Just posted on Sutter's Mill, "Reader Q&A: How to write a CAS loop using std::atomics." From the post:

A colleague asked:

How should one write the following “conditional interlocked” function in the new C++ atomic<> style?

[...]

The short answer is:

T old = a;
while( old >= 0 && !a.compare_exchange_weak( old, old+val ) )
    { }

For the longer answer, continue reading...