Re: implementation of "Double-Checked Locking" Pattern in C++
sk.smawsk@gmail.com ha scritto:
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.
According to the recently proposed paper N2171
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2171.htm) this
particular kind of reordering will be forbidden. See, in particular, the
proposed amendement to 5.17p1: "[...] In all cases, the assignment is
sequenced after the value computation of the right and left operands,
[...]".
HTH,
Ganesh
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