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\renewcommand{\runningtitle}{A C++ interface to SWI-Prolog}

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\begin{document}

\title{A C++ interface to SWI-Prolog}
\author{Jan Wielemaker \& Peter Ludemann \\
	SWI-Prolog Solutions b.v. \\
	E-mail: \email{jan@swi-prolog.org}}

\maketitle

\begin{abstract}
This document describes a C++ interface to SWI-Prolog. SWI-Prolog could
be used with C++ for a very long time, but only by calling the extern
"C" functions of the C-interface. The interface described here
provides a true C++ layer around the C-interface for much more concise
and natural programming from C++. The interface deals with automatic
type-conversion to and from native C data-types, transparent mapping of
exceptions, making queries to Prolog and registering foreign predicates.

At this moment there are \textbf{two} versions of the C++ interface.

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

\end{abstract}

\vfill

\vfill
\vfill

\newpage

\tableofcontents

\newpage

\chapter{A C++ interface to SWI-Prolog (Version 1)}
\label{sec:cpp}

\section{Introduction}
\label{sec:cpp-intro}

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 (\jargon{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.

\subsection*{Competing interfaces}

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


\subsection*{Acknowledgements}

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


\section{Overview}
\label{sec:cpp-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 \type{term_t}. In C++, \type{term_t}
is embedded in the \jargon{lightweight} class \class{PlTerm}.
Constructors and operator definitions provide flexible operations and
integration with important C-types (\type{char *}, \type{wchar_t*},
\type{long} and \type{double}).

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

\begin{description}
    \classitem{PlTerm}
Generic Prolog term.  Provides constructors and operators for conversion
to native C-data and type-checking.
    \classitem{PlString}
Subclass of \class{PlTerm} with constructors for building
Prolog string objects.
    \classitem{PlCodeList}
Subclass of \class{PlTerm} with constructors for building
Prolog lists of ASCII values.
    \classitem{PlCharList}
Subclass of \class{PlTerm} with constructors for building
Prolog lists of one-character atoms (as \index{atom_chars/2}\predref{atom_chars}{2}).
    \classitem{PlCompound}
Subclass of \class{PlTerm} with constructors for building compound
terms.
    \classitem{PlTail}
SubClass of \class{PlTerm} for building and analysing Prolog lists.
    \classitem{PlTermv}
Vector of Prolog terms. See \cfuncref{PL_new_term_refs}{}. the \const{\Snil} operator
is overloaded to access elements in this vector.  \class{PlTermv} is used
to build complex terms and provide argument-lists to Prolog goals.
    \classitem{PlException}
Subclass of \class{PlTerm} representing a Prolog exception.  Provides
methods for the Prolog communication and mapping to human-readable text
representation.
    \classitem{PlTypeError}
Subclass of \class{PlException} for representing a Prolog
\except{type_error} exception.
    \classitem{PlDomainError}
Subclass of \class{PlException} for representing a Prolog
\except{domain_error} exception.
    \classitem{PlExistenceError}
Subclass of \class{PlException} for representing a Prolog
\except{existence_error} exception.
    \classitem{PlPermissionError}
Subclass of \class{PlException} for representing a Prolog
\except{permission_error} exception.
    \classitem{PlAtom}
Allow for manipulating atoms in their internal Prolog representation
for fast comparison.
    \classitem{PlQuery}
Represents opening and enumerating the solutions to a Prolog query.
    \classitem{PlFrame}
This utility-class can be used to discard unused term-references as well
as to do `\jargon{data-backtracking}'.
    \classitem{PlEngine}
This class is used in \jargon{embedded} applications (applications
where the main control is held in C++).  It provides creation and
destruction of the Prolog environment.
    \classitem{PlRegister}
The encapsulation of \cfuncref{PL_register_foreign}{} is defined to be able to
use C++ global constructors for registering foreign predicates.
\end{description}

