---

This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 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. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. ________________________________________________________________________

Design pattern: Null object

Description: "Instead of using a null reference to convey absence of an object (for instance, a non-existent customer), one uses an object which implements the expected interface, but whose method body is empty. The advantage of this approach over a working default implementation is that a null object is very predictable and has no side effects: it does nothing."

This pattern can be used with both classes and prototypes. The description above is from the Wikipedia page on this pattern:

https://en.wikipedia.org/wiki/Null_object_pattern

This pattern is not described in the GoF book but is found in books by Martin Fowler and Joshua Kerievsky. See the Wikipedia page for details and references.

Logtalk doesn't provide a "null" built-in object by design. When the equivalent to a null object is necessary, one can be defined as e.g. defined in this pattern. Note that Logtalk provides a library implementation of "optionals", which are often a better solution.