std call once std once flag

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std::call_once ensures execution of a function exactly once by competing threads. It throws std::system_error in case it cannot complete its task.

Used in conjunction with std::once_flag.

#include <mutex>
#include <iostream>

std::once_flag flag;
void do_something(){
      std::call_once(flag, [](){std::cout << "Happens once" << std::endl;});
      std::cout << "Happens every time" << std::endl;

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