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


\section{Examples}
\label{sec:cpp-examples}

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


\subsection{Hello(World)}
\label{sec:cpp-hello-world}

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

\begin{code}
PREDICATE(hello, 1)
{ cout << "Hello " << (char *)A1 << endl;

  return TRUE;
}
\end{code}

\noindent
The arguments to PREDICATE() are the name and arity of the predicate.
The macros A\bnfmeta{n} provide access to the predicate arguments by position
and are of the type \class{PlTerm}. Casting a \class{PlTerm} to a
\type{char *} or \type{wchar_t *} provides the natural type-conversion
for most Prolog data-types, using the output of \index{write/1}\predref{write}{1} otherwise:

\begin{code}
?- hello(world).
Hello world

Yes
?- hello(X)
Hello _G170

X = _G170
\end{code}

\noindent
\subsection{Adding numbers}
\label{sec:cpp-ex-adding-numbers}

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

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

\noindent
Casting a \class{PlTerm} to a \type{long} performs a \cfuncref{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 \type{long}. The
\const{\Seq} operator of \class{PlTerm} is defined to perform unification
and returns \const{TRUE} or \const{FALSE} depending on the result.

\begin{code}
?- add(1, 2, X).

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

\noindent
\subsection{Average of solutions}
\label{sec:cpp-ex-average}

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

\class{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
\const{TRUE} if there was a solution and \const{FALSE} otherwise. If
the goal yielded a Prolog exception it is mapped into a C++ exception.


\begin{code}
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;
}
\end{code}

\noindent
\section{The class PlTerm}
\label{sec:cpp-plterm}

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

\subsection{Constructors}
\label{sec:cpp-plterm-constructurs}

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

\begin{code}
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;
}
\end{code}

\noindent
\end{description}


\subsection{Casting PlTerm to native C-types}
\label{sec:cpp-plterm-casting}

\ctype{PlTerm} can be cast to the following types:

\begin{description}
    \cppcast{PlTerm}{term_t}
This cast is used for integration with the C-interface primitives.
    \cppcast{PlTerm}{long}
Yields a \type{long} if the \class{PlTerm} is a Prolog integer or
float that can be converted without loss to a long.  throws a
\except{type_error} exception otherwise.
    \cppcast{PlTerm}{int}
Same as for \type{long}, but might represent fewer bits.
    \cppcast{PlTerm}{double}
Yields the value as a C double if \class{PlTerm} represents a
Prolog integer or float.
    \cppcast{PlTerm}{wchar_t *}
    \nodescription
    \cppcast{PlTerm}{char *}
Converts the Prolog argument using \cfuncref{PL_get_chars}{} using the flags
\const{CVT_ALL|CVT_WRITE|BUF_RING}, which implies Prolog atoms and
strings are converted to the represented text.  All other data is
handed to \index{write/1}\predref{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.
    \cppcast{PlTerm}{void *}
Extracts pointer value from a term. The term should have been created
by PlTerm::PlTerm(void*).
\end{description}

\subsection{Unification}
\label{sec:cpp-plterm-unification}

\begin{description}
    \cfunction{int}{PlTerm::operator =}{Type}
The operator \const{\Seq} is defined for the \arg{Types} \class{PlTerm},
\type{long}, \type{double}, \type{char *}, \type{wchar_t*} and
\class{PlAtom}. It performs Prolog unification and returns \const{TRUE}
if successful and \const{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.

\begin{code}
PREDICATE(hostname, 1)
{ char buf[32];

