在结构中定义常量

Question

I'm trying to create a struct with some constants in it like this:

#include <CoreAudio/CoreAudio.h>
...
struct properties {
    //Volume control
    const AudioObjectPropertyAddress volume = {
        kAudioDevicePropertyVolumeScalar, //mSelector
        kAudioDevicePropertyScopeOutput, //mScope
        0 //mElement
    };
    //Mute control
    const AudioObjectPropertyAddress mute = { 
        kAudioDevicePropertyMute,
        kAudioDevicePropertyScopeOutput,
        0
    };
};

However, I cannot access the constants in this class;

//Function used to for example set volume level or set mute status
//bool setProperty(UInt32 data_to_write, AudioObjectPropertyAddress addr_to_write_to);
//Following line should mute the system audio
setProperty(1, properties::mute);

This will make the compiler return the following error:

error: invalid use of non-static data member 'mute'

So, I tried making the constants static like this:

const static AudioObjectPropertyAddress volume = { ...

But now, I get a different error:

error: in-class initializer for static data member of type 'const AudioObjectPropertyAddress' requires 'constexpr' specifier

The last thing I tried is changing const static to static constexpr , however again, I cannot access the constants. Every time I try to access them, the compiler show this error:

Undefined symbols for architecture x86_64:
  "properties::mute", referenced from:
      _main in main-fefb9f.o
ld: symbol(s) not found for architecture x86_64

I'm not really sure what's going on here, I tried converting my struct into a class, but I ended up getting the same Undefined symbols error. I know I could just define the two constants as global variables, but I thought putting them into a struct/class would make the code look "nicer" or just more organized. Could someone please explain what's wrong here and provide a possible solution?

Answer 1

why not just properties.volume and properties.mute or use namespace otherwise...

namespace properties 
{
    //Volume control
    const AudioObjectPropertyAddress volume = {
        kAudioDevicePropertyVolumeScalar, //mSelector
        kAudioDevicePropertyScopeOutput, //mScope
        0 //mElement
    };

    //Mute control
    const AudioObjectPropertyAddress mute = { 
        kAudioDevicePropertyMute,
        kAudioDevicePropertyScopeOutput,
        0
    };
};

Answer 2

•  struct properties { //Volume control const AudioObjectPropertyAddress volume = { kAudioDevicePropertyVolumeScalar, //mSelector kAudioDevicePropertyScopeOutput, //mScope 0 //mElement }; };

  is valid, but volume is not a static member, so usage requires an instance:

   setProperty(1, properties{}.volume);

• With static const , it would be:

   struct properties { //Volume control static const AudioObjectPropertyAddress volume; }; const properties::AudioObjectPropertyAddress volume { kAudioDevicePropertyVolumeScalar, //mSelector kAudioDevicePropertyScopeOutput, //mScope 0 //mElement };

  and usage might be the expected:

   setProperty(1, properties::volume);

• with static constexpr in C++11/C++14:

   struct properties { //Volume control static constexpr AudioObjectPropertyAddress volume{ kAudioDevicePropertyVolumeScalar, //mSelector kAudioDevicePropertyScopeOutput, //mScope 0 //mElement }; }; const properties::AudioObjectPropertyAddress volume; // Definition when ODR-used

• with static /*inline*/ constexpr since C++17:

   struct properties { //Volume control static /*inline*/ constexpr AudioObjectPropertyAddress volume{ kAudioDevicePropertyVolumeScalar, //mSelector kAudioDevicePropertyScopeOutput, //mScope 0 //mElement }; };

Answer 3

Here are a list of things you can try:

You could.... redesign your templating/types. Struct vs Namespace . Without seeing the whole project its tough to guide in a specific direction. Its up to you to decide if you need to use a struct, enum, namespace, class to encapsulate your code.

As for const static declaration. Here is a post describing valid methods of static class membder declarations. Pay close attention to your c++ version.

"Prior to C++11, only static const data members of integral or enumeration type could have initializers in the class definition."

Lastly, the static constexpr undefined. Is your static var in the same header/source file as where you are calling it? (Likely not, as the error shows the call from main). Static variables are not visible to other files .

If you want to modify static vars you could do so using a public get/set from a class function.

Suppose 'mute' is a single uint8_t value. Consider using a namespace with all static data declared as: static const uint8_t mute.