atomicity in C++

Without the requirements of crossplatforming you can use WInAPI and its Interlocked* functions.

Specifically to your question can not be answered. But the writing of the framework U++ where all these things are present in a cross-platform way:
Atomic t;
AtomicWrite(t, 1);

In GCC have built-in functions of the form _sync_val_compare_and_swap().
Under Solaris you can do #include where there is a sort function atomic_cas().
Windows — API from the category of InterlockedCompareExchange().
\r
Universal solutions are not know. I met several times for this case inline assembler #ifdef'AMI for different platforms.