Singleton Pattern: Initialization, Concurrency, and Trade-offs

All flashcards

What is the singleton pattern?

It is a design pattern that restricts a class to one instance and provides a way to access that instance.

How can a singleton differ from a global variable?

A singleton can avoid occupying the global namespace and may delay allocating and initializing its object until it is needed.

Why might a logging component use a singleton?

It can offer many objects a uniform access point for writing messages to one shared logging destination.

What is a facade object?

An object that provides a simplified interface to a subsystem; one facade may be sufficient for an application.

What does singleton mean as a dependency injection service lifetime?

The dependency injection container creates one instance of a service and shares that same instance throughout the application. The service can be an ordinary class rather than a class with a private constructor and static instance method.

Quiz questions

1. When does an eagerly instantiated singleton create its instance?Answer: During static initialization, before the access method is first called.
2. What thread-safety guarantee does C++11 provide for a function-local static used in the Meyers singleton?Answer: Its initialization is thread-safe without manual locks.
3. Why can a globally accessible singleton make unit testing harder?Answer: Code that looks it up directly can acquire hidden dependencies.
4. What does dependency injection focus on, rather than requiring a class to decide how many instances exist?Answer: How dependencies are passed to an object.
5. What design concern arises because a singleton both performs its normal work and enforces uniqueness?Answer: It can conflict with the single-responsibility principle.