Documentation
- Reference manual
- Packages
  - A C++ interface to SWI-Prolog
    - A C++ interface to SWI-Prolog (Version 1)
    - A C++ interface to SWI-Prolog (Version 2)

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, SWI-cpp2-plx.h, and SWI-cpp2-atommap.h.

The overall structure of the API has been retained - that is, it is a thin layer 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operators have been removed or deprecated, and replaced by "getter" methods; the overloaded constructors have been replaced by subclasses for the various types.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, an exception type PlFail has been added, together with PlCheckFail(), to allow more compact code by "short circuit" return to Prolog on failure.

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

More specifically:

SWI-cpp2.cpp has been added, containing the implementation of some functions. This is included by default from SWI-cpp2.h or can be compiled separately.
The constructor PlTerm() is restricted to a few unambiguous cases - instead, you should use the appropriate subclass constructors (PlTerm_var(), PlTerm_atom(), PlTerm_term_t(), PlTerm_integer(), PlTerm_int64(), PlTerm_uint64(), PlTerm_size_t(), PlTerm_float(), or PlTerm_pointer()).
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 2.5.4. Many of these wrapper functions are also methods in the PlAtom and PlTerm classes, with the arguments changed from atom_t and term_t to PlAtom and PlTerm. 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}; PlCheckFail(rc) is used to additionally throw PlFail, similar to returning false from the top-level of a foreign predicate - Prolog will check for an error and call throw/1 if appropriate.
The PlException class is a subclass of std::exception and encapsulates a Prolog error. Prolog errors are converted into throw PlException(...). If the user code does not catch the PlException, the PREDICATE() macro converts the error to a Prolog error upon return to the Prolog caller.
The C++ constructors, functions, and methods use the wrapper functions to 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, static_cast<char*>(t)) have been deprecated, replaced by "getters" (e.g., t.as_string(), t.as_int64_t()).
The overloaded assignment operator for unification is deprecated, replaced by 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.^{5If you
want to return a char* from a function, you should not do return
t.as_string().c_str() because that will return a pointer to local
or stack memory. Instead, you should change your interface to return a std::string
and apply the c_str() method to it. These lifetime errors
can sometimes be caught by specifying the Gnu C++ or Clang
options -Wreturn-stack-address or -Wreturn-local-addr
- as of 2023-04, Clang seems to do a better analysis.}
Most constructors, methods, and functions that accept char* or wchar_t* arguments also accept std::string or std::wstring arguments. Where possible, encoding information can also be specified.
Type-checking methods have been added: PlTerm::type(), PlTerm::is_variable(), PlTerm::is_atom(), etc.
PlString has been renamed to PlTerm_string to make it clear that it's a term that contains a Prolog string.
More PL_...(term_t, ...) methods have been added to PlTerm, and PL_...(atom_t, ...) methods have been added to PlAtom. Where appropriate, the arguments use PlTerm, PlAtom, etc. instead of term_t, atom_t, etc.
Most functions/methods that return an int for true/false now return a C++ bool.
The wrapped C types fields (term_t, atom_t, etc.) have been renamed from handle, ref, etc. to C_.^{6This is done by
subclassing from Wrapped<term_t>, Wrapped<atom_t>,
etc., which define the field C_, standard constructors, the
methods is_null(), not_null(), reset(), reset(v), reset_wrapped(v), plus
the constant null.} This value can be accessed by the unwrap() and unwrap_as_ptr() methods. There is also a "friend" function PlUnwrapAsPtr().
A convenience function PlControl::context_unique_ptr<ContextType>() has been added, to simplify dynamic memory allocation in non-deterministic predicates.
A convenience function PlRewindOnFail() has been added, to simplify non-deterministic code that does backtracking by checking unification results.
PlStringBuffers provides a simpler interface for allocating strings on the stack than PL_STRINGS_MARK() and PL_STRINGS_RELEASE(). See section 2.5.7.1.
PlStream provides a simpler interface for streams than PL_get_stream(), PL_acquire_stream(), and PL_release_stream(). See section 2.5.7.2.
Wrapper classes for record_t have been added. The PlRecordExternalCopy class contains the opaque handle, as a convenience.
Wrapper class for control_t has been added and the PREDICATE_NONDET() has been modified to use it.

More details are given in section 2.7 and section 2.8.

2.2 Sample code (version 2)

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

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

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

2.3 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.

