Needs cleanup and an example.

### 9.2 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

## 10 Exceptions

Prolog exceptions are mapped to C++ exceptions using the subclass PlException of PlTerm 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, subclasses of PlException have been created to exploit both their constructors for easy creation of these exceptions as well as selective trapping in C++. Currently, these are PlTypeEror and PlDomainError.

To throw an exception, create an instance of PlException and use throw() or PlException::cppThrow(). The latter refines the C++ exception class according to the represented Prolog exception before calling throw().

      char *data = "users";

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

### 10.1 The class PlException

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

PlException :: PlException(const PlTerm &t)
  Create an exception from a general Prolog term. This is provides the interface for throwing any Prolog terms as an exception.
PlException ::operator wchar_t *(void)
PlException ::operator char *(void)
  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 << (char *) ex << endl;
        }

int plThrow()
  Used in the PREDICATE() wrapper to pass the exception to Prolog. See PL_raise_exeption().
int cppThrow()
  Used by PlQuery::next_solution() to refine a generic PlException representing a specific class of Prolog exceptions to the corresponding C++ exception class and finally then executes throw(). Thus, if a PlException represents the term

  > `error(`type_error(Expected, Actual)`, Context)`

  PlException::cppThrow() throws a PlTypeEror exception. This ensures consistency in the exception-class whether the exception is generated by the C++-interface or returned by Prolog.

  The following example illustrates this behaviour:

      PREDICATE(call_atom, 1)
      { try
        { return PlCall((char *)A1);
        } catch ( PlTypeError &ex )
        { cerr << "Type Error caugth in C++" << endl;
          cerr << "Message: \"" << (char *)ex << "\"" << endl;
          return FALSE;
        }
      }

### 10.2 The class PlTypeError

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

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

### 10.3 The class 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 char *expected, const PlTerm &actual)
  Creates an ISO standard Prolog error term expressing a the expected domain and the actual term found.

## 11 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.1For 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 4.11 has a simple example using this class.

## 12 Considerations

### 12.1 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 automatic type-conversion between the two, this interface can be freely mixed with the functions defined for plain C.

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.

### 12.2 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.

### 12.3 Status and compiler versions

The current interface is 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.

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.

## 13 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
  3.2
arg/3
  4.5
assert
  8.1
atom_chars/2
  2 4.9
average/3
  3.3
entry/1
  4.11
hello/1
  3.1
load_foreign_library/1
  9.2
open/3
  10.3
print_message/2
  10.1
read/1
  4.10
word/1
  8.1
write/1
  3.1 4.2
NAMED_PREDICATE()
NAMED_PREDICATE_NONDET()
P
PlAtom
  4.3 6
PlAtom::operator==()
PlCall()
PlCompound
  4.5
PlDomainError
  10
PlEngine
  11
PlException
  2 2 2 2 10 10 10 10.1 10.1
PlFrame
  8.1 8.1 8.1 8.1 8.1 12.1
PlFrame::rewind()
PlQuery
  3.3 5 8
PlQuery::next_solution()
PlRegister
  9
PREDICATE0()
PlTail
  4.11 4.11 4.11 4.11 4.11 4.11 4.11 4.11 4.11 4.11
PlTail::append()
PlTail::close()
PlTail::next()
PlTerm
  2 2 2 2 2 2 2 3.1 3.1 3.2 3.2 4 4.1 4.2 4.2 4.2 4.3 4.4 4.4 4.4 4.5 4.5 4.5 4.6 10 10.1 12.1
PlTerm::arity()
PlTerm::name()
PlTerm::operator <()
PlTerm::operator <=()
PlTerm::operator >()
PlTerm::operator >=()
PlTerm::operator!=()
PlTerm::operator=()
PlTerm::operator==()
PlTerm::operator[]()
PlTerm::type()
PlTermv
  2 4.10 5 9
PlTypeEror
  10 10.1
T
cppThrow()
plThrow()