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, SWI-cpp2.cpp, and SWI-cpp2-plx.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 and some of the comparison operators have been removed or deprecated, and replaced by "getter" methods. The overloaded constructors have been replaced by subclasses for the various types. Some changes were also made to ensure that the [] operator for PlTerm and PlTermv doesn't cause unexpected implicit conversions. ^{2If there is an implicit conversion operator from PlTerm to term_t and also to char*, then the [] operator is ambiguous if f is overloaded to accept a term_t or char* in the code PlTerm t=...; f(t[0])}

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

More specifically:

• SWI-cpp2.cpp has been added, containing the implementation of some functions that are too long to inline. The user must either #include <SWI-cpp2.cpp> or compile it separately and link it with the other foreign function code.
• The constructor PlTerm() is restricted to a few unambiguous cases - instead, you should use the appropriate subclass' constructors (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)).
• Wrapper functions have been provided for almost all the PL_*() functions in SWI-Prolog.h, and have the same names with the “PL'' replaced by “Plx''.^{3 “Pl'' is used throughout the SWI-cpp2.h interface, and the “x'' is for “eXtended with eXception handling.''} Where appropriate, these check return codes and throw a C++ exception (created from the Prolog error). See section 2.4.4 Many of these wrapper functions have been added to the PlAtom and PlTerm classes, with the arguments changed from atom_t and term_t to PlAtom and PlTerm. These wrappers are available if you include SWI-cpp2.h (they are in a separate SWI-cpp2-plx.h file for ease of maintenance).
• Instead of returning false from a foreign predicate to indicate failure, you can use throw PlFail(). The convenience function PlCheckFail(rc) can be used to throw PlFail() if false is returned from a function in SWI-Prolog.h. If the wrapper functions or class methods are used, Prolog errors result in a C++ PlException exception.^{4If a “Plx_'' wrapper is used to call a SWI-Prolog.h function, a Prolog error will have already resulted in throwing PlException;‘cfuncrefPlCheckFailrc is used to additionally throw PlFail, similar to returning false from the top-level of a foreign predicate.}
• The PlException class is a subclass of std::excxeption and encapsulates a Prolog error. Prolog errors are converted into throw PlException(...). If the user code does not catch the PlException, the PREDICATE() macro converts the error to a Prolog error upon return to the Prolog caller.
• The C++ constructors, functions, and methods use the wrapper functions to a C++ exception on error (and the C++ exception is converted to a Prolog exception when control returns to Prolog).
• 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()).^{5The 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 PlCheckFail().
• Many of the equality and inequality operators are deprecated; replaced by the as_string() method and the associated std::string, comparison operators. The as_string() method allows specifying the encoding to use whereas the == and similar operators do not allow for this.
• Methods that return char* have been replaced by methods that return std::string to ensure that lifetime issues don't cause subtle bugs.^{6If you want to return a char* from a function, you should not do return t.as_string().c_str() because that will return a pointer to local or stack memory. Instead, you should change your interface to return a std::string and apply the c_str() method to it. These lifetime errors can sometimes be caught by specifying the Gnu C++ or Clang options -Wreturn-stack-address or -Wreturn-local-addr - as of 2023-04, Clang seems to do a better analysis.}
• Most constructors, methods, and functions that accept char* arguments also accept std::string or std::wstring arguments. Where possible, encoding information can also be specified.
• 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, and PL_...(atom_t, ...) methods have been added to PlAtom. Where appopriate, the arguments use PlTerm, PlAtom, etc. instead of term_t, atom_t, etc.
• 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_.^{7This 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().
• Wrapper classes for record_t have been added. The PlRecordExternalCopy class contains the opaque handle, as a convenience.
• Wrapper class for control_t has been added and the PREDICATE_NONDET() has been modified to use it.

More details 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 easily translated into appropriate Prolog types, automatic destructors can be used to deal with most of the cleanup required and C++ exception handling can be used to map Prolog exceptions and interface conversion errors to C++ exceptions, which are automatically mapped to Prolog exceptions as control is turned back to Prolog.

