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 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.

At this moment there are two versions of the C++ interface.

- Version 1 is implemented by `SWI-cpp.h` and described in chapter 1. This version is old, suffers from several ambiguities, covers only the core part of the C interface and does not support character encoding issues, which implies `char*` can only be used to exchange text in ISO-Latin-1 encoding. We hope to deprecate this interface soon.
- Version 2 is implemented by `SWI-cpp2.h` and described in chapter 2. This is a much more mature C++ interface has been designed and implemented by Peter Ludemann. We plan to make this the preferred interface soon. There are still several issues that need to be resolved before this can happen, notably related to handling text encoding.

# Table of Contents

# 1 A C++ interface to SWI-Prolog (Version 1)

## 1.1 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.

## 1.2 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().

## 1.3 Examples

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

### 1.3.1 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

### 1.3.2 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) ?

### 1.3.3 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;
    }

## 1.4 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.

### 1.4.1 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;
      }

### 1.4.2 Casting PlTerm to native C-types

`PlTerm` can be cast 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*).

### 1.4.3 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;
      }

### 1.4.4 Comparison

int PlTerm::operator ==(const PlTerm &)
int PlTerm::operator !=(const PlTerm &)
int PlTerm::operator <(const PlTerm &)
int PlTerm::operator >(const PlTerm &)
int PlTerm::operator <=(const PlTerm &)
int PlTerm::operator >=(const PlTerm &)
  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 1.6 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''.

### 1.4.5 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.

### 1.4.6 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.

### 1.4.7 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.

### 1.4.8 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.

### 1.4.9 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.

### 1.4.10 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"))

### 1.4.11 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 &)
  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;
      }

## 1.5 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";
        ...

## 1.6 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 1.6, 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 &)
  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 &)
  Compares the two atom-handles, returning `TRUE` or `FALSE`.

## 1.7 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.

## 1.8 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.

### 1.8.1 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;
    }

## 1.9 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.

### 1.9.1 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.

### 1.9.2 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

## 1.10 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().

      char *data = "users";

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

### 1.10.1 The class PlException

The C++ model of exceptions and the Prolog model of exceptions are different. Wherever the underlying function returns a "fail" return code, the C++ API does a further check for whether there's an exception and, if so, does a C++ `throw` of a PlException object. You can use C++ try-catch to intercept this and examine the

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

PlException :: PlException()
  Create an exception term using PL_exception(0). The method is_null() succeeds if there was simple failure (e.g., from unification failing) and not_null() succeeds if there was an exception.
PlException :: PlException(const PlTerm &)
  Create an exception from a general Prolog term. This 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;
        }
      }

### 1.10.2 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.

### 1.10.3 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.

## 1.11 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 1.4.11 has a simple example using this class.

## 1.12 Considerations

### 1.12.1 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.

### 1.12.2 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.

### 1.12.3 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.

## 1.13 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.

# 2 A C++ interface to SWI-Prolog (Version 2)

## 2.1 Summary of changes between Versions 1 and 2

Version 1 is in `SWI-cpp.h`; version 2 is in `SWI-cpp2.h`.

The overall structure of the API has been retained - that is, it is a thin layer on top of the interface provided by `SWI-Prolog.h`. Based on experience with the API, most of the conversion 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. 2If there is an implicit conversion operator from `PlTerm` to `term_t` and also to `char*`, then the ``[]`` operator is ambiguous in `PlTerm t=...; f(t[0])` if `f` is overloaded to accept a `term_t` or `char*`.

