winapivisual-c++c++-cxc++-winrt

C++/CX using abstract + final keywords together

I'm working on a C++/CX project and just came across the following definition:

#pragma once

struct ColorHelper abstract final {
    static COLORREF Lighten(COLORREF color, int amount);
    static COLORREF Darken(COLORREF color, int amount);
    static bool IsSystemThemeDark();
};

I'm a bit confused. The abstract keyword tells me that I cannot instatiate this class but the final keyword says that I cannot derive from it, so what gives? Is this supposed to be like a namespace? Or maybe something like a static class in C#?

Solution

  • The abstract keyword tells me that I cannot instatiate this class but the final keyword says that I cannot derive from it, so what gives?

    That's correct. The only available operation for this type is accessing its static class members.

    Is this supposed to be like a namespace?

    Yes, pretty much. If ColorHelper was a C++ namespace it would have an identical interface as far as clients are concerned.

    So then, why is it not a namespace and rather a type with a somewhat obscure definition?

    Because not all programming languages (such as C#) support calling free functions. Since ColorHelper is a Windows Runtime type there are restrictions on how it can be modeled to support cross-language interoperability. Specifically, all functions need to be implemented on a type, even when that means introducing a type that can never be instantiated.

    In essence, an abstract final type with static class members exclusively is the moral equivalent of a C++ namespace expressed in a way that makes it accessible across all supported Windows Runtime languages.