2.3.1 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).

2.4 The life of a PREDICATE (version 2)

A foreign predicate is defined using the PREDICATE() macro, plus a few variations on this, such as PREDICATE_NONDET(), NAMED_PREDICATE(), and NAMED_PREDICATE_NONDET(). These define an internal name for the function, register it with the SWI-Prolog runtime (where it will be picked up by the use_foreign_library/1 directive), and define the names A1, A2, etc. for the arguments.^{7You can define
your own names for the arguments, for example: auto dir=A1, db=A2,
options=A3;.} If a non-deterministic predicate is being defined, an additional parameter handle is defined (of type PlControl).

The foreign predicate returns a value:

true - success
false - failure or an error (see section 2.18 and Prolog exceptions in foreign code).
"retry" - for non-deterministic predicates, gives a "context" for backtracking / redoing the call for the next solution.

If a predicate fails, it could be simple failure (the equivalent of calling the builtin fail/0 predicate) or an error (the equivalent of calling the throw/1 predicate). When a Prolog exception is raised, it is important that a return be made to the calling environment as soon as possible. In C code, this requires checking every call for failure, which can become cumbersome. C++ has exceptions, so instead the code can wrap calls to PL_*() functions with PlCheckFail() 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 essentially the same thing (for implementing the equivalent of =/2); however the forst option (with PlTerm::unify_term()) and third option (with Plx_unify()) throw a C++ PlExceptionFail exception if there's an error and return true or false; the second option (with PlCheckFail()) throws a PlFail exception for both failure and an error and otherwise returns true - the PREDICATE() wrapper handles all of these appropriately and reports the same result back to Prolog; but you might wish to distinguish the two situations in more complex code.

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

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

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

2.5 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).

2.5.1 Design philosophy of the classes

2.5.2 Summary of files

The following files are provided:

SWI-cpp2.h Include this file to get the C++ API. It automatically includes SWI-cpp2-plx.h and SWI-cpp2.cpp, unless the macro _SWI_CPP2_CPP_SEPARATE is defined, in which case you must compile SWI-cpp2.cpp separately.
SWI-cpp2.cpp Contains the implementations of some methods and functions. If you wish to compile this separately, you must define the macro _SWI_CPP2_CPP_SEPARATE before your include for SWI-cpp2.h.
SWI-cpp2-plx.h Contains the wrapper functions for the most of the functions in SWI-Prolog.h. This file is not intended to be used by itself, but is #included by SWI-cpp2.h.
SWI-cpp2-atommap.h Contains a utility class for mapping atom-to-atom or atom-to-term.
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.

2.5.3 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 2.10.3.

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.^{8PL_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 PlExceptionBase, 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.
PlExceptionBase: A "do nothing" subclass of std::exception, to allow catching PlException, PlExceptionFail or PlFail in a single "catch" clause.

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 when control returns to Prolog from the PREDICATE() macros.

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.

2.5.4 Wrapper functions

The various PL_*() functions in SWI-Prolog.h have corresponding Plx_*() functions, defined in SWI-cpp2-plx.h, which is always included by SWI-cpp2.h. There are three kinds of wrappers:

"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 PlEx(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.

Important: You should use the Plx_*() wrappers only in the context of a PREDICATE() call, which will handle any C++ exceptions. Some blob callbacks can also handle an exception. Everywher else, results are unpredicatable (probably a crash).

2.5.5 Naming conventions, utility functions and methods (version 2)

2.5.6 Blobs

Disclaimer:

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

The blob contains the foreign object (e.g., contains a pointer to a database connection), plus optionally some other data.
The blob is created by a predicate that makes the foreign object and stores it (or a pointer to it) within the blob - for example, making a connection to a database or compiling a regular expression into an internal form.
Optionally, there is a predicate that deletes the foreign object, such as a file or database connection close().
The blob will not be subclassed.
The blob is defined as a subclass of PlBlob, which provides a number of fields and methods, of which a few can be overridden in the blob (notably: write_fields(), compare_fields(), save(), load(), and the destructor).
The blob must have a default constructor that sets all the fields to appropriate initial values.^{10This
is used by the load() method, which by default throws an error.}
The blob's constructor throws an exception and cleans up any resources if it cannot create the blob.^{11This
is not a strong requirement, but the code is simpler if this style is
used.}
The foreign object can be deleted when the blob is deleted. That is, the foreign object is created using the new operator and passes ownership to the blob.
The blob's allocation is controlled by Prolog and its destructor is envoked when the blob is garbage collected. Optionally, the predicate that deletes the foreign object deletes the foreign object and the Prolog garbage collector only frees the blob.