More specifically:

- The constructor PlTerm() is not available - instead, you should use the appropriate subclass' constructor (PlTerm_var(), PlTerm_atom(a), PlTerm_term_t(t), PlTerm_integer(i), PlTerm_int64(i), PlTerm_uint64(i), PlTerm_size_t(i), PlTerm_float(v), or PlTerm_pointer(p)).
- Instead of returning `false` from a predicate to indicate failure, you can use `throw PlFail()`. The convenience function PlCheck(rc) can be used to throw `PlFail()`, if a `false` is returned from a function in `SWI-Prolog.h`
- The "cast" operators (e.g., `(char*)t`, `(int64_t)t`) have been deprecated, replaced by "getters" (e.g., `t.as_string()`, `t.as_int64_t()`).3The form `(char*)t` is a C-style cast; C++'s preferred form is more verbose: `static_cast<char*>(t)`.
- The overloaded assignment operator for unification is deprecated; replaced by unify_term(), unify_atom(), etc., and the helper PlCheck().
- 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 will need to change your interface to return a `std::string` and apply the `c_str()` method to it. These errors can sometimes be caught by specifying the Gnu C++ or Clang options `-Wreturn-stack-address` or `-Wreturn-local-addr` - Clang seems to do a better analysis.
- Type-checking methods have been added: type(), is_variable(), 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`.
- `std::string` and `std::wstring` are now supported in most places where `char*` or `wchar_t*` are allowed.
- 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(), and reset(v), plus the constant `null`.
- A convenience class `PlForeignContextPtr<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().

More details are given in section 2.6 and section 2.7.

## 2.2 Introduction (version 2)

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 programmable type-conversion (casting) and overloading, 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.

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

It would be tempting to use C++ 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 easily 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.

### 2.2.1 Acknowledgements (version 2)

I would like to thank Anjo Anjewierden for comments on the definition, implementation and documentation of this package. Peter Ludemann modified the interface to remove some pitfalls, and also added some convenience functions (see section 2.1).

## 2.3 The life of a PREDICATE (version 2)

A foreign predicate is defined using the PREDICATE() macro.6Plus a few variations on this, such as PREDICATE_NONDET(), NAMED_PREDICATE(), and NAMED_PREDICATE_NONDET(). This defines an internal name for the function, registers it with the SWI-Prolog runtime (where it will be picked up by the use_foreign_library/1 directive), and defines the names `A1`, `A2`, etc. for the arguments.7You can define your own names for the arguments, for example: `auto x=A1, y=A2, result=A3;`. If a non-deterministic predicate is being defined, an additional parameter `handle` is defined (of type `control_t`).

The foreign predicate returns a value of `true` or `false` to indicate whether it succeeded or failed.8Non-deterministic predicates can also return a "retry" value. If a predicate fails, it could be simple failure (the equivalent of calling the builtin fail/0) or an error (the equivalent of calling throw/1). When an 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 to check for failure, which can become cumbersome. C++ has exceptions, so instead the code can wrap calls to PL_*() functions with PlCheck(), which will do `throw PlFail()` to exit from the top level of the foreign predicate, and handle the failure or exception appropriately.

The following three snippets do the same thing (for implementing the equivalent of =/2):

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

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

    PREDICATE(eq, 2)
    { PlCheck(PL_unify(A1.C_, A2.C_));
      return true;
    }

## 2.4 Overview (version 2)

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`), plus the C++-types (`std::string`, `std::wstring`).

### 2.4.1 Design philosophy of the classes

See also section 2.4.3.

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 2.16.)

The various 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`). You can check whether the object is "fully created" by using the verify() method - it will throw an exception if the object is `null`.

However, 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.

To help avoid programming errors, most 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 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). Sometimes, it's desirable to create them "lazily", such as:

    static PlAtom foo(PlAtom::null};
       ...
    if ( foo.is_null() )
      foo = PlAtom("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 2.9.1. Note that the default constructor is not public; to create a "variable" term, you should use the subclass constructor PlTerm_var().

