Creational pattern · Visual logic
Singleton Pattern: One Instance, One Hidden Cost
Singleton guarantees one globally reachable instance, but the convenience behaves like shared global state and must be treated as a trade-off.
A process appears to need exactly one coordinator, yet unrestricted construction could create conflicting instances.
Control construction and expose one access point only when uniqueness is a real invariant of the running process.
- FIRST ACCESS checks whether the shared instance already exists.
- CONSTRUCTOR creates the instance only when no instance is available.
- LATER CALLS receive the same instance and therefore the same state.
The process observes one coordinated instance instead of accidentally creating competing copies.
Run it in the real world
A desktop application may coordinate one window registry, while a web service should usually inject scoped dependencies instead of assuming one global process.
Go deeper
This is an original explanation and example inspired by Head First Design Patterns. The book covers the complete pattern, design principles, and implementation detail.
Retail links are not affiliate links yet. If that changes, this page will clearly disclose it.