built-in predicate

create_object/4

Description

create_object(Identifier, Relations, Directives, Clauses)

Creates a new, dynamic object. The word object is used here as a generic term. This predicate can be used to create new prototypes, instances, and classes. This predicate is often used as a primitive to implement high-level object creation methods.

Note that, when opting for runtime generated object identifiers, it’s possible to run out of identifiers when using a backend Prolog compiler with bounded integer support. The portable solution, when creating a large number of dynamic objects in long-running applications, is to recycle, whenever possible, the identifiers.

When creating a new dynamic parametric object, access to the object parameters must use the parameter/2 built-in execution context method.

Declared predicates (using scope clauses in the Directives argument) are implicitly declared also as dynamic.

When using Logtalk multi-threading features, predicates calling this built-in predicate may need to be declared synchronized in order to avoid race conditions.

Modes and number of proofs

create_object(?object_identifier, @list(object_relation), @list(object_directive), @list(clause)) - one

Errors

Relations, Directives, or Clauses is a variable:

instantiation_error

Identifier is neither a variable nor a valid object identifier:

type_error(object_identifier, Identifier)

Identifier is already in use:

permission_error(modify, category, Identifier)

permission_error(modify, object, Identifier)

permission_error(modify, protocol, Identifier)

Relations is neither a variable nor a proper list:

type_error(list, Relations)

Repeated entity relation clause:

permission_error(repeat, entity_relation, implements/1)

permission_error(repeat, entity_relation, imports/1)

permission_error(repeat, entity_relation, extends/1)

permission_error(repeat, entity_relation, instantiates/1)

permission_error(repeat, entity_relation, specializes/1)

Directives is neither a variable nor a proper list:

type_error(list, Directives)

Clauses is neither a variable nor a proper list:

type_error(list, Clauses)

Examples

% create a stand-alone object (a prototype):
| ?- create_object(
         translator,
         [],
         [public(int/2)],
         [int(0, zero)]
     ).

% create a prototype derived from a parent prototype:
| ?- create_object(
         mickey,
         [extends(mouse)],
         [public(alias/1)],
         [alias(mortimer)]
     ).

% create a class instance:
| ?- create_object(
         p1,
         [instantiates(person)],
         [],
         [name('Paulo Moura'), age(42)]
     ).

% create a subclass:
| ?- create_object(
         hovercraft,
         [specializes(vehicle)],
         [public([propeller/2, fan/2])],
         []
     ).

% create an object with an initialization goal:
| ?- create_object(
         runner,
         [instantiates(runners)],
         [initialization(::start)],
         [length(22), time(60)]
     ).

% create an object supporting dynamic predicate declarations:
| ?- create_object(
         database,
         [],
         [set_logtalk_flag(dynamic_declarations, allow)],
         []
     ).

See also

abolish_object/1, current_object/1, object_property/2, extends_object/2-3, instantiates_class/2-3, specializes_class/2-3, complements_object/2