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

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

It would be tempting to use C++ implicit conversion operators and method overloading to automatically convert between C++ types such as std::string and int64_t and Prolog foreign language interface types such as term_t and atom_t. However, types such as term_t are unsigned integers, so many of the automatic type conversions can 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, pPlus 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.^{8You 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 PlControl).

The foreign predicate returns a value of true or false to indicate whether it succeeded or failed.^{9Non-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 predicate) or an error (the equivalent of calling the throw/1 predicate). When a Prolog exception is raised, it is important that a return be made to the calling environment as soon as possible. In C code, this requires checking every call for failure, which can become cumbersome. C++ has exceptions, so instead the code can wrap calls to PL_*() functions with PlCheck_PL() or PlCheckEx(), which will throw a PlException() 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)
{ PlCheckFail(A1.unify_term(A2));
  return true;
}

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

PREDICATE(eq, 2)
{ return PlWrap<int>(PL_unify(A1.C_, A2.C_));
}

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 2.4.5.

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 2.17.)

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). Normally, you shouldn't need to check whether the object is "fully created", but if you do, you can use the methods is_null() or not_null().

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

PlAtom foo("foo");

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

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).

C code sometimes creates objects "lazily" on first use:

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

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

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

The class PlTerm (which wraps term_t) is the most used. Although a PlTerm object can be created from a term_t value, it is intended to be used with a constructor that gives it an initial value. The default constructor calls PL_new_term_ref() and throws an exception if this fails. The various constructors are described in section 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 files

The following files are provided:

• SWI-cpp2.h Include this file to get the C++ API. It automatically includes SWI-cpp2-plx.h but does not include SWI-cpp2.cpp.

• SWI-cpp2.cpp Contains the implementations of some methods and functions. It must be compiled as-is or included in the foreign predicate's source file. Alternatively, it can be included with each include of SWI-cpp2.h with this macro definition:

      #define _SWI_CPP2_CPP_inline inline
      

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

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

2.4.3 Summary of classes

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

PlTerm

Generic Prolog term that wraps term_t (for more details on term_t, see Interface Data Types). This is a "base class" whose constructor is protected; subclasses specify the actual contents. Additional methods allow checking the Prolog type, unification, comparison, conversion to native C++-data types, etc. See section 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.^{10PL_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 std::exception, representing a Prolog exception. Provides methods for the Prolog communication and mapping to human-readable text representation.

PlTerm PlTypeError()

Creates a PlException object for representing a Prolog type_error exception.

PlTerm PlDomainError()

Creates a PlException object for representing a Prolog domain_error exception.

PlTerm PlExistenceError()

Creates a PlException object for representing a Prolog existence_error exception.

PlTerm PlPermissionError()

Creates a PlExceptionobject 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. An error can be signaled by calling Plx_raise_exception() or one of the PL_*_error() functions and then throwing PlFail; but it's better style to create the error throwing one of the subclasses of PlException e.g., throw PlTypeError("int", t).

PlException

If a call to Prolog results in an error, the C++ interface converts the error into a PlException object and throws it. If the enclosing code doesn't intercept the exception, the PlException object is turned back into a Prolog error.

PlExceptionFail

In some situations, a Prolog error cannot be turned into a PlException object, so a PlExceptionFail object is thrown. This is turned into failure by the PREDICATE() macro, resulting in normal Prolog error handling.

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().

2.4.4 Wrapper functions

The various PL_*() functions in SWI-Prolog.h have corresponding Plx_*() functions. There are three kinds of wrappers:

• "as-is" - the PL_*() function cannot cause an error. If it has a return value, the caller will want to use it. (These are defined using the PLX_ASIS() and PLX_VOID() macros.)