A Prolog blob consists of five parts:

A PL_blob_t structure that defines the callbacks.
A structure that contains the blob data.
A "create" or "open" predicate that unifies one of its arguments with a newly created blob that contains the foreign object.
(Optionally) a "close" predicate that does the opposite of the "create" or "open" predicate.
Predicates that manipulate the foreign object (e.g., for a file-like object, these could be read, write, seek, etc.).

For the PL_blob_t structure, the C++ API provides a set of template functions that allow easily setting up the callbacks, and defining the corresponding methods in the blob "contents" class. The C interface allows more flexibility by allowing some of the callbacks to default; however, the C++ API for blobs provides suitable callbacks for all of them, so usually the programmer will specify all the template callbacks using the PL_BLOB_DEFINITION(blob_class,blob_name) macro.

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

There is a mismatch between how Prolog does memory management (and garbage collection) and how C++ does it. In particular, Prolog assumes that cleanup will be done in the release() function associated with the blob whereas C++ typically does cleanup in a destructor. The blob interface gets around this mismatch by providing a default release() function that assumes that the blob was created using PL_BLOB_NOCOPY and manages memory using a std::unique_ptr.

The C blob interface has a flag that determines how memory is managed: PL_BLOB_NOCOPY. The PL_BLOB_DEFINITION() macro sets this, so Prolog does not do a call to free() when the blob is garbage collected; instead, it lets the blob's release() free the memory, which is done by calling the C++ destructor.

The C++ API for blobs only supports blobs with PL_BLOB_NOCOPY.^{12The
API can probably also support blobs with PL_BLOB_UNIQUE,
but there seems to be little point in setting this flag for non-text
blobs.}

2.5.6.1 How to define a blob using C++

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

2.5.6.2 The life of a PlBlob

In this section, the blob is of type MyBlob, a subclass of PlBlob.

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

Creates the blob using auto ref = std::unique_ptr<PlBlob>(new MyBlob>(...)) (std::make_unique() can't be used because it returns type std::unique_ptr<MyBlob> but PlTerm::unify_blob() requires a std::unique_ptr<PlBlob> and C++'s type inferencing can't figure out that this is a covariant type).
Calls PlTerm::unify_blob(ref), using PlCheckFail() to throw an exception if it fails. This, in turn, calls:
- PlBlobV<MyBlob>acquire(), which calls
- MyBlob::acquire(), which sets MyBlob::symbol_ (MyBlob::symbol_ is usually accessed using the method MyBlob::symbol_term()). If this all succeeds, PlTerm::unify_blob(ref) calls ref.release() to pass ownership to the Prolog blob. If you wish to use std::make_unique<MyBlob>(), you could instead do:
```
      auto ref = std::make_unique<MyBlob>(...);
         ... // code that accesses fields in *ref
      std::unique_ptr<PlBlob> refb(ref.release()); // transfer ownership of ptr
         // from here on, can't access fields in *ref
      return A2.unify_blob(refb);
      
```

At this point, the blob is owned by Prolog and will be freed by its atom garbage collector, which will call the blob's destructor.

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

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

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

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

2.5.6.3 C++ exceptions and blobs

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

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

acquire(), which is called from PlBlobV<MyBlob>::acquire(), can throw a C++ exception.
compare_fields(), which is called from PlBlobV<MyBlob>::compare(), should not throw an exception. A Prolog error won't work as it uses "raw pointers" and thus a GC or stack shift triggered by creating the exception will upset the system.
write_fields(), which is called from PlBlobV<MyBlob>::write(), can throw an exception, just like code inside a PREDICATE(). In particular, you can wrap calls to Sfprintf() in PlCheckFail(), although the calling context will check for errors on the stream, so checking the Sfprintf() result isn't necessary.
save() can throw a C++ exception, including PlFail().
load() can throw a C++ exception, which is converted to a return value of PlAtom::null, which is interpreted by Prolog as failure.