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

  return FALSE;
}
\end{code}

\noindent
\end{description}


\subsection{Comparison}
\label{sec:cpp-plterm-comparison}


\begin{description}
    \cfunction{int}{PlTerm::operator ==}{const PlTerm \&t}
    \nodescription
    \cfunction{int}{PlTerm::operator !=}{const PlTerm \&t}
    \nodescription
    \cfunction{int}{PlTerm::operator $<$}{const PlTerm \&t}
    \nodescription
    \cfunction{int}{PlTerm::operator $>$}{const PlTerm \&t}
    \nodescription
    \cfunction{int}{PlTerm::operator $<=$}{const PlTerm \&t}
    \nodescription
    \cfunction{int}{PlTerm::operator $>=$}{const PlTerm \&t}
Compare the instance with \arg{t} and return the result according to
the Prolog defined \jargon{standard order of terms}.
    \cfunction{int}{PlTerm::operator ==}{long num}
    \nodescription
    \cfunction{int}{PlTerm::operator !=}{long num}
    \nodescription
    \cfunction{int}{PlTerm::operator $<$}{long num}
    \nodescription
    \cfunction{int}{PlTerm::operator $>$}{long num}
    \nodescription
    \cfunction{int}{PlTerm::operator $<=$}{long num}
    \nodescription
    \cfunction{int}{PlTerm::operator $>=$}{long num}
Convert \class{PlTerm} to a \type{long} and perform standard
C-comparison between the two long integers. If \class{PlTerm} cannot be
converted a \except{type_error} is raised.

    \cfunction{int}{PlTerm::operator ==}{const wchar_t *}
    \nodescription
    \cfunction{int}{PlTerm::operator ==}{const char *}
Yields \const{TRUE} if the \class{PlTerm} is an atom or string
representing the same text as the argument, \const{FALSE} if the
conversion was successful, but the strings are not equal and an
\except{type_error} exception if the conversion failed.
\end{description}

Below are some typical examples.  See \secref{cpp-dirplatom} for direct
manipulation of atoms in their internal representation.

\begin{center}
\begin{tabularlp}{\tt A1 == PlCompound("a(1)")}
\hline
\tt A1 $<$ 0	& Test \arg{A1} to hold a Prolog integer or float
		  that can be transformed lossless to an integer
		  less than zero. \\
\tt A1 $<$ PlTerm(0) &
		  \arg{A1} is before the term `0' in the `standard
		  order of terms'. This means that if \arg{A1}
		  represents an atom, this test yields \const{TRUE}. \\
\tt A1 == PlCompound("a(1)") &
		  Test \arg{A1} to represent the term
		  \exam{a(1)}. \\
\tt A1 == "now" &
		  Test \arg{A1} to be an atom or string holding the
		  text ``now''. \\
\hline
\end{tabularlp}
\end{center}


\subsection{Analysing compound terms}
\label{sec:cpp-plterm-compound}

Compound terms can be viewed as an array of terms with a name and arity
(length). This view is expressed by overloading the \const{\Snil} operator.

A \except{type_error} is raised if the argument is not compound and a
\except{domain_error} if the index is out of range.

In addition, the following functions are defined:

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

\begin{code}
   ...,
   if ( x[1] == "gnat" )
     ...
\end{code}

\noindent
    \cfunction{const char *}{PlTerm::name}{}
Return a \type{const char *} holding the name of the functor of the
compound term.  Raises a \except{type_error} if the argument is not
compound.
    \cfunction{int}{PlTerm::arity}{}
Returns the arity of the compound term. Raises a \except{type_error} if
the argument is not compound.
\end{description}

\subsection{Miscellaneous}
\label{sec:cpp-plterm-misc}

\begin{description}
    \cfunction{int}{PlTerm::type}{}
Yields the actual type of the term as \cfuncref{PL_term_type}{}. Return values are
\const{PL_VARIABLE}, \const{PL_FLOAT}, \const{PL_INTEGER},
\const{PL_ATOM}, \const{PL_STRING} or \const{PL_TERM}
\end{description}

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

\subsection{The class PlString}
\label{sec:cpp-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.


\begin{description}
    \constructor{PlString}{const wchar_t *text}
    \nodescription
    \constructor{PlString}{const char *text}
Create a SWI-Prolog string object from a 0-terminated C-string.  The
\arg{text} is copied.

