BloombergLP::bslmt::Once::callOnce

If no other thread has yet called enter or callOnce, then call the specified function and set this object to the state where pending and future calls to enter or callOnce will return false or do nothing, respectively. Otherwise, wait for the one‐time code to complete and return without calling function where function is a function or functor that can be called with no arguments. Note that one‐time code is considered not to have run if function terminates with an exception.