2.5.6.4 Sample PlBlob code

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

struct MyBlob;

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

struct MyBlob : public PlBlob
{ std::unique_ptr<MyConnection> connection;
  std::string name_; // Used for error terms

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

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

  PL_BLOB_SIZE

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

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

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

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

  bool write_fields(IOSTREAM *s, int flags) const override
  { if ( !Sfprintf(s, ",name=%s", name_.c_str()) )
      return false;
    if ( !connection )
      return Sfprintf(s, ",closed");
    return true;
  }
};

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

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

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

2.5.6.5 Discussion of the sample PlBlob code

PL_BLOB_DEFINITION(MyBlob, "my_blob") creates a PL_blob_t structure with the wrapper functions and flags set to PL_BLOB_NOCOPY. It should be declared outside the PlBlob class and should not be marked const - otherwise, a runtime error can occur.^{13The cause of the
runtime error is not clear, but possibly has to do with the order of
initializing globals, which is unspecified for C++.}
The MyBlob struct is a subclass of PlBlob. See below for a discussion of the default behaviors.
- MyBlob contains a pointer to a MyConnection object and keeps a copy of the connection's name. The MyConnection object is handled by a std::unique_ptr smart pointer, so that it is automatically freed when the MyBlob object is freed.
- A default constructor is defined - this is needed for the load() and save() methods; it invokes the PlBlob constructor.
- The MyBlob class must not provide a copy or move constructor, nor an assignment operator (PlBlob has these as delete, so if you try to use one of these, you will get a compile-time error).
- PlBlob’s constructor sets blob_t_ to a pointer to the my_blob definition. This is used for run-time consistency checking by the various callback functions and for constructing error terms (see PlBlob::symbol_term()).
- PlBlob’s acquire() is called by PlBlobV<MyBlob>::awcuire() and fills in the symbol field. MyBlob must not override this - it is not a virtual method.
- PlBlob::symbol_term() Creates a term from the blob, for use in error terms. It is always safe to use this; if the symbol hasn't been set (because acquire() hasn't been called), symbol_term() returns a "var" term.
- The MyBlob(connection_name) constructor creates a MyConnection object. If this fails, an exception is thrown. The constructor then calls MyConnection::open() and throws an exception if that fails. (The code would be similar if instead the constructor for MyConnection also did an open and threw an exception on failure.)
- The PL_BLOB_SIZE is boilerplate that defines a blob_size_() method that is used when the blob is created.
- The destructor MyBlob() is called when the blob is released by the garbage collector and in turn calls the MyBlob::close(), throwing away the result. If there is an error, a message is printed because there is no other way report the error. For this reason, it is preferred that the program explicitly calls the close_my_blob/1 predicate, which can raise an error. One way of doing this is by using the at_halt/1 hook.
- The MyBlob::close() method is called by either the destructor or by the close_my_blob/1 predicate. Because it can be called by the garbage collector, which does not provide the usual environment and which may also be in a different thread, the only Prolog function that can be called is PlAtom::unregister_ref(); and the MyBlob::close() method must not throw an exception.^{14It isn't enough to
  just catch exceptions; for example, if the code does throw
  PlUnknownError("..."), that will try to create a Prolog term,
  which will crash because the environment for creating terms is not
  available.} Because there is no mechanism for reporting an error, the destructor prints a message on failure (calling PL_warning() would cause a crash).
  PlBlob::close() calls MyConnection::close() and then frees the object. Error handling is left to the caller because of the possibility that this is called in the context of garbage collection. It is not necessary to free the MyConnection object here - if it is not freed, the std::unique_ptr<MyConnection>’s destructor would free it.
- PlBlob::MyBlobError() is a convenience method for creating errror terms.
- PlBlob::compare_fields() makes the blob comparison function more deterministic by comparing the name fields; if the names are the same, the comparison will be done by comparing the addresses of the blobs (which is the default behavior for blobs defined using the C API). PlBlob::compare_fields() is called by PlBlobV<PlBlob>::compare(), which provides the default comparison if PlBlob::compare_fields() returns 0 ("equal").
  The _b_data argument is of type const PlBlob* - this is cast to const MyBlob* using a static_cast. This is safe because Prolog guarantees that PlBlobV<PlBlob>::compare() will only be called if both blobs are of the same type.
- PlBlob::write_fields() outputs the name and the status of the connection, in addition to the default of outputting the blob type and its address. This is for illustrative purposes only; an alternative is to have a my_blob_properties/2 predicate to provide the information.
  The flags argument is the same as given to PlBlobV<PlBlob>::write(), which is a bitwise or of zero or more of the PL_WRT_* flags that were passed in to the caling PL_write_term() (defined in SWI-Prolog.h). The flags do not have the PL_WRT_NEWLINE bit set, so it is safe to call PlTerm::write() and there is no need for writing a trailing newline.
  
