Detecting if an expression is constant in C

The Linux kernel has even a way to determine whether the expression is compile-time, WITHOUT aborting compilation in either case.

The trick is this (copied vebratim from Linux):

#define __is_constexpr(x) (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

Explanation: if x is a constant expression, then multiplying it by zero yields a constant 0, and casting a constant 0 to void* makes a null pointer constant. And the ternary expression, if one of its sides is a null pointer constant, collapses to the type of the other side (thus the type of the returned pointer will be int*, and the sizeof will match). And if x was not constant, then the lefthand side would not be considered a null pointer constant by type inference, the type of the ternary expression will be void*, and the sizeof check will not match.

With a few more clever tricks, it's even possible to implement a compile-time "type ternary expression", like this: TYPE_IF(2 * 2 == 4, int, long). This is left as an exercise for the reader.

> This works. But both gcc and clang warn about the enum being anonymous... even though that's exactly what I wanted to do. And this cannot be silenced with #pragma since it's a macro, so the warning occurs at the location where the macro is invoked.

You can use _Pragma instead of #pragma. E.g.

  #define C(x) ( \
    _Pragma("clang diagnostic push") \
    _Pragma("clang diagnostic ignored \"-Wvisibility\"") \
    (x) + 0*sizeof(void (*)(enum { tmp = (int)(x) })) \
    _Pragma("clang diagnostic pop") \
  )

I thought this would work:

#define C(x) (sizeof(char[x]), x)

sizeof is a compile-time operation so x need to be known at compile time.

It didn't work as expected. It turns out there is an exception and the standard says that sizeof is actually calculated at runtime specifically for variable length arrays:

> The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

It's remarkable that people will say that doing this kind of thing is better than learning a language which actually lets you enforce this with the type system.

(or even just insist that users use the version of the language which supports "constexpr"!)