### 2.4.2 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 2.9.3.

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

  PlTerm_atom
    Subclass of `PlTerm` with constructors for building a term that contains an atom.
  PlTerm_var
    Subclass of `PlTerm` with constructors for building a term that contains an uninstantiated variable. Typically this term is then unified with another object.
  PlTerm_term_t
    Subclass of `PlTerm` with constructors for building a term from a C `term_t`.
  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
    Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `int64_t`.
  PlTerm_uint64
    Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `uint64_t`.
  PlTerm_size_t
    Subclass of `PlTerm` with constructors for building a term that contains a Prolog integer from a `size_t`.
  PlTerm_float
    Subclass of `PlTerm` with constructors for building a term that contains a Prolog float.
  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.
  PlTerm_string
    Subclass of `PlTerm` with constructors for building a term that contains a Prolog string object.
  PlTerm_list_codes
    Subclass of `PlTerm` with constructors for building Prolog lists of character integer values.
  PlTerm_chars
    Subclass of `PlTerm` with constructors for building Prolog lists of one-character atoms (as atom_chars/2).
  PlTerm_tail
    SubClass of `PlTerm` for building and analysing Prolog lists.

  Additional subclasses of `PlTerm` are:

  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.
  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 (`atom_t`) in their internal Prolog representation for fast comparison. (For more details on `atom_t`, see Interface Data Types).
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.
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.
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().

### 2.4.3 Naming conventions, utility functions and methods (version 2)

See also section 2.4.1.

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`) all contain a field `C_` that contains the wrapped value (`functor_t`, `atom_t`, `term_t` respectively).

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

- default constructor (sets the wrapped value to `null`)
- 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`: `Plcheck(PL_put_atom(t.C_,a.C_))`.
- `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
- `verify()` - if the wrapped value (`C_`) is `null`, throw a PlFail() exception. Typically, this check is done after an allocation function such as Plnew_term_ref() returns a null value, so the PlFail() is turned into a a resource error. However, if there is no pending exception, this results in simple failure (see section 2.18.2).
- The `bool` operator is turned off - you should use not_null() instead.10The reason: a `bool` conversion causes ambiguity with `PlAtom(PlTterm)` and `PlAtom(atom_t)`.

The `C_` field 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.C_` 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;

      PlCheck(PL_scan_options(options, 0, "mypred_options", mypred_options,
                              &quoted, &length, &callback.C_));
      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`, PlCheck() is a convenience function that checks the return code and throws a `PlFail` exception on failure. The 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. Where possible, the "getters" have the same name as the underlying type; but this isn't possible for types such as `int` or `float`, so for these the name is prepended with "as_".

"Getters" for integers have an additionnal problem, in that C++ doesn't define the sizes of `int` and `long`, nor for `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++, 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.

## 2.5 Examples (version 2)

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

### 2.5.1 Hello(World) (version 2)

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;11The 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. 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

### 2.5.2 Adding numbers (version 2)

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());
    }

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) ?

### 2.5.3 Average of solutions (version 2)

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 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.

## 2.6 Rational for changes from version 1 (version 2)

### 2.6.1 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 " << A1.as_string() << endl;
      return true;
    }

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

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 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 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");

- 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 ctypebool. 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 PlCheck() to raise an exception on unification failure.

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, some 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.

### 2.6.2 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. The buffer stack is abstracted into `PlStringBuffers`, 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.12As of 2022-11, this had only been partially implemented. Some of the details for this - such as the default encoding - may change slightly in the future.

## 2.7 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.

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 " << A1.as_string().c_str()() << 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 PlCheck(rc). Note that throwing an exception is significantly slower than returning `false`, so performance-critical code should avoid PlCheck(rc).

- You can use the PlCheck(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.

- `PlAtom::handle` has been replaced by `PlAtom::C_`.

- `PlTerm::ref` has been replaced by `PlAtom::C_`.

- `PlFunctor::functor` has been replaced by `PlAtom::C_`.

- The operator ``=`` for unification has been deprecated, replaced by various `unify_XXX`‘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()`; `static_cast<int32_t>(t)` is replaced by `t.as_int32_t()`.

- 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.

## 2.8 The class PlFail (version 2)

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 2.18.2).

