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A class diagram exemplifying the singleton pattern. 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 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]
In software engineering, the initialization-on-demand holder (design pattern) idiom is a lazy-loaded singleton. In all versions of Java, the idiom enables a safe, highly concurrent lazy initialization of static fields with good performance. [1] [2]
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 Java, the multiton pattern can be implemented using an enumerated type, with the values of the type corresponding to the instances. In the case of an enumerated type with a single value, this gives the singleton pattern. In C#, we can also use enums, as the following example shows:
Design Patterns: Elements of Reusable Object-Oriented Software (1994) is a software engineering book describing software design patterns. The book was written by Erich Gamma , Richard Helm , Ralph Johnson , and John Vlissides , with a foreword by Grady Booch .
See Singleton pattern. A singleton is also a variable that is only used once in a program, usually indicating a programming mistake. See singleton variable. From the start of the term 'Singleton pattern': In software engineering, the singleton design pattern is designed to restrict instantiation of a class to one (or a few) objects.
Singleton object: only instance of its class for the lifetime of the program; Filter object: receives a stream of data as its input and transforms it into the object's output; As an example of an object anti-pattern, the God object knows or does too much.