C++ provides a number of features that make it possible to define a more natural and concise interface to dynamically typed languages than plain C does. Using programmable type-conversion (casting) and overloading, native data-types can be translated automatically into appropriate Prolog types, automatic destructors can be used to deal with most of the cleanup required and C++ exception handling can be used to map Prolog exceptions and interface conversion errors to C++ exceptions, which are automatically mapped to Prolog exceptions as control is turned back to Prolog.
More information on the SWI-Prolog native types is given in Interface Data Types.
It would be tempting to use C++ conversion operators and method
overloading to automatically convert between C++ types such as
std::string and int64_t and Prolog foreign
language interface types such as term_t and atom_t.
However, types such as term_t are unsigned integers, so
many of the automatic type conversions can easily do something other
than what the programmer intended, resulting in subtle bugs that are
difficult to find. Therefore Version 2 of this interface reduces the
amount of automatic conversion and introduces some redundancy, to avoid
these subtle bugs, by using "getter" methods rather than conversion
operators, and using naming conventions for explicitly specifying
constructors.
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).