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c++thread-safetysingletonc++20

Leaky Meyers Singleton: is it threadsafe?

I implemented a Meyers Singleton, then realized it could be vulnerable to the destructor fiasco problem.

As a result, I changed the code to be:

Instance *getInstance()
{
    static Instance* singleton = new Instance();
    return singleton;
}

After implementing this, and no apparent bugs occuring, a coworker was implementing a different singleton and used std::call_once instead.

I've now realized that after much searching, I couldn't find if the "Leaky Meyers Singleton" is a threadsafe pattern. Should the leaky singleton be changed to std::call_once? Or is it threadsafe as-is?

Is the pointer considered a "block-scope" variable? If so I think it would be thread-safe, but if not there's significant bugs introduced with the current leaky singleton approach.

Solution

  • Yes, this is thread-safe: no two threads will ever try to initialize the same variable with static storage duration at the same time. That includes the entirety of evaluating the initializer.