Single bit manipulations with guaranteed atomicity

Question

Is there a way to set, clear, test and flip a single bit as an atomic operation in c++? For example bitwise variants to "compare_and_swap".

Answer 1

Manipulating bits atomically requires a compare_exchange RMW to avoid touching other bits in the atomic variable. Testing a bit is not a modifying operation, therefore a load() suffices.

You will have to add range error checking

template<typename T, typename OP>
T manipulate_bit(std::atomic<T> &a, unsigned n, OP bit_op)
{
    static_assert(std::is_integral<T>::value, "atomic type not integral");

    T val = a.load();
    while (!a.compare_exchange_weak(val, bit_op(val, n)));

    return val;
}

auto set_bit = [](auto val, unsigned n) { return val | (1 << n); };
auto clr_bit = [](auto val, unsigned n) { return val & ~(1 << n); };
auto tgl_bit = [](auto val, unsigned n) { return val ^ (1 << n); };


int main()
{
    std::atomic<int> a{0x2216};

    manipulate_bit(a, 3, set_bit);  // set bit 3

    manipulate_bit(a, 7, tgl_bit);  // toggle bit 7

    manipulate_bit(a, 13, clr_bit);  // clear bit 13

    bool isset = (a.load() >> 5) & 1;  // testing bit 5
}