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Singleton

Singleton in Rust

Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.

Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break the modularity of your code.

You can’t just use a class that depends on a Singleton in some other context, without carrying over the Singleton to the other context. Most of the time, this limitation comes up during the creation of unit tests.

Rust specifics

By definition, Singleton is a global mutable object. In Rust this is a static mut item. Thus, to avoid all sorts of concurrency issues, the function or block that is either reading or writing to a mutable static variable should be marked as an unsafe block.

For this reason, the Singleton pattern can be percieved as unsafe. However, the pattern is still widely used in practice. A good read-world example of Singleton is a log crate that introduces log!, debug! and other logging macros, which you can use throughout your code after setting up a concrete logger instance, such as env_logger. As we can see, env_logger uses log::set_boxed_logger under the hood, which has an unsafe block to set up a global logger object.

  • In order to provide safe and usable access to a singleton object, introduce an API hiding unsafe blocks under the hood.
  • See the thread about a mutable Singleton on Stackoverflow for more information.

Starting with Rust 1.63, Mutex::new is const, you can use global static Mutex locks without needing lazy initialization. See the Singleton using Mutex example below.

Singleton in Other Languages

Singleton in C# Singleton in C++ Singleton in Go Singleton in Java Singleton in PHP Singleton in Python Singleton in Ruby Singleton in Swift Singleton in TypeScript