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In object-oriented programming, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance. It is one of the well-known "Gang of Four" design patterns , which describe how to solve recurring problems in object-oriented software. [ 1 ]
The book includes examples in C++ and Smalltalk. It has been influential to the field of software engineering and is regarded as an important source for object-oriented design theory and practice. More than 500,000 copies have been sold in English and in 13 other languages. [1] The authors are often referred to as the Gang of Four (GoF). [2] [3 ...
The original form of the pattern, appearing in Pattern Languages of Program Design 3, [2] has data races, depending on the memory model in use, and it is hard to get right. Some consider it to be an anti-pattern. [3] There are valid forms of the pattern, including the use of the volatile keyword in Java and explicit memory barriers in C++. [4]
This pattern can be implemented in several ways depending on the host programming language, such as the singleton design pattern, object-oriented static members in a class and procedural global functions. In Python, the pattern is built into the language, and each .py file is automatically a module.
In a software design pattern view, lazy initialization is often used together with a factory method pattern.This combines three ideas: Using a factory method to create instances of a class (factory method pattern)
For example, several GoF patterns, like the Factory method pattern, the Builder or even the Singleton are implementations of this concept. The Abstract factory pattern instead is a method to build collections of factories. In some design patterns, a factory object has a method for every kind of object it can create
Singleton pattern, a design pattern that allows only one instance of a class to exist; Singleton bound, used in coding theory; Singleton variable, a variable that is referenced only once; Singleton, a character encoded with one unit in variable-width encoding schemes for computer character sets
Lets take two singleton examples. On my 8bit Amstrad CPC running Symbos (awesome multiprogramming OS using the Z80s unprotected memory space), I write an application in C++ that has a singleton in it. But... it isn't a singleton from the 'system' point of view, it has 2 instances when I run 2 instances of the C application.