Struct is non-literal type

Question

struct rgb_color {
    constexpr rgb_color(std::uint8_t nr, std::uint8_t ng, std::uint8_t nb) :
        r(nr), g(ng), b(nb) { }

    std::uint8_t r; // red
    std::uint8_t g; // green
    std::uint8_t b; // blue

    constexpr static rgb_color black = rgb_color(0, 0, 0);
    constexpr static rgb_color white = rgb_color(255, 255, 255);
};

The constexpr static constant definitions fail to compile:

constexpr variable cannot have non-literal type 'const rgb_color'

However according to http://en.cppreference.com/w/cpp/concept/LiteralType, const rgb_color should be a literal type, because it has only literal types as data members (std::uint8_t), and the constexpr constructor.

Why does the code not compile?

Also, is it necessary to define the constexpr static members in a .cc file, like

constexpr rgb_color rgb_color::black;

Answer 1

This doesn't work, because you are instantiating a type that is not fully declared yet (you have not reached the closing brace and semicolon yet, so rgb_color is still an incomplete type).

You can work around this by declaring your constants out of the class, maybe in their own namespace:

namespace rgb_color_constants {
    constexpr static rgb_color black = rgb_color(0, 0, 0);
    constexpr static rgb_color white = rgb_color(255, 255, 255);
}