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_errorIdentifier 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)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)],
[]
).