compile-time evaluation of a function to compute a constant

Question

Preamble: I've looked at constexpr initializing static member using static function, but (thanks to Oleg Bogdanov's answer) I'm not trying to initialize a static.

I'd like to know how to make the following work:

typedef uint32_t color_t;       // represent color as 00rrggbb

class Color {
  static color_t makeColor(const uint8_t r,
                           const uint8_t g,
                           const uint8_t b) {
    return (((color_t)r << 16) | ((color_t)g << 8) | (color_t)b);
  }  
  static const color_t kRed = makeColor(255, 0, 0);
}

As I see it, the compiler needs to be told that it can evaluate makeColor() at compile time, so I think this is a job for constexpr. Despite my best efforts at sprinkling around constexpr and const, I still get

error: field initializer is not constant

What am I missing?

P.S.: I can certainly accomplish what I want with #define:

#define makeColor(r, g, b) (((color_t)(r) << 16) | ((color_t)(g) << 8) | (color_t)(b))

... but that seems so 20th century!

Answer 1

Actually you were very close to your goal, making makeColor constexpr would be enough if you remove static from kRed

Demo

Why exactly static is problematic is explained in this answer