design patterns · high

Thread-Safe Singleton

Why the naive lazy singleton is a data race, and how a function-local static (Meyers' singleton) gives thread-safe, lazy, single init.

The code

// Naive (BROKEN under threads)
Singleton* getInstance() {
    if (!instance) // race: two threads see null
        instance = new Singleton();
    return instance;
}

// Meyers' singleton (thread-safe since C++11)
Singleton& getInstance() {
    static Singleton inst; // initialized once, thread-safely
    return inst;
}

What this lesson walks through

1. 01Naive lazy singleton
2. 02Both threads see null
3. 03Both create -> TWO instances
4. 04Fix: Meyers' singleton
5. 05C++11 guarantees one-time init
6. 06Both get the SAME instance
7. 07When you can't use a static local

The classic lazy singleton: if the instance pointer is null, create it. This looks fine single-threaded.