    \constructor{PlString}{const wchar_t *text, size_t len}
    \nodescription
    \constructor{PlString}{const char *text, size_t len}
Create a SWI-Prolog string object from a C-string with specified length.
The \arg{text} may contain 0-characters and is copied.
\end{description}

\subsection{The class PlCodeList}
\label{sec:cpp-codelist}

\begin{description}
    \constructor{PlCodeList}{const wchar_t *text}
    \nodescription
    \constructor{PlCodeList}{const char *text}
Create a Prolog list of ASCII codes from a 0-terminated C-string.
\end{description}


\subsection{The class PlCharList}
\label{sec:cpp-plcharlist}

Character lists are compliant to Prolog's \index{atom_chars/2}\predref{atom_chars}{2} predicate.

\begin{description}
    \constructor{PlCharList}{const wchar_t *text}
    \nodescription
    \constructor{PlCharList}{const char *text}
Create a Prolog list of one-character atoms from a 0-terminated
C-string.
\end{description}


\subsection{The class PlCompound}
\label{sec:cpp-plcompound}

\begin{description}
    \constructor{PlCompound}{const wchar_t *text}
    \nodescription
    \constructor{PlCompound}{const char *text}
Create a term by parsing (as \index{read/1}\predref{read}{1}) the \arg{text}.  If the \arg{text}
is not valid Prolog syntax, a \except{syntax_error} exception is raised.
Otherwise a new term-reference holding the parsed text is created.

    \constructor{PlCompound}{const wchar_t *functor, PlTermv args}
    \nodescription
    \constructor{PlCompound}{const char *functor, PlTermv args}
Create a compound term with the given name from the given vector of
arguments.  See \class{PlTermv} for details.  The example below
creates the Prolog term \exam{hello(world)}.

\begin{code}
PlCompound("hello", PlTermv("world"))
\end{code}

\noindent
\end{description}


\subsection{The class PlTail}		\label{sec:pltail}

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

\begin{description}
    \constructor{PlTail}{PlTerm list}
A \class{PlTail} is created by making a new term-reference pointing to
the same object.  As \class{PlTail} is used to enumerate or build a
Prolog list, the initial \arg{list} term-reference keeps pointing to
the head of the list.
    \cfunction{int}{PlTail::append}{const PlTerm \&element}
Appends \arg{element} to the list and make the \class{PlTail} reference
point to the new variable tail.  If \arg{A} is a variable, and this
function is called on it using the argument \exam{"gnat"}, a list of
the form \exam{[gnat|B]} is created and the \class{PlTail} object
now points to the new variable \arg{B}.

This function returns \const{TRUE} if the unification succeeded and
\const{FALSE} otherwise.  No exceptions are generated.

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

\begin{code}
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;
}
\end{code}

\noindent
    \cfunction{int}{PlTail::close}{}
Unifies the term with \const{\Snil} and returns the result of the
unification.
    \cfunction{int}{PlTail::next}{PlTerm \&t}
Bind \arg{t} to the next element of the list \class{PlTail} and advance
\class{PlTail}. Returns \const{TRUE} on success and \const{FALSE} if
\class{PlTail} represents the empty list. If \class{PlTail} is neither a
list nor the empty list, a \except{type_error} is thrown. The example
below prints the elements of a list.

\begin{code}
PREDICATE(write_list, 1)
{ PlTail tail(A1);
  PlTerm e;

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

  return TRUE;
}
\end{code}

\noindent
\end{description}


\section{The class PlTermv}
\label{sec:cpp-pltermv}

The class \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 \cfuncref{PlTerm::PlTerm}{const char *name,
PlTermv arguments}) and to create queries (see \class{PlQuery}).

The only useful member function is the overloading of \const{\Snil},
providing (0-based) access to the elements.  Range checking is performed
and raises a \except{domain_error} exception.

The constructors for this class are below.

