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

C++ does have a builtin control flow primitive that consists of "(before-block; condition; after-block)" already:

for (static bool b = true; b; b = false)

Or hackier, but shorter:

for (static bool b; !b; b = !b)

However, I think any of the techniques presented here should be used with care, as they are not (yet?) very common.