host_data.md
In rswipl: Embed 'SWI'-'Prolog'

class host_data {#class-host_data}

Class host_data deals with representing native host-data in XPCE objects. A host_data object contains a void * handle that can be used by the interface to the host-language to pack native data in an XPCE object.

Objects of class host_data are only acceptable to types that explicitely include this type. I.e. the generic types any and object do not accept instances of this class. This is primarily for backward compatibility reasons: a Prolog term should be translated as quickly as possible into an XPCE object to avoid multiple places where this translation happens, and thus unexpected and unintended creation of multiple different objects from the same Prolog term.

Except for class host, no methods in XPCE accept objects of class host_data. Usage of this class and its associated type therefore only makes sense in the context of classes defined in the host-language.

See class prolog_term and ``Programming in XPCE/Prolog.

host_data->initialise: alien:void * Instances of class host_data are created using special interface code between the host and XPCE. Explicit creation of instances is not allowed. The following example stores host-data in an instance-variable:
```
:- pce_begin_class(term, object,
                   "Represent a prolog term").

variable(value, prolog, both, "represented value").

initialise(T, Term:prolog) :->
    send_super(T, initialise),
    send(T, value, Term).

:- pce_end_class.
```