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

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

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

\begin{code}
{ PlTermv av(10);

  av[0] = "hello";
  ...
\end{code}

\noindent
\end{description}


\section{Supporting Prolog constants}
\label{sec:cpp-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:

\subsection*{Direct comparision to char *}

Example:

\begin{code}
PREDICATE(test, 1)
{ if ( A1 == "read" )
    ...;
\end{code}

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

\subsection*{Direct comparision to PlAtom}		\label{sec:cpp-dirplatom}

Example:

\begin{code}
static PlAtom ATOM_read("read");

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

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

\subsection*{Extraction of the atom and comparison to PlAtom}

Example:

\begin{code}
static PlAtom ATOM_read("read");

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

  if ( a1 == ATOM_read )
    ...;
\end{code}

\noindent
This approach is basically the same as \secref{cpp-dirplatom}, but in
nested if-then-else the extraction of the atom from the term is
done only once.

\subsection*{Extraction of the atom and comparison to char *}

Example:

\begin{code}
PREDICATE(test, 1)
{ PlAtom a1(A1);

  if ( a1 == "read" )
    ...;
\end{code}

\noindent
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.

\begin{description}
    \constructor{PlAtom}{atom_t handle}
Create from C-interface atom handle.  Used internally and for
integration with the C-interface.
    \constructor{PlAtom}{const wchar_t *text}
    \nodescription
    \constructor{PlAtom}{const char *text}
Create an atom from a string.  The \arg{text} is copied if a new atom
is created.
    \constructor{PlAtom}{const PlTerm \&t}
If \arg{t} represents an atom, the new instance represents this
atom.  Otherwise a \except{type_error} is thrown.
    \cfunction{int}{PlAtom::operator ==}{const wchar_t *text}
    \nodescription
    \cfunction{int}{PlAtom::operator ==}{const char *text}
Yields \const{TRUE} if the atom represents \arg{text}, \const{FALSE}
otherwise.  Performs a strcmp() for this.
    \cfunction{int}{PlAtom::operator ==}{const PlAtom \&a}
Compares the two atom-handles, returning \const{TRUE} or
\const{FALSE}.
\end{description}


\section{The class PlRegister}
\label{sec:cpp-plregister}

This class encapsulates \cfuncref{PL_register_foreign}{}.  It is defined as a class
rather then a function to exploit the C++ \jargon{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.

\begin{description}
    \constructor{PlRegister}{const char *module,
			     const char *name,
			     int arity,
			     foreign_t (f)(term_t t0, int a, control_t ctx)}
Register \arg{f} as a the implementation of the foreign predicate
\bnfmeta{name}/\bnfmeta{arity}.  This interface uses the \const{PL_FA_VARARGS} calling
convention, where the argument list of the predicate is passed using an
array of \type{term_t} objects as returned by \cfuncref{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.
    \constructor{PlRegister}{const char *module,
			     const char *name,
			     foreign_t (*f)(PlTerm a0, \ldots)}
Registers functions for use with the traditional calling conventional,
where each positional argument to the predicate is passed as an argument
to the function \arg{f}. This can be used to define functions as
predicates similar to what is used in the C-interface:

\begin{code}
static foreign_t
pl_hello(PlTerm a1)
{ ...
}

PlRegister x_hello_1(NULL, "hello", 1, pl_hello);
\end{code}

\noindent
This construct is currently supported upto 3 arguments.
\end{description}


\section{The class PlQuery}
\label{sec:cpp-plquery}

This class encapsulates the call-backs onto Prolog.

\begin{description}
    \constructor{PlQuery}{const char *name, const PlTermv \&av}
Create a query where \arg{name} defines the name of the predicate and
\arg{av} the argument vector.  The arity is deduced from \arg{av}.  The
predicate is located in the Prolog module \module{user}.
    \constructor{PlQuery}{const char *module, const char *name,
			  const PlTermv \&av}
Same, but performs the predicate lookup in the indicated module.
    \cfunction{int}{PlQuery::next_solution}{}
Provide the next solution to the query.  Yields \const{TRUE} if
successful and \const{FALSE} if there are no (more) solutions.
Prolog exceptions are mapped to C++ exceptions.
\end{description}

