Most elegant way to write a one-shot 'if'

Question

Since C++ 17 one can write an if block that will get executed exactly once like this:

#include 

        

Answer 1

You could wrap the one-time action in the constructor of a static object that you instantiate in place of the conditional.

Example:

#include 
#include 

struct do_once {
    do_once(std::function fun) {
        fun();
    }
};

int main()
{
    for (int i = 0; i < 3; ++i) {
        static do_once action([](){ std::cout << "once\n"; });
        std::cout << "Hello World\n";
    }
}

Or you may indeed stick with a macro, that may look something like this:

#include 

#define DO_ONCE(exp) \
do { \
  static bool used_before = false; \
  if (used_before) break; \
  used_before = true; \
  { exp; } \
} while(0)  

int main()
{
    for (int i = 0; i < 3; ++i) {
        DO_ONCE(std::cout << "once\n");
        std::cout << "Hello World\n";
    }
}