For example, this code:

    PREDICATE(unify_zero, 1)
    { if ( !PL_unify_integer(A1.C_, 0) )
        return false;
      return true;
    }

can instead be written this way:

    void
    PREDICATE(unify_zero, 1)
    { if ( !PL_unify_integer(A1.C_, 0) )
        throw PlFail();
      return true;
    }

or:

    PREDICATE(unify_zero, 1)
    { PlCheck(PL_unify_integer(t.C_, 0));
      return true;
    }

or:

    PREDICATE(unify_zero, 1)
    { PlCheck(A1.unify_integer(0));
      return true;
    }

or:

    PREDICATE(unify_zero, 1)
    { return A1.unify_integer(0);
    }

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.

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);
    }

### 2.8.1 PlCheck() convenience function

In general, wherever there is a method that wraps a C "PL_" function, PlCheck() can be used to return failure to Prolog from the "PL_" function.

The code for PlCheck() is very simple - it checks the return code and throws `PlFail` if the return code isn't "true". If the return code is from a Prolog function (that is, a function starting with "PL_"), the return code can be "false" either because of failure or because an exception happened. If the cause is an exception, then the only sensible thing is to return to Prolog immediately; throwing `PlFail` will do this. See also section 2.18.2.

## 2.9 The class PlTerm (version 2)

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.

### 2.9.1 Constructors (version 2)

The 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 an atom; `PlTerm_string` creates a Prolog string). All of the constructors are "explicit" because implicit creation of `PlTerm` objects can lead to subtle and difficult to debug errors.

PlTerm :: PlTerm()
  Creates a new initialised "null" term (holding a Prolog variable).
PlTerm_term_t :: PlTerm_term_t(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_atom :: PlTerm_atom(const char *text)
  Creates a term-references holding a Prolog atom representing text.
PlTerm_atom :: PlTerm_atom(const wchar_t *text)
  Creates a term-references holding a Prolog atom representing text.
PlTerm_atom :: PlTerm_atom(const PlAtom &atom)
  Creates a term-references holding a Prolog atom from an atom-handle.
PlTerm_int :: PlTerm_int(long n)
  Creates a term-references holding a Prolog integer representing n.
PlTerm_int :: PlTerm_int(int64_t n)
  Creates a term-references holding a Prolog integer representing n (up to 64 bits signed).
PlTerm_int :: PlTerm_int(uint64_t n)
  Creates a term-references holding a Prolog integer representing n (up to 64 bits unsigned).
PlTerm_float :: PlTerm_float(double f)
  Creates a term-references holding a Prolog float representing f.
PlTerm_pointer :: PlTerm_pointer(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 *`. Also note that in general blobs are a better way of doing this (see the section on blobs in the Foreign Language Interface part of the SWI-Prolog manual).

      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.pointer());
        return A2.unify_string(myobj->contents);
      }

      PREDICATE(free_my_object, 1)
      { auto myobj = static_cast<MyClass*>(A1.pointer());

        delete myobj;
        return true;
      }

### 2.9.2 Overview of accessing and changing values (version 2)

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.

### 2.9.3 Converting PlTerm to native C and C++ types (version 2)

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 type()
  See PL_term_type()
bool is_variable()
  See PL_is_variable()
bool is_ground()
  See PL_is_ground()
bool is_atom(S)
  ee PL_is_atom()
bool is_integer(S)
  ee PL_is_integer()
bool is_string(S)
  ee PL_is_string()
bool is_float(S)
  ee PL_is_float()
bool is_rational(S)
  ee PL_is_rational()
bool is_compound(S)
  ee PL_is_compound()
bool is_callable(S)
  ee PL_is_callable()
bool is_list(S)
  ee PL_is_list()
bool is_dict(S)
  ee PL_is_dict()
bool is_pair(S)
  ee PL_is_pair()
bool is_atomic(S)
  ee PL_is_atomic()
bool is_number(S)
  ee PL_is_number()
bool is_acyclic(S)
  ee PL_is_acyclic()
bool is_functor(PlFunctor)
  See PL_is_functor()

### 2.9.4 Unification (version 2)

See also section 2.12.

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(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 PlCheck() to raise an exception if the unification fails.

    PREDICATE(hostname2, 1)
    { char buf[256];
      PlCheck(gethostname(buf, sizeof buf) == 0);
      PlCheck(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`.

