Singleton

Definition

• A component created only once

Motivation

• For some component, it only makes sense to have one instance of it
  • Database repository
  • Object Factory
• Constructor call is expensive
  • Only do it once
  • Provide any one with the same instance
• Prevent anyone from creating additional copy
• Need to take care of lazy instantiation and thread safety

Implementation

• Make a safe singleton:
  • Hide or delete type’s constructor, copy constructor or assignment operator
  • Create a static method that returns a reference to a static member
  • Guaranteed to be thread-safe since C++11
• Types with hard dependencies Singleton are difficult to test
  • Cannot decouple singleton and supply a fake object
• Consider depending on abstraction
• Consider defining singleton lifetime using DI container

Singleton In Dependency Injection