  If anything in PlBlob::write_fields() throws a C++ exception, it will be caught by the calling PlBlobV<PlBlob>::write() and handled appropriately.
- PlBlob::save() and PlBlob::load() are not defined, so the defaults are used - they throw an error on an attempt to save the blob (e.g., by using
  indexqsave_program/[1,2]qsave_program/[1,2]).^{15If
  these weren't specified, the defaults would save the internal form of
  the blob, which is probably not what you want.}
create_my_blob/2 predicate:
- std::unique_ptr<PlBlob>() creates a MyBlob that is deleted when it goes out of scope. If an exception occurs between the creation of the blob or if the call to unify_blob() fails, the pointer will be automatically freed (and the MyBlob destructor will be called).
  If PlTerm::unify_blob() is called with a pointer to a std::unique_ptr, it takes ownership of the object by calling std::unique_ptr<PlBlob>::release(). This sets ref to nullptr, so any attempt to use ref after a successful call to PlTerm::unify_blob() will be an error.
  
  If you wish to create a MyBlob object instead of a PlBlob object, a slightly different form is used:
```
      auto ref = std::make_unique<MyBlob>(...);
        ...
      std::unique_ptr<PlBlob> refb(ref.release());
      PlCheckFail(A2.unify_blob(&refb));
      return true;
      
```
close_my_blob/1 predicate:
- The argument is turned into a MyBlob pointer using the PlBlobV<MyBlob>::cast_ex() function, which will throw a type_error if the argument isn't a blob of the expected type.
- The MyBlob::close() method is called - if it fails, a Prolog error is thrown.

2.5.6.6 Identifying blobs by atoms

Passing a blob around can be inconvenient; there is an easy way to identify a blob by an atom. An example of this is with streams, which are identified by atoms such as user_input.

A utility class AtomMap is provided for this situation. See section 2.20.4.

2.5.7 Limitations of the interface

The C++ API remains a work in progress.

2.5.7.1 Strings

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

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

For return values, char* is dangerous because it can point to local or stack memory. For this reason, wherever possible, the C++ API returns a std::string, which contains a copy of the 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.^{16If
we wish to minimize the overhead of passing strings, this can be done by
passing in a pointer to a string rather than returning a string value;
but this is more cumbersome and modern compilers can often optimize the
code to avoid copying the return value.}

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

2.5.7.2 Stream I/O

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

The methods are:

PlStream :: PlStream(term_t t, int flags)

- see PL_get_stream() for documentation of the flags. Throws a C++ exception on error.

PlStream :: PlStream(IOSTREAM *s)

- calls PL_acquire_stream() to lock the stream. Throws a C++ exception on error.

~ PlStream()

- calls PlStream::release(). See below for caveats if there are exceptions.

void release()

- calls PL_release_stream(), throwing an exception if there has been an I/O error on the stream, and sets the PlStream object to an invalid stream (see PlStream::check_stream()).

IOSTREAM* ::operator PlStream(void)

- when used in a context that requires an IOSTREAM*, PlStream is implicitly converted to IOSTREAM*.
cfunctionvoidcheck_stream - checks that the PlStream object contains a valid stream and throws an exception if it doesn't. This is used to ensure that PlStream::release() hasn't been called.
Most of the stream I/O functions have corresponding methods in PlStream. For example, Sfprintf() corresponds to PlStream::printf().

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

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

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

The following two code examples do essentially the same thing:

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

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

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

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

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

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

2.5.7.3 Object handles

Many of the "opaque object handles", such as atom_t, term_t, and functor_t are integers.^{17Typically uintptr_t
values, which the C standard defines as “an unsigned integer type
with the property that any valid pointer to void can be converted to
this type, then converted back to pointer to void, and the result will
compare equal to the original pointer.''} As such, there is no compile-time detection of passing the wrong handle to a function.

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

A subclass for each kind of initializer;
A tag for each kind of intializer;
Change the 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.

