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

Based on @Bathsheba's great answer for this - just made it even simpler.

In C++ 17, you can simply do:

if (static int i; !i++) {
  cout << "Execute once";
}

(In previous versions, just declare int i outside the block. Also works in C :) ).

In simple words: you declare i, which takes default value of zero (0). Zero is falsey, therefore we use exclamation mark (!) operator to negate it. We then take into account the increment property of the ++ operator, which first gets processed (assigned, etc) and then incremented.

Therefore, in this block, i will be initialized and have the value 0 only once, when block gets executed, and then the value will increase. We simply use the ! operator to negate it.