### 2.9.5 Comparison (version 2)

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 2.11.2 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''.

### 2.9.6 Analysing compound terms (version 2)

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.

### 2.9.7 Miscellaneous (version 2)

bool is_null()
  `t.is_null()` is the same as `t.C_ == PlTerm::null`
bool not_null()
  `t.not_null()` is the same as `t.C_ != PlTerm::null`
bool reset()
  `t.reset()` is the same as `t.C_ = PlTerm::null`
bool reset(term_t)
  `t.reset(x)` is the same as `t.C_ = 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`

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.

### 2.9.8 The class PlTermString (version 2)

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)`).

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.

### 2.9.9 The class PlCodeList (version 2)

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

### 2.9.10 The class PlCharList (version 2)

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.

### 2.9.11 The class PlCompound (version 2)

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"))

### 2.9.12 The class PlTail (version 2)

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++)
          PlCheck(l.append(argv[i]));
        PlCheck(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 &)
  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 << e.as_string() << endl;

        return true;
      }

## 2.10 The class PlTermv (version 2)

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";
        ...
      }

## 2.11 The class PlAtom - Supporting Prolog constants (version 2)

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:

### 2.11.1 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.

### 2.11.2 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.

### 2.11.3 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 2.11.2, but in nested if-then-else the extraction of the atom from the term is done only once.

### 2.11.4 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.13If 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().

## 2.12 Unification and foreign frames (version 2)

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(), 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:

    { PlFrame frame;
      ...
      if ( !t1.unify_term(t2) )
        frame.rewind();
      ...
    }

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

## 2.13 The class PlRegister (version 2)

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.

## 2.14 The class PlQuery (version 2)

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.
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 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.

### 2.14.1 The class PlFrame (version 2)

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);
      PlCheck(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;
    }

PlRewindOnFail(f) is a convenience function that does a frame rewind if unification fails. Here is an example, where `name_to_term` contains a map from names to terms (which are made global by using the PL_record() function):

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

    bool lookup_term(const std::string name, PlTerm result)
    { const auto it = name_to_term.find(name);
      if ( it == name_to_term.cend() )
        return false;

      PlTerm t = PlTerm_recorded(it->second);
      return PlRewindOnFail([result,t]() -> bool { return result.unify_term(t); });
    }

## 2.15 The PREDICATE and PREDICATE_NONDET macros (version 2)

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:14There 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.

### 2.15.1 Variations of the PREDICATE macro (version 2)

The PREDICATE() macros have 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;
          }

PREDICATE_NONDET(name, arity)
  Define a non-deterministic Prolog predicate in C++. See also section 2.15.2.
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 2.15.2.

### 2.15.2 Non-deterministic predicates (version 2)

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 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. To simplify this, a template helper class `PlForeignContextPtr<ContextType>` provides a "smart pointer" that frees the context on normal return or an exception; if PlForeignContextPtr<ContextType>::keep() is called, the pointer isn't freed on return or exception.

The skeleton for a typical non-deterministic predicate is:

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

    PREDICATE_NONDET(pred, <arity>)
    { PlForeignContextPtr<PredContext> ctxt(handle);
      switch( PL_foreign_control(handle) )
      { case PL_FIRST_CALL:
          ctxt.set(new PredContext(...));
          ...
          break;
        case PL_REDO:
          break;
        case PL_PRUNED:
          return true;
      }

      if ( ... )
        return false; // Failure (and no more solutions)
        // or throw PlFail();

      if ( ... )
        return true;  // Success (and no more solutions)

      ...

      ctxt.keep();
      PL_retry_address(ctxt.get()); // Succeed with a choice point
    }

### 2.15.3 Controlling the Prolog destination module (version 2)

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

## 2.16 Exceptions (version 2)

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`, `PlTermvDomainError`, `PlInstantiationError`, `PlExistenceError`, `PermissionError`, `PlResourceError`, and `PlException_qid`.

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 2.18.2.

      char *data = "users";

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

### 2.16.1 The class PlException (version 2)

This subclass of `PlTerm` 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().