2.5.8 Linking embedded applications using swipl-ld

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

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

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

2.6 Examples (version 2)

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

2.6.1 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;^{18The
C-string values can be extracted from std::string by using
c_str(), but you must be careful to not return a pointer to a
local/stack value, so this isn't recommende.} and similar access methods provide an easy type-conversion for most Prolog data-types, using the output of write/1 otherwise:

?- hello(world).
Hello world

Yes
?- hello(X)
Hello _G170

X = _G170

2.6.2 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());
}

or more compactly:

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

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

?- add(1, 2, X).

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

2.6.3 Average of solutions - calling a Prolog goal (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.

2.7 Rationale for changes from version 1 (version 2)

2.7.1 Implicit constructors and conversion operators

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

PREDICATE(hello, 1)
{ cout << "Hello " << (char *)A1 << endl;
  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 implicit constructors and conversion operators, combined with the C++ conversion rules for integers and floats, could sometimes lead to subtle bugs that were difficult to find -- in one case, a typo resulted in terms being unified with floating point values when the code intended them to be atoms. This was mainly because the underlying C types for terms, atoms, etc. are unsigned integers, leading to confusion between numeric values and Prolog terms and atoms.
The overloaded assignment operator for unification changed the usual C++ semantics for assignments from returning a reference to the left-hand-side to returning a bool. In addition, the result of unification should always be checked (e.g., an "always succeed" unification could fail due to an out-of-memory error); the unify_XXX() methods return a bool and they can be wrapped inside a PlCheckFail() to raise an exception on unification failure.
The C-style of casts is deprecated in C++, so the expression (char*)A1 becomes the more verbose static_cast<std::string>(A1), which is longer than A1.as_string(). Also, the string casts don't allow for specifying encoding.
The implicit constructors and conversion operators were attractive because they allowed directly calling the foreign language interface functions, for example:
```
PlTerm t;
Pl_put_atom_chars(t, "someName");
```
whereas this is now required:
```
PlTerm t;
Pl_put_atom_chars(t.as_term_t(), "someName");
```
However, this is mostly avoided by methods and constructors that wrap the foreign language functions:
```
PlTerm_atom t("someName");
```
or
```
auto t = PlTerm_atom("someName");
```
Additionally, there are now wrappers for most of the PL_*() functions that check the error return and throw a C++ exception as appropriate.

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

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

2.7.2 Strings

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

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

There still remains the problems of Unicode and encodings. std::wstring is one way of dealing with this. And for interfaces that use std::string, an encoding can be specified.^{19As 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.

2.8 Porting from version 1 to version 2

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

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

Here is a list of typical changes:

Replace PlTerm() constructor with PlTerm_var() for uninstantiated variables, PlTerm_atom(a) for atoms, PlTerm_term_t(t) for the raw term_t, PlTerm_integer(i), PlTerm_float(v), or PlTerm_pointer(p).
Examine uses of char* or wchar_t and replace them by std::string or std::wstring if appropriate. For example, cout << "Hello " << (char*)A1 << endl can be replaced by cout << "Hello " << A1.as_string() << endl. In general, std::string is safer than char* because the latter can potentially point to freed memory.
Instead of returning false from a predicate for failure, you can do throw PlFail(). This mechanism is also used by PlCheckFail(rc). Note that throwing an exception is slower than returning false, so performance-critical code should avoid PlCheckFail(rc) if failure is expected to happen often.
You can use the 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_, which should be accessed by PlAtom::unwrap().
PlTerm::ref has been replaced by PlTerm::C_, which should be accessed by PlTerm::unwrap().
PlFunctor::functor has been replaced by PlFunctor::C_, which should be accessed by PlFunctor::unwrap().
The operator = for unification has been deprecated, replaced by various unify_*() methods (PlTerm::unify_term(t2), PlTerm::unify_atom(a), etc.).
The various "cast" operators have been deprecated or deleted; you should use the various "getter" methods. For example, static_cast<char*>(t) is replaced by t.as_string().c_str() (and you should prefer t.as_striong(); static_cast<int32_t>(t) is replaced by t.as_int32_t(), etc.
It is recommended that you do not use int or long because of problems porting between Unix and Windows platforms; instead, use int32_t, int64_t, uint32_t, uint64_t, etc.

2.9 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 2.20.2).

For example, this code, using the C API:

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

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

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

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

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

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

2.9.1 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. PlCheckFail() calls PlCheckEx() and additionally throws PlFail() if the failure is for Prolog failure.

