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

In C++17 you can write

if (static int i; i == 0 && (i = 1)){

in order to avoid playing around with i in the loop body. i starts with 0 (guaranteed by the standard), and the expression after the ; sets i to 1 the first time it is evaluated.

Note that in C++11 you could achieve the same with a lambda function

if ([]{static int i; return i == 0 && (i = 1);}()){

which also carries a slight advantage in that i is not leaked into the loop body.