c++ static const struct initialization

Question

I have a class that changes the color of something based on button inputs. I'd like to predefine a bunch of "colors" so that a state machine can simply copy the constant structs defined in header to some variable to change colors.

I don't understand why the compiler is hunky-dory with the below code but the Linker can't deal with it. Is there a better pattern for defining constant structs? Should I just make them non-static non-const and live with it?

class ColorManager {
 public:

  ColorManager () {
  }

  // ProcessButton uses the static constexpr structs defined below
  void ProcessButton(uint8_t button_state);

 private:
  // Color_t is defined elsewhere as a struct of R, G, B fields
  static constexpr Color_t kColorWhite  = {100, 100, 100};
  static constexpr Color_t kColorRed    = {100, 0, 0};
  static constexpr Color_t kColorPurple = {100, 0, 70};

};

Edit: Here's an example error message:

src/color_manager.o: In function `ColorManager::ProcessButton(uint8_t button_state)':
color_manager.cc:(.text+0x224): undefined reference to `ColorManager::kColorWhite'
collect2: error: ld returned 1 exit status

Where and how is "Color_t is defined elsewhere as a struct of R, G, B fields"? How is that source being linked with the final program? — David C. Rankin
– David C. Rankin, Commented Dec 27, 2018 at 6:08
It's defined in another header file that is included in the color_manger.hh — kkemper
– kkemper, Commented Dec 27, 2018 at 6:09
Is it defined or simply declared as a forward declaration with the definition in another source? If the latter, you would need to link the additional source. As it sits, there is nothing inherently wrong with what you have posted and where you say the struct is defined. (not even from a -std=c++11 standpoint) — David C. Rankin
– David C. Rankin, Commented Dec 27, 2018 at 6:14

StoryTeller - Unslander Monica · Accepted Answer · 2018-12-27 06:10:49Z

3

Prior to C++17 those constexpr static member declarations are not defintions. You need to provide a definition in some translation unit.

Color_t ColorManager::kColorWhite;

In C++17 those would be implicitly inline variables.

edited Dec 27, 2018 at 6:10

answered Dec 27, 2018 at 6:08

StoryTeller - Unslander Monica

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4 Comments

kkemper Over a year ago

I see so if I move to C++17 all will be well? Is there any usual pattern people follow for this situation prior to C++17?

StoryTeller - Unslander Monica Over a year ago

@kkemper - We would add a definition in a cpp file and proceed to compile and link it.

kkemper Over a year ago

I was afraid of that. It just seems messy but I might have to live with it.

StoryTeller - Unslander Monica Over a year ago

@kkemper - Mind you that you only need a definition if those constans are odr-used (i.e. need to have an address)

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