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 2.18.

2.10 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.

2.10.1 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;
}

2.10.2 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.

2.10.3 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()

2.10.4 Unification (version 2)

2.10.5 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 2.12.2 for direct manipulation of atoms in their internal representation.

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

2.10.6 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.

2.10.7 Miscellaneous (version 2)

bool is_null(): t.is_null() is the same as t.unwrap() == PlTerm::null
bool not_null(): t.not_null() is the same as t.unwrap() != PlTerm::null
bool reset(): t.reset() is the same as t.unwrap() = PlTerm::null
bool reset(term_t): t.reset(x) is the same as t.unwrap() = x
int PlTerm::type(): Yields the actual type of the term as PL_term_type(). Return values are PL_VARIABLE, PL_FLOAT, PL_INTEGER, PL_ATOM, PL_STRING or PL_TERM

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.

2.10.8 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.

2.10.9 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.

2.10.10 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.

2.10.11 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"))

2.10.12 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;
}

2.11 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";
  ...
}

2.12 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:

2.12.1 Direct comparision to char *

Example:

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

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

2.12.2 Direct comparision to PlAtom

Example:

static PlAtom ATOM_read("read");

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

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

2.12.3 Extraction of the atom and comparison to PlAtom

Example:

static PlAtom ATOM_read("read");

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

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

This approach is basically the same as section 2.12.2, but in nested if-then-else the extraction of the atom from the term is done only once.

2.12.4 Extraction of the atom and comparison to char *

Example:

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

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

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

PlAtom :: PlAtom(atom_t handle)

Create from C-interface atom handle (atom_t). Used internally and for integration with the C-interface.

PlAtom :: PlAtom(const char_t *text)

PlAtom :: PlAtom(const wchar *text)

PlAtom :: PlAtom(const std::string& text)

PlAtom :: PlAtom(const std::wstring& text)

Create an atom from a string. The text is copied if a new atom is created. See PL_new_atom(), PL_new_atom_wchars(), PL_new_atom_nchars(), PL_new_atom_wchars().

PlAtom :: PlAtom(const PlTerm &)

If t represents an atom, the new instance represents this atom. Otherwise a type_error is thrown.

int PlAtom::operator ==(const wchar_t *text)

int PlAtom::operator ==(const char *text)

int PlAtom::operator ==(const std::string& text)

int PlAtom::operator ==(const std::wstring& text)

Yields true if the atom represents text, false otherwise. Performs a strcmp() or similar for this.

int PlAtom::operator ==(const PlAtom &)

Compares the two atom-handles, returning true or false. Because atoms are unique, there is no need to use strcmp() for this.

int PlAtom::operator !=(const wchar_t *text)

int PlAtom::operator !=(const char *text)

int PlAtom::operator !=(const std::string& text)

int PlAtom::operator !=(const std::wstring& text)

int PlAtom::operator !=(const PlAtom &)

The inverse of the == operator.

bool is_valid()

Verifies that the handle is valid. This can be used after calling a function that returns an atom handle, to check that a new atom was created.

void reset()

Sets the handle to an invalid valid - a subsequent call to is_null() will return true.

const std::string as_string(PlEncoding enc=EncLocale)

Returns the string representation of the atom.^{20If
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().

2.13 Classes for the recorded database: PlRecord and PlRecordExternalCopy

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

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

The class PlRecord supports the following methods:

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

The class PlRecord provides direct access to the reference counting aspects of the recorded term (through the duplicate() and erase() methods), but does not connect these with C++'s copy constructor, assignment operator, or destructor. If the recorded term is encapsulated within an object, then the containing object can use the duplicate() and erase() methods in its copy and move constructors and assignment operators (and the erase() method in the destructor).^{21The copy constructor
and assignment use the duplicate() method; the move constructor and
assignment use the duplicate() method to assign to the destination and
the erase() method on the source; and the destructor uses erase().}

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

For example:

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

The class PlRecordExternalCopy keeps the external record as an uninterpreted string. 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()).

2.14 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.unwrap(),t2.unwrap()).

2.15 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.

2.16 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.

2.16.1 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); });
}

2.17 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:^{22There
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.

2.17.1 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 2.17.2.

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 2.17.2.

