CODEWITHSUNDEEP
c++c++17if-constexpr
ge45mue
ge45mue

Reputation: 707

Wrong understanding of 'if constexpr'

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?

Upvotes: 2

Views: 383

Answers (2)

Cow Corporation
Cow Corporation

Reputation: 409

As you've observed, you can't have "garbage code" like that x codeline inside of a if constexpr clause.

The major advantage of having if constexpr instead of #if is that if constexpr is able to reason about types in a way that the preprocessor wasn't able to. This advantage is often exploited within templates.

For example, you could do something like the following:

template <typename T>
T squareOnlyInt(T num) {
    if constexpr (std::is_same<T,int>::value) {
      return num * num;
    }
    else {
      return num;
    }
}

Upvotes: -2

lubgr
lubgr

Reputation: 38267

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.

Upvotes: 8

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