• "exception wrapper" - the PL_*() function can return false, indicating an error. The Plx*() function checks for this and throws a PlException object containing the error. The wrapper uses template<typename C_t> C_t PlExce(C_t rc), where C_t is the return type of the PL_*() function. (These are defined using the PLX_WRAP() macro.)

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

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

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

2.4.5 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), etc. 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(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
• The bool operator is turned off - you should use not_null() instead.^{11The 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_L(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_PL() is a convenience function that checks the return code and throws a PlFail exception on failure or PlException if there was an exception. 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.

2.4.6 Limitations of the interface

The C++ API remains a work in progress.

2.4.6.1 Strings

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

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

However, there is no native "string" type in C++; the char* strings can be automatically cast to string. If a C++ interface provides only std::string arguments or return values, that can introduce some inefficiency; therefore, many of the functions and constructors allow either a char* or std::string as a value (also wchar_t* or std::wstring.

For return values, char* is dangerous because it can point to local or stack memory. For this reason, wherever possible, the C++ API returns a std::string, which contains a copy of the the string. This can be slightly less efficient that returning a char*, but it avoids some subtle and pervasive bugs that even address sanitizers can't detect.^{12If we wish to minimize the overhead of passing strings, this can be done by passing in a pointer to a string rather than returning a string value; but this is more cumbersome and modern compilers can often optimize the code to avoid copying the return value.}

Many of the classes have a as_string() method - this might be changed in future to to_string(), to be consistent with std::to_string(). However, the method names such as as_int32_t() were chosen istntead of to_int32_t() because they imply that the representation is already an int32_t, and not that the value is converted to a int32_t. That is, if the value is a float, int32_t will fail with an error rather than (for example) truncating the floating point value to fit into a 32-bit integer.

2.4.6.2 Object handles

Many of the "opaque object handles", such as atom_t, term_t, and functor_t are integers.^{13Typically uintptr_t values, which the C standard defines as “an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer.''} As such, there is no compile-time detection of passing the wrong handle to a function.

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

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

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

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

2.4.7 Linking embedded applications using swipl-ld

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

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

To avoid incompatibilities amongst the various C++ compilers' ABIs, the object file from compiling SWI-cpp2.cpp is not included in the shared object libswipl; instead, it must be compiled along with any foreign predicate files. You can do this in three ways:

• Compile SWI-cpp2.cpp separately.
• Add #include SWI-cpp2.cpp to one of the foreign predicate files.
• Wherever you have #include SWI-cpp2.h%, add

        #define _SWI_CPP2_CPP_inline inline
        #include <SWI-cpp2.cpp>
    

  This will cause the compiler to attempt to inline all the functions and methods, even those that are rarely used, resulting in some code bloat.

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;^{14The 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 PlCheckFail() 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.^{15As of 2023-04, this had only been partially implemented}. Some of the details for this - such as the default encoding - may change slightly in the future.

2.7 Porting from version 1 to version 2

SWI-cpp2.h is not complete; it needs‘fileSWI-cpp2.cpp to implement some functions. The easiest way of taking care of this is to add #include <SWI-cpp2.cpp> in your "main" file; alternatively, you can create another source file that contains the "include" statement.

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

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

Here is a list of typical changes:

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

• Examine uses of char* or wchar_t and replace them by std::string or std::wstring if appropriate. For example, cout << "Hello " << 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 PlCheckFail(rc). Note that throwing an exception is significantly slower than returning false, so performance-critical code should avoid PlCheckFail(rc).

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

• PlAtom::handle has been replaced by PlAtom::C_.

• 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 2.19.2).

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(PL_unify_integer(t.C_, 0));
  return true;
}

or:

PREDICATE(unify_zero, 1)
{ PlCheckFail(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 PlCheckFail(), PlCheckEx(), and PlCheck_PL() convenience functions

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

The code for PlCheck() is just

void PlCheck(int rc)
{ if ( !PlCheckEx(rc) ) throw PlFail(); }

PlCheckEx() calls PL_exception() to see if there is a Prolog exception; if so, the Prolog exception is converted to a PlException object, which is then thrown. For more details on the C++ exceptions, see section 2.17.