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

The skeleton for a typical non-deterministic predicate is:

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

PREDICATE_NONDET(pred, <arity>)
{
  auto ctxt = handle.context_unique_ptr<PredContext>();
  switch( PL_foreign_control(handle) )
  { case PL_FIRST_CALL:
      ctxt.reset(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)

  ...

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

2.17.3 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

2.18 Exceptions (version 2)

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

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

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

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

To throw an exception, create an instance of PlException and use throw. This is intercepted by the PREDICATE macro and turned into a Prolog exception. See section 2.20.2.

  char *data = "users";

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

2.18.1 The class PlException (version 2)

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

PlException :: PlException(const PlTerm &)

Create an exception from a general Prolog term. This provides the interface for throwing any Prolog terms as an exception.

std::string as_string()

The exception is translated into a message as produced by print_message/2. The character data is stored in a ring. Example:

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

int plThrow()

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

2.18.2 The class PlTypeError (version 2)

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

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

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

2.19 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.^{23For
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.

2.20 Considerations (version 2)

2.20.1 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 unwrap() method), this interface can be freely mixed with the functions defined for plain C. For checking return codes from C functions, it is recommended to use PlCheckFail() or PlCheck_PL().

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

Global terms and atoms need to be handled slightly differently in C++ than in C - see section 2.20.3

2.20.2 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.

2.20.3 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();
}

2.20.4 Atom map utilities

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

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

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

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

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

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

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

The constructor and methods are as follows:

template<ValueType, StoredValueType> AtomMap::AtomMap(const std::string& insert_op)
const std::string& insert_type Construct an AtomMap. The ValueType and StoredValueType specify what type you wish for the value. Currently, two value types are supported:
- PlAtom - the StoredValueType should be PlAtom.
- PlTerm - the StoredValueType shoud be PlRecord (because the term needs to be put on the global stack).
The insert_op and insert_type values are used in constructing error terms - these correspond to the operation and type arguments to Pl_permission_error().
insert PlAtom key, ValueType value(I)

nserts a new value; raises a permission_error if the value is already in the map, unless the value is identical to the value in the map. The insert() method converts the value to the StoredValueType. The insertion code takes care of atom reference counts.

ValueType find(PlAtom key)

Look up an entry. Success/failure can be determined by using ValueType::is_null() or ValueType::not_null(). The stored value is converted from StoredValueType to ValueType.

erase PlAtom(r)

emoves the entry from the map. If there was no entry in the map with that key, this is a no-op. The erasure code takes care of atom reference counts.

2.20.5 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.

2.20.6 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.unwrap()); throw PlFail(). See section 2.20.2.

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.

2.21 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 2.6.2
arg/3: 1.4.5 2.10.6
as_string()
as_wstring()
assert: 1.8.1 2.16.1
at_halt/1: 2.5.6.5
atom_chars/2: 1.2 1.4.9 2.5.3 2.10.10
average/3: 1.3.3 2.2 2.6.3
between/3: 2.2
blob_data()
can_unify/2: 2.2
cappend/3: 2.2
close_my_blob/1: 2.5.6.5 2.5.6.5 2.5.6.5
create_my_blob/2: 2.5.6.5
duplicate()
entry/1: 1.4.11 2.10.12
eq1/1: 2.2
eq2/2: 2.2
eq3/2: 2.2
erase()
error/2: 2.20.2
fail/0: 2.4
find()
functor/3: 2.2
happy/1: 2.2
hello/1: 1.3.1 2.6.1
int_info/2: 2.2
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()
likes/2: 2.2
load_foreign_library/1: 1.9.2 2.17.3
my_blob_properties/2: 2.5.6.5
name_arity/3: 2.2
not_null()
open/3: 1.10.3 2.18.3
print_message/2: 1.10.1 2.18.1
range_cpp/3: 2.2
read/1: 1.4.10 2.10.11
register_atom()
release()
reset()
square_roots/2: 2.2
term()
throw/1: 2.1 2.4
type()
unifiable/3: 2.2
unregister_atom()
use_foreign_library/1: 2.4
word/1: 1.8.1 2.16.1
write/1: 1.3.1 1.4.2 2.6.1 2.10.3
write_list/1: 2.2
NAMED_PREDICATE()
NAMED_PREDICATE_NONDET()
P
PlAtom: 1.4.3 1.6
PlAtom!=()
PlAtom()
PlAtom==()
PlAtomkey,ValueTypevalue()
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()
V
template<ValueType,StoredValueType>AtomMap::AtomMap()