Singleton
Ensures a class has only one instance and provides a global point of access to it.
errorWhat Problem Does the Singleton Pattern Solve?
Some resources—database connections, configuration managers, logging services—should exist as a single shared instance. Creating multiple instances wastes memory, causes inconsistent state, and can lead to race conditions in concurrent environments.
check_circleHow the Singleton Pattern Works
Make the constructor private and provide a static method that always returns the same instance. On first call, the instance is created; on subsequent calls, the existing instance is returned. Thread-safe variants use double-checked locking or language-level guarantees.
Singleton Pattern Architecture
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Implementation by Language
Singleton Pattern in the Real World
“Think of a country’s president. There can only be one at any time. When anyone needs to communicate with the president, they don’t create a new one—they access the existing one through the official channel (the static method).”
Frequently Asked Questions
helpWhat is the difference between a Singleton and a static class?
A static class cannot implement interfaces, be passed as a parameter, or support lazy initialization. A Singleton is a regular object that can participate in polymorphism, dependency injection, and testing. Use Singleton when you need instance-level features; use static classes for pure utility methods.
helpWhy is Singleton considered an anti-pattern by some developers?
Singleton introduces global state, which makes unit testing difficult because tests share the same instance. It also creates hidden dependencies — classes use the Singleton without declaring it in their constructor. Modern alternatives include dependency injection containers that manage object lifetimes.
helpHow do I make a Singleton thread-safe?
In TypeScript/JavaScript, the single-threaded event loop makes Singletons inherently safe. In multi-threaded languages like C++ or C#, use double-checked locking, a static initializer, or language-specific features like C#'s Lazy<T> or C++'s std::call_once.
How the Singleton Pattern Relates to Other Patterns
The Singleton pattern is frequently combined with the Factory Method to control how instances are created. When a singleton becomes a bottleneck, consider the Flyweight pattern for sharing fine-grained state instead.