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

\begin{code}
PREDICATE(list_modules, 0)
{ PlTermv av(1);

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

  return TRUE;
}
\end{code}

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

\begin{description}
    \cfunction{int}{PlCall}{const char *predicate, const PlTermv \&av}
Creates a \class{PlQuery} from the arguments generates the
first next_solution() and destroys the query.  Returns the
result of next_solution() or an exception.
    \cfunction{int}{PlCall}{const char *module, const char *predicate,
			    const PlTermv \&av}
Same, locating the predicate in the named module.
    \cfunction{int}{PlCall}{const wchar_t *goal}
    \nodescription
    \cfunction{int}{PlCall}{const char *goal}
Translates \arg{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.
\end{description}

\subsection{The class PlFrame}
\label{sec:cpp-plframe}

The class \class{PlFrame} provides an interface to discard unused
term-references as well as rewinding unifications
(\jargon{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 \class{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 \class{PlFrame} is the
only way to reclaim them.

\begin{description}
    \constructor{PlFrame}{}
Creating an instance of this class marks all term-references created
afterwards to be valid only in the scope of this instance.
    \destructor{PlFrame}
Reclaims all term-references created after constructing the instance.
    \cfunction{void}{PlFrame::rewind}{}
Discards all term-references {\bf and} global-stack data created as well
as undoing all unifications after the instance was created.
\end{description}

\index{assert}%
A typical use for \class{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 \index{word/1}\predref{word}{1}:

\begin{code}
void
assertWord(const char *word)
{ PlFrame fr;
  PlTermv av(1);

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

\noindent
This example shows the most sensible use of \class{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.

\begin{code}
PREDICATE(can_unify, 2)
{ PlFrame fr;

  int rval = (A1=A2);
  fr.rewind();
  return rval;
}
\end{code}

\noindent
\section{The PREDICATE macro}
\label{sec:cpp-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

\begin{code}
PREDICATE(hello, 1)
\end{code}

\noindent
is the same as writing:

\begin{code}
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)
\end{code}

\noindent
The first function converts the parameters passed from the Prolog
kernel to a \class{PlTermv} instance and maps exceptions raised in
the body to Prolog exceptions.  The \class{PlRegister} global
constructor registers the predicate.  Finally, the function header
for the implementation is created.

\subsection{Variations of the PREDICATE macro}
\label{sec:cpp-predicate-macro-variations}

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

\begin{description}
    \cmacro{}{PREDICATE0}{name}
This is the same as PREDICATE(name, 0).  It avoids a compiler warning
about that \const{PL_av} is not used.

    \cmacro{}{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.

    \begin{code}
    NAMED_PREDICATE("#", hash, 2)
    { A2 = (wchar_t*)A1;
    }
    \end{code}

\noindent
    \cmacro{}{NAMED_PREDICATE_NONDET}{plname, cname, arity}
Define a non-deterministic Prolog predicate in C++. See
\file{SWI-cpp.h}.  FIXME: Needs cleanup and an example.
\end{description}


\subsection{Controlling the Prolog destination module}
\label{sec:cpp-module}

With no special precautions, the predicates are defined into the
module from which \index{load_foreign_library/1}\predref{load_foreign_library}{1} was called, or in the module
\const{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, {\em 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:

\begin{code}
#define PROLOG_MODULE "math"
#include <SWI-Prolog.h>
#include <math.h>

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

\noindent
\begin{code}
?- math:pi(X).

X = 3.14159
\end{code}

\noindent
\section{Exceptions}
\label{sec:cpp-exceptions}

Prolog exceptions are mapped to C++ exceptions using the subclass
\class{PlException} of \class{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 \class{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 \class{PlTypeEror} and \class{PlDomainError}.

