Singleton

Ensure a class only has one instance and provide a global point of access to it

• Structural Pattern

Singleton

Singleton: Motivation

• Sometimes, a class must have exactly one instance, e.g., a class that maps to a physical resource
• How do we ensure that a class has only one instance and that the instance is easily accessible? A global variable makes an object accessible but doesn’t keep you from instantiating multiple objects.
• A better solution is to make the class itself responsible for keeping track of its sole instance, i.e., a Singleton

Singleton: Applicability

• Use the Singleton pattern when:
• There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
• When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code

Singleton: Structure

Singleton: Participants

• Singleton
• Defines an instance operation that lets clients access a unique instance
• The instance (in C++) is a static member function
• Class may be responsible for creating a unique instance (typical)

Singleton: Consequences

• Controls access to the sole instance
• In place of global variables, and avoids polluting the global namespace
• The Singleton class can be subclassed
• Once in place, you can change your mind and allow more than one instance
• More flexible than static member functions, which are not polymorphic (i.e., cannot make them virtual)

Implementation

Thread-Safe Implementation

DCL (Double-Checked Locking) Pattern

DCL Comparison

std::call_once()

std::call_once() Comparison

Related Patterns

• Abstract Factory, Builder, Prototype
• Can be implemented using the Singleton pattern

Discussion

• Some strong opinions against the necessity of using the Singleton pattern
• Singleton may be used to get around a poor design
• Why? A singleton is basically a global variable (object)
• Note that the implementations of many standard data structures are not entirely thread-safe