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   SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
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       http://www.apache.org/licenses/LICENSE-2.0

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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: create_object/4; Built-in predicate
.. _predicates_create_object_4:

``create_object/4``
===================

Description
-----------

::

   create_object(Identifier, Relations, Directives, Clauses)

Creates a new, dynamic object. The word :term:`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 :term:`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 :ref:`methods_parameter_2` built-in execution context method.

Declared predicates (using scope directives 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 a valid object identifier but one of its arguments, ``Parameter``, is not a variable:
|     ``type_error(variable, Parameter)``
| ``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)],
            []
        ).

.. seealso::

   :ref:`predicates_abolish_object_1`,
   :ref:`predicates_current_object_1`,
   :ref:`predicates_object_property_2`,
   :ref:`predicates_extends_object_2_3`,
   :ref:`predicates_instantiates_class_2_3`,
   :ref:`predicates_specializes_class_2_3`,
   :ref:`predicates_complements_object_2`
