Assigning static const within a class and outside, why the difference?

Question

This is so wierd, i am coding a pixel manipulation library in C++. So i want to have a class filled with color presets. Color is a class with a struct that looks like:

Color(int r, int g, int b, int a)

And if i declare it outside of any classes as:

static const Color COL_BLACK = Color(0, 0, 0, 255);

That shows no error in Visual Studio Code. But however when i structure like this:

class Colors { public: static const Color Black = Color(0, 0, 0, 255); };

The equal sign has a red squiggly line under it, but when i hover my mouse over it doesn't tell me what is wrong. What is wrong?

Note that i want it to be called like so:

Color newCol = Colors::Black;

Try static inline const Color Black = Color(0, 0, 0, 255); for the static member in Colors. ;-) — Scheff's Cat
– Scheff's Cat, Commented Nov 18, 2021 at 16:45
static members in a class declaration are a potential violation of the One Definition Rule (ODR) because the class declaration (in a header) could be repeated. In older standards, it was necessary to move the definition of static members into one translation unit (a .cpp file). Since recent standards (I believe C++17), an alternative option is the static inline (with the requirement that the static inlined hast to be equal at every time when its seen by the compiler again). — Scheff's Cat
– Scheff's Cat, Commented Nov 18, 2021 at 16:48
Though, a static definition of a global variable in a header is not that clever as well. Each time the header is seen, a new symbol will be generated... This symbol is exclusively visible in the corresponding object file but it still appears to be a waste (to me). — Scheff's Cat
– Scheff's Cat, Commented Nov 18, 2021 at 16:52

jdt · Accepted Answer · 2021-11-18 17:06:24Z

Since C++ 17 you can simply use inline like in the following:

#include <iostream> class Color { public: Color(int r, int g, int b, int a) : r{ r }, g{ g }, b{ b }, a{ a } { } int r, g, b, a; }; class Colors { public: inline static const Color Black = Color(0, 0, 0, 255); }; int main() { Color c = Colors::Black; std::cout << c.r << "," << c.g << "," << c.b << "," << c.a << "\n"; }

Jojo · Accepted Answer · 2021-11-18 16:50:41Z

Read up a little on static initialization. In the meantime, in this case you could define Color with a constexpr ctor, like this:

struct Color { const unsigned int r; const unsigned int g; const unsigned int b; const unsigned int a; constexpr Color( const unsigned int r, const unsigned int g, const unsigned int b, const unsigned int a ) : r(r), g(g), b(b), a(a) {} };

And then you would initialize Colors like this:

class Colors { public: static constexpr Color Black = Color(0, 0, 0, 255); };

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Assigning static const within a class and outside, why the difference?

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