How to atomically update a maximum value?

Question

In serial code, updating a maximum could be accomplished simply by

template<typename T> void update_maximum(T& maximum_value, T const& value) noexcept {   if(value > maximum_value) maximum_value = value; } 

However, how should this be done for an atomic<T> variable holding the maximum value:

template<typename T> void update_maximum(std::atomic<T>& maximum_value, T const& value) noexcept {    // want a lock-free, thread-safe implementation } 

Obviously, the code from the serial version doesn't work, as another thread may alter maximum_value between the load and the store. Can one use the compare_exchange (which compares == rather than >) to implement this? how?

Note that explicit locks are not allowed (the only lock allowed is that which may come with the implementation of std::atomic<T>).

Answer 1

It doesn't seem to be possible in a single operation, but you can make a loop, that tries to do this, until it finally succeeds or value in atomic variable becomes bigger than value:

template<typename T> void update_maximum(std::atomic<T>& maximum_value, T const& value) noexcept {     T prev_value = maximum_value;     while(prev_value < value &&             !maximum_value.compare_exchange_weak(prev_value, value))         {} }