### 2.16.2 The class PlTypeError (version 2)

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.

### 2.16.3 The class PlDomainError (version 2)

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.

## 2.17 Embedded applications (version 2)

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.15For 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 1.4.11 has a simple example using this class.

## 2.18 Considerations (version 2)

### 2.18.1 The C++ versus the C interface (version 2)

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 `C_` field), 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 PlCheck().

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.

### 2.18.2 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 envoked as part of handling another exception.

### 2.18.3 Static linking and embedding (version 2)

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.

### 2.18.4 Status and compiler versions (version 2)

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.

As of 2022-11, 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 2022-11, 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 calling that would happen from `PL_instantiation_error(A1.C_); throw PlFail()`. See section 2.18.2.

## 2.19 Conclusions (version 2)

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
  1.3.2 2.5.2
arg/3
  1.4.5 2.9.6
as_string()
as_wstring()
assert
  1.8.1 2.14.1
atom_chars/2
  1.2 1.4.9 2.4.2 2.9.10
average/3
  1.3.3 2.5.3
blob_data()
entry/1
  1.4.11 2.9.12
error/2
  2.18.2
fail/0
  2.3
hello/1
  1.3.1 2.5.1
is_acyclic()
is_atom()
is_atomic()
is_callable()
is_compound()
is_dict()
is_float()
is_functor()
is_ground()
is_integer()
is_list()
is_null()
is_number()
is_pair()
is_rational()
is_string()
is_valid()
is_variable()
load_foreign_library/1
  1.9.2 2.15.3
not_null()
open/3
  1.10.3 2.16.3
print_message/2
  1.10.1 2.16.1
read/1
  1.4.10 2.9.11
register_atom()
reset()
throw/1
  2.3
type()
unregister_atom()
use_foreign_library/1
  2.3
word/1
  1.8.1 2.14.1
write/1
  1.3.1 1.4.2 2.5.1 2.9.3
NAMED_PREDICATE()
NAMED_PREDICATE_NONDET()
P
PlAtom
  1.4.3 1.6
PlAtom!=()
PlAtom==()
PlCall()
PlCompound
  1.4.5
PlDomainError
  1.10
PlEngine
  1.11
PlException
  1.2 1.2 1.2 1.2 1.10 1.10 1.10 1.10.1 1.10.1 1.10.1
PlFrame
  1.8.1 1.8.1 1.8.1 1.8.1 1.8.1 1.12.1
PlFrame::rewind()
PlQuery
  1.3.3 1.5 1.8
PlQuery::cut()()
PlQuery::next_solution()
PlRegister
  1.9
PREDICATE0()
PREDICATE_NONDET()
PlTail
  1.4.11 1.4.11 1.4.11 1.4.11 1.4.11 1.4.11 1.4.11 1.4.11 1.4.11 1.4.11
PlTail::append()
PlTail::close()
PlTail::next()
PlTerm
  1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.3.1 1.3.1 1.3.2 1.3.2 1.4 1.4.1 1.4.2 1.4.2 1.4.3 1.4.4 1.4.4 1.4.4 1.4.5 1.4.5 1.4.5 1.4.6 1.10 1.10.1 1.12.1
PlTerm!=()
PlTerm::arity()
PlTerm::compare()
PlTerm::name()
PlTerm::type()
PlTerm::unify_atom()
PlTerm::unify_blob()
PlTerm::unify_chars()
PlTerm::unify_float()
PlTerm::unify_functor()
PlTerm::unify_integer()
PlTerm::unify_list_chars()
PlTerm::unify_list_codes()
PlTerm::unify_nil()
PlTerm::unify_pointer()
PlTerm::unify_string()
PlTerm::unify_term()
PlTerm<()
PlTerm<=()
PlTerm=()
PlTerm==()
PlTerm>()
PlTerm>=()
PlTerm[]()
PlTermv
  1.2 1.4.10 1.5 1.9
PlTypeEror
  1.10 1.10.1
T
cppThrow()
plThrow()