To throw an exception, create an instance of \class{PlException} and
use throw().

\begin{code}
  char *data = "users";

  throw PlException(PlCompound("no_database", PlTerm(data)));
\end{code}

\noindent
\subsection{The class PlException}
\label{sec:cpp-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++ \exam{throw} of a \class{PlException}
object. You can use C++ try-catch to intercept this and examine the


This subclass of \class{PlTerm} is used to represent exceptions.
Currently defined methods are:

\begin{description}
    \constructor{PlException}{}
Create an exception term using \cfuncref{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.

    \constructor{PlException}{const PlTerm \&t}
Create an exception from a general Prolog term.  This provides the
interface for throwing any Prolog terms as an exception.
    \cppcast{PlException}{wchar_t *}
    \nodescription
    \cppcast{PlException}{char *}
The exception is translated into a message as produced by
\index{print_message/2}\predref{print_message}{2}.  The character data is stored in a ring.  Example:

\begin{code}
  ...;
  try
  { PlCall("consult(load)");
  } catch ( PlException &ex )
  { cerr << (char *) ex << endl;
  }
\end{code}

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

\begin{quote}
\term{error}{\term{type_error}{Expected, Actual}, Context}
\end{quote}

PlException::cppThrow() throws a \class{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:

\begin{code}
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;
  }
}
\end{code}

\noindent
\end{description}



\subsection{The class PlTypeError}
\label{sec:cpp-pl-type-error}

A \jargon{type error} expresses that a term does not satisfy the
expected basic Prolog type.

\begin{description}
    \constructor{PlTypeError}{const char *expected, const PlTerm \&actual}
Creates an ISO standard Prolog error term expressing the
\arg{expected} type and \arg{actual} term that does not satisfy this
type.
\end{description}


\subsection{The class PlDomainError}
\label{sec:cpp-pl-domain-error}

A \jargon{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 \index{open/3}\predref{open}{3}
call expect an \const{io_mode} (read, write, append, ...). If an integer
is provided, this is a \jargon{type error}, if an atom other than one
of the defined io-modes is provided it is a \jargon{domain error}.

\begin{description}
    \constructor{PlDomainError}{const char *expected, const PlTerm \&actual}
Creates an ISO standard Prolog error term expressing a the
\arg{expected} domain and the \arg{actual} term found.
\end{description}


\section{Embedded applications}
\label{sec:cpp-embedding}

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 \jargon{main-program} and we want to use Prolog as a
\jargon{logic server}.  For these applications, the class
\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.%
    \footnote{For 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.}

\begin{description}
    \constructor{PlEngine}{int argc, char **argv}
Initialises the Prolog engine.  The application should make sure to
pass \exam{argv[0]} from its main function, which is needed in the
Unix version to find the running executable.  See \cfuncref{PL_initialise}{}
for details.
    \constructor{PlEngine}{char *argv0}
Simple constructure using the main constructor with the specified
argument for \exam{argv[0]}.
    \destructor{PlEngine}
Calls \cfuncref{PL_cleanup}{} to destroy all data created by the Prolog engine.
\end{description}

\Secref{pltail} has a simple example using this class.


\section{Considerations}
\label{sec:cpp-considerations}

\subsection{The C++ versus the C interface}
\label{sec:cpp-vs-c}

Not all functionality of the C-interface is provided, but as
\class{PlTerm} and \type{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 \class{PlFrame}). Use of some intermediate types
(\type{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.


\subsection{Static linking and embedding}
\label{sec:cpp-linking}

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

\subsection{Status and compiler versions}
\label{sec:cpp-status}

The current interface is entirely defined in the \fileext{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.

\section{Conclusions}				\label{sec:conclusions}
\label{sec:cpp-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.

\input{pl2cpp2}

\printindex

\end{document}