How to implement thread safe reference counting in C++

do
   read the count
   perform mathematical operation
   interlockedcompareexchange( destination, updated count, old count)
until the interlockedcompareexchange returns the success code.

a = b;

if (a != null)
    if (InterlockedDecrement(ref a.m_ref) == 0)
            a.FinalRelease();

if (b != null)
    InterlockedIncrement(ref b.m_ref);

a = b;