Re: New Singleton Scheme
It has been known for many years that the double-checked locking pattern
is fundamentally broken. It cannot be implemented portably. Google
Groups will yield thread upon thread on this topic.
What about this implementation:
class Singleton
{
public:
static Singleton *instance();
private:
Singleton() { }
static volatile bool mInitialized;
static Singleton *mInstance;
};
volatile bool Singleton::mInitialized = false;
Singleton *Singleton::mInstance = 0;
Singleton *Singleton::instance()
{
if (not mInitialized)
{
/* scope */
{
Lock lock;
if (mInstance == 0)
{
mInstance = new Singleton;
}
}
mInitialized = true;
}
return mInstance;
}
Precisely it is not double check, that's the idea. The scope is
probably not necessary - it causes, that lock will be released before
setting mInitialized - another "sequence point" added. Also mInstance
is not declared as volatile, because protected by Lock.
--
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[ comp.lang.c++.moderated. First time posters: Do this! ]
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