Re: implementation of "Double-Checked Locking" Pattern in C++
Pedro Lamar?o wrote:
On 24 abr, 09:28, sk.sma...@gmail.com wrote:
What will happen when thread A is suspended just after it finishes
step two and thread B enters the ?Instance()? function?
It will detect that pInstance is not 0 anymore and may exit the
function, returning the address to a section of memory that has not
been initialized. Thus the code that was calling the Instance function
could dereference a Singleton object that has not been constructed
yet! Chaos ?
Singleton* Singleton::Instance() {
if (0 == pInstance) {
Guard lock(m_mutex);
if (0 == pInstance) {
Singleton* tmp = new Singleton(); // pInstance still zero
pInstance = tmp; // pointer assignment is atomic (right?)
}
}
return pInstance;
}
Do we still need the inner if-clause?
Wrong question, it's the outer if that becomes redundant. The idea is
to avoid an unnecessary lock it the instance has already been allocated.
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