How do I declare a static const struct in C++?

Question

I have an LEDControl class which sets the color of an LED using a method. Many classes use this LEDControl class to pass it some colors to assign to an LED. Therefore, I wanted to define the colors somewhere as a constant.

I thought I'd make a struct in my LEDControl class called Color, since I really only need to access the members directly and never anything else:

struct Color{
    bool r;
    bool g;
    bool b;
};

I then added a #define in the .cpp

#define RED Color{true, false, false};

But this didn't work; it is not declared in the scope of any other class.

What can I do to store a set of colors somewhere in my program so that every class using the LEDControl can use keywords or variable names like RED and GREEN?

Possible duplicate of [Defining global constant in C++](https://stackoverflow.com/questions/2268749/defining-global-constant-in-c) — underscore_d, Oct 23 '17 at 13:35
As a rule of thumb, #defines are evil. if you really want to do something make a `const Color red{}` but this isn't really what you want either... — UKMonkey, Oct 23 '17 at 13:35
Unless you include the header that defines `Color`, you can't use it. There is no portable way to make an identifier "globally available". — StoryTeller - Unslander Monica, Oct 23 '17 at 13:39
Wonder why the down voters did so ... they didn't seem to comment on why. — UKMonkey, Oct 23 '17 at 13:43

Vittorio Romeo · Answer 1 · 2017-10-23T13:40:53.033

4

Instead of using macros, provide inline constexpr instances of your struct:

namespace color
{
    inline constexpr Color red{true, false, false};
    inline constexpr Color green{false, true, false};
    inline constexpr Color blue{false, false, true};
}

You can provide these in an header file so that they will be accessible from any file including it.

edited Oct 23 '17 at 13:40

answered Oct 23 '17 at 13:39

Vittorio Romeo

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is `inline` variables live yet? thought it was for '17 – sp2danny Oct 23 '17 at 13:41
Where would I define the struct itself? – Zimano Oct 23 '17 at 13:41
@Zimano: anywhere you want, as long as it's before the `red`/`green`/`blue` definitions and visible there. – Vittorio Romeo Oct 23 '17 at 13:42
@sp2danny: yes, `inline` is in 17. – Vittorio Romeo Oct 23 '17 at 13:42
@VittorioRomeo And the namespace I define in a separate header, right? – Zimano Oct 23 '17 at 13:44
@Zimano: sure. You can define it anywhere you want. If you want multiple `.cpp` files to use it, define it in an header and include it in every `.cpp` file. – Vittorio Romeo Oct 23 '17 at 13:49

score 0 · Answer 2 · answered Oct 23 '17 at 13:39

0

put the definition in a header. Please avoid #define
it could look like:

static constexpr Color RED{true, false, false};

answered Oct 23 '17 at 13:39

sp2danny

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2

That `static` is unnecessary, actually. – StoryTeller - Unslander Monica Oct 23 '17 at 13:40

How do I declare a static const struct in C++?

2 Answers2