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 2.13.

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

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

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

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++)
    PlCheckFail(l.append(argv[i]));
  PlCheckFail(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.^{16If 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 Classes for the recorded database: PlRecord and PlRecordExternalCopy

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

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

The class PlRecord supports the following methods:

PlRecord(PlTerm)

Constructor.

PlRecord(PlRecord)

Copy and move constructors. Currently these do not do any reference counting. The assignment operator is currently not supported.

 PlRecord()

Destructor. Currently this does not call PL_erase().

PlTerm term()

- creates a term from the record, using PL_recorded().

void erase()

- decrements the reference count of the record and deletes it if the count goes to zero, using PL_erase(). It is safe to do this multiple times on the same PlRecord object.

PlRecord duplicate()

- increments the reference count of the record, using PL_duplicate_record().

The class PlRecord provides direct access to the reference counting aspects of the recorded term (through the duplicate() and erase() methods), but does not connect these with C++'s copy constructor, assignment operator, or destructor. If the recorded term is encapsulated within an object, then the containing object can use the duplicate() and erase() methods in its copy and move constructors and assignment operators (and the erase() method in the destructor).^{17The copy constructor and assignment use the duplicate() method; the move constructor and assignment use the duplicate() method to assign to the destination and the erase() method on the source; and the destructor uses erase().}

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

For example:

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

The class PlRecordExternalCopy keeps the external record as an uninterpreted string. It supports the following methods.

PlRecordExternalCopy()

Constructor. Creates a string using Pl_record_external(), copies it into the object, then deletes the reference using PL_erase_external().

PlTerm term()

- creates a term from the record, using PL_recorded_external()).

2.13 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_).

2.14 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.

2.15 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.

2.15.1 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);
  PlCheckFail(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); });
}

2.16 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:^{18There 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.

2.16.1 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 2.16.2.

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 2.16.2.

2.16.2 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
}

2.16.3 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

2.17 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 2.19.2.

  char *data = "users";

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

2.17.1 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().

2.17.2 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.

2.17.3 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.

2.18 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.^{19For 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.

2.19 Considerations (version 2)

2.19.1 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 PlCheckFail() or PlCheck_PL().

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

Global terms and atoms need to be handled slightly differently in C++ than in C - see section 2.19.3

2.19.2 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.

2.19.3 Global terms, atoms, and functors

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

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

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

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

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

2.19.4 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.

2.19.5 Status and compiler versions (version 2)

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

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

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

As of 2023-04, the various error convenience classes do not fully match what the equivalent C functions do. That is, throw PlInstantiationError(A1) does not result in the same context and traceback information that would happen from Plx_instantiation_error(A1.C_); throw PlFail(). See section 2.19.2.

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

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

2.20 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 2.15.1

atom_chars/2

1.2 1.4.9 2.4.3 2.9.10

average/3

1.3.3 2.5.3

blob_data()

duplicate()

entry/1

1.4.11 2.9.12

erase()

error/2

2.19.2

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 2.16.3

not_null()

open/3

1.10.3 2.17.3

print_message/2

1.10.1 2.17.1

read/1

1.4.10 2.9.11

register_atom()

reset()

term()

throw/1

2.3

type()

unregister_atom()

use_foreign_library/1

2.3

word/1

1.8.1 2.15.1

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

PlDomainError()

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

PlExistenceError()

PlFrame

1.8.1 1.8.1 1.8.1 1.8.1 1.8.1 1.12.1

PlFrame::rewind()

PlPermissionError()

PlQuery

1.3.3 1.5 1.8

PlQuery::cut()()

PlQuery::next_solution()

PlRecord()

PlRegister

1.9

 PlRecord()

PlRecordExternalCopy()

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

PlTypeError()

T

cppThrow()

plThrow()