C++ increment std::atomic_int if nonzero

Question

I'm implementing a pointer / weak pointer mechanism using std::atomics for the reference counter (like this). For converting a weak pointer to a strong one I need to atomically

• check if the strong reference counter is nonzero
• if so, increment it
• know whether something has changed.

Is there a way to do this using std::atomic_int? I think it has to be possible using one of the compare_exchange, but I can't figure it out.

Answer 1

Given the definition std::atomic<int> ref_count;

int previous = ref_count.load();
for (;;)
{
    if (previous == 0)
        break;
    if (ref_count.compare_exchange_weak(previous, previous + 1))
        break;
}

previous will hold the previous value. Note that compare_exchange_weak will update previous if it fails.

Answer 2

This should do it:

bool increment_if_non_zero(std::atomic<int>& i) {
    int expected = i.load();
    int to_be_loaded = expected;

    do {
        if(expected == 0) {
            to_be_loaded = expected;
        }
        else {
            to_be_loaded = expected + 1;
        }
    } while(!i.compare_exchange_weak(expected, to_be_loaded));

    return expected;
}