Re: Please disprove this Double-Checked Locking "fix"
On 4/26/2011 10:50 AM, Pete Becker wrote:
pinstance points to. The C++0x solution is:
#include <atomic>
std::atomic<Singleton*> pinstance;
if (pinstance == 0) {
Lock lock;
if (pinstance == 0)
pinstance = new Singleton;
}
return pinstance;
Making pinstance an atomic variable ensures that all writes that "happen
before" the assignment to pinstance in one thread are seen before the
result of that assignment is seen in any other thread.
True, but that's not enough to make this code work. You also need to
know that assignment to pinstance can't take place until the Singleton
constructor has run to completion. You have that guarantee in C++0x,
too, so the code is correct, but it's not just the use of atomics that
gives you that correctness. You also need the additional guarantees
that C++0x offers regarding the behavior of new expressions (compared to
the weaker guarantees that C++03 gives you).
Scott
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