Question

I have been reading about thread safe singletons and the implementation I find everywhere has a getInstance() method something like this:

Singleton* getInstance()
{
    if ( !initialized )
    {
        lock();
        if ( !initialized )
        {
            instance = new Singleton();
            initialized = true;
        }
        unlock();
    }

    return instance;
}

• Is this actually thread safe?
• Have I missed something or is there a small chance this function will return an uninitialized instance because ‘initialized’ may be reordered and set before instance?

This article is on a slightly different topic but the top answer describes why I think the above code is not thread safe:

Why is volatile not considered useful in multithreaded C or C++ programming?

Answer 1

Not a good idea. Look for double check locking. For instance:

http://www.drdobbs.com/cpp/c-and-the-perils-of-double-checked-locki/184405726

http://www.drdobbs.com/cpp/c-and-the-perils-of-double-checked-locki/184405772