Properly deleting a singleton

Question

I have the following code:

MyClass.h:

static MyMutex instanceMutex;
static MyClass* getInstance();
static void deleteInstance();

MyClas

Answer 1

Here be a problem:

if (theInstance == 0)  // <- Some other thread might see a non-null theInstance 
{                      // before the constructor below returns

    instanceMutex.acquire();
    if (theInstance == 0)
    {
        theInstance  = new MyClass();  // <- This might set theInstance to something 
                                       // before the constructor returns.
    }
    instanceMutex.release();
}

You may want to implement some sort of reference counting system (like using shared_ptr) and initializing it in a similar manner, though taking care to ensure that its instance pointer is not set before the initialization completes.

If you are feeling adventurous, you could also try:

if (theInstance == 0)
{
    instanceMutex.acquire();
    if (theInstance == 0)
    {
        MyClass * volatile ptr = new MyClass();
        __FuglyCompilerSpecificFenceHintThatMightNotActuallyDoAnything();
        theInstance = ptr;  // <- Much hilarity may ensue if this write is not atomic.
    }
    instanceMutex.release();
}

This might fix it, and it might not. In the second case, it depends on how your compiler handles volatile, and weather or not pointer-sized writes are atomic.