Wrong understanding of 'if constexpr'

问题

I have following code

static constexpr bool condition = true;

int square(int num) {
    if constexpr (condition) {
      return num * num;
    } else {
      x
      return num;
    }
}

int main() {
    return square(3);
}

compiled with

-std=gnu++17

My assumption for

if constexpr (condition)

was that during compilation the part

    } else {
      x
      return num;
    }

get discarded and I get no error about the undefined

x

Is my understanding wrong that this 'if constexpr' is something like

#ifdef CONDITION
  return num * num;
#else
  x
  return num;
#endif

How do I modify this code that I can compile this?

Thanks for the help

回答1:

How do I modify this code that I can compile this?

To fix your code, simply remove the line with x.

Is my understanding wrong that this 'if constexpr' is something like [...]

Yes, your understanding is wrong. From cppreference:

Outside a template, a discarded statement is fully checked. if constexpr is not a substitute for the #if preprocessing directive.

This means every branch in an if constexpr block must be valid C++, even if it's never going to be used.

来源：https://stackoverflow.com/questions/58300760/wrong-understanding-of-if-constexpr