error LNK2001: unresolