Re: implementation of "Double-Checked Locking" Pattern in C++
On Apr 24, 8:28 am, sk.sma...@gmail.com wrote:
Hi All,
The crux of the problem lies in the way C++ objects are created. They
are not always created in the same manner.
With "pInstance = new Singleton", three things happen:
1. Memory is allocated to hold a Singleton object.
2. The Singleton is constructed inside the allocated memory.
3. The address of the allocated memory is assigned to pInstance.
But the compiler is not constrained to perform these steps in this
order! This creates a problem when step two and three are swapped:
1. Memory is allocated to hold a Singleton object.
2. The address of the allocated (un?initialized) memory is assigned to
pInstance.
3. The Singleton is constructed inside the allocated memory.
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 ?
I don't think that is particularly the problem. To solve that, all
you would have to do is something along these lines:
if (pInstance == 0)
{
Singleton * pTmp = new Singleton;
pInstance = pTmp;
}
Now, pInstance isn't set until the object is fully constructed. I
think the bigger problem is that the second if() statement can be
totally elided using the as if rule because the compiler already knows
that pInstance is 0 from the first check. Of course, a compiler that
clever might also get rid of my temporary, but the temporary could be
scoped differently to prevent that (i.e made global or static).
The problem is that C++ isn't thread aware and there isn't a way to
signal "I'm being tricky here and order is important, don't you be
tricky too" to the compiler. Some compiler's have been doing some
things with volatile to try to help out with that, but with mixed
results.
Hope something in there helps.
joe
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