C++ 'multiple definition' compile error with static enum class members

Question

This is an Arduino project compiled in Visual Studio (using visual micro plugin). I am getting the following error:

AutonomyHandler.cpp.o (symbol from plugin): In function AutonomyHandler::setup() const (.text+0x0): multiple definition of Module::AvailableCommandKeys ArduinoProject.cpp.o (symbol from plugin)*: (.text+0x0): first defined here

I am using an enum of CmdKeys within the class definition, and I can use the line of code below to get the available set of keys, but when I try to use it, I get multiple compile errors as seen above for each place I have used it.

Module::AvailableCommandKeys

My Module.h looks as follows:

#ifndef _MODULE_h
#define _MODULE_h

class Module {
public:
    enum CmdKeys { Forward, Left, Back, Right, Stop };
    static const CmdKeys AvailableCommandKeys[2];

    // other definitions...
};
const Module::CmdKeys Module::AvailableCommandKeys[] = { Forward, Back };

#endif

Does anyone know what is happening? I have had this issue before and making the members non-static fixed the issue but I want to keep these enum arrays static.

Move the line const Module::CmdKeys Module::AvailableCommandKeys[] = { Forward, Back }; to a .cpp file. Otherwise, every cpp file that #incluides Module.h will have a definition of Module::AvailableCommandKeys. — R Sahu
Yes, thank you for your help, I have posted the answer below. — oli_taz
This isn’t the problem, but names that begin with an underscore followed by a capital letter (_MODULE_h) and names that contain two consecutive underscores are reserved for use by the implementation. Don’t use them in your code. — Pete Becker

oli_taz oli_taz · Accepted Answer · 2017-12-14T22:44:24

Since writing this post I found the answer so I thought I would post to help others anyway.

To fix the issue, you just need to move the initialisation of the static members from the definition file (.h) to the declaration file (.cpp)

Module.h looks as follows:

#ifndef _MODULE_h
#define _MODULE_h

class Module {
public:
    enum CmdKeys { Forward, Left, Back, Right, Stop };
    static const CmdKeys AvailableCommandKeys[2];

    // other definitions...
}
const Module::CmdKeys Module::AvailableCommandKeys[] = { Forward, Back };

#endif

Module.cpp looks as follows:

#include "Module.h"

const Module::CmdKeys Module::AvailableCommandKeys[] = { Forward, Back };

// Other code...

C++ 'multiple definition' compile error with static enum class members

2 Answers