if constexpr statements

if constexpr statements

There is an alternative usage of constexpr in terms of “if” statement conditions (since C++17):

   if constexpr(cond)

This new syntax tags the condition as being amenable to computation at compile-time. Hence, the compiler should optimize the if statement to a constant value, and it can then determine at compile-time which branch should be executed. So, there is a double speedup from:

(a) the condition computation is removed at run-time, and

(b) code size reduction from unexecuted “dead code” being removed.

In fact, this determines at compile-time which code block will be parsed, so there are cases where you can avoid a compile-time error in templates by wrapping it inside an “if constexpr” check. This can be useful in compile-time situations such as template expansion, where you can prevent some expressions from being compiled, and also code bloat can be reduced.

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