CODEWITHSUNDEEP
c++visual-studioconstexpr
Hans Olsson
Hans Olsson

Reputation: 12517

How to use constexpr bool instead of preprocessor flag without warnings

I tried to use a constexpr bool instead of a repeating a pre-processor flag in the following code (simplified):

int main()
{
   constexpr bool b
#if SOME_FLAG
         =true;
#else
         =false;
#endif
    bool a=foo(1);
    if (b && a) bar(2);
}

https://godbolt.org/z/ear8qYxYY

The problem is the line if (b && a) where Visual Studio 2019 with warnings enabled (and preprocessor flag not defined) say:

warning C4127: conditional expression is constant note: consider using 'if constexpr' statement instead

Gcc and Clang seems to accept it.

I know that I can avoid the warning by using if (a && b) instead (without generating worse code), but it looks a bit odd. I also understand that I could write if constexpr(a) if (b), but it would look even weirder and risk problems, and similarly for ignoring or silencing the warning.

Is there a better way to avoid the warning for Visual Studio?

Upvotes: 1

Views: 304

Answers (1)

user1196549
user1196549

Reputation:

Why not

int main()
{
    bool a=foo(1);
#if SOME_FLAG
    if (a) bar(2);
#endif
}

?

You can also just disable the warning with a #pragma. https://learn.microsoft.com/en-us/cpp/preprocessor/warning?view=msvc-170

Upvotes: 2

Related Questions