lst-0032-godb.cpp
//#(compile) c++; compiler:g132; options:-O3 -std=c++23; libs:-
// https://godbolt.org/z/9PMcz1cqP
friend void swap(MxStack& re, MxStack& li) {
if(&re==&li) return; // Same address? Swapping with itself is unnecessary
std::lock( re.mx_, li.mx_ ); // multiple locks simultaneously
std::lock_guard lkre{re.mx_, std::adopt_lock}; // already locked
std::lock_guard lkli{li.mx_, std::adopt_lock}; // already locked
std::swap(li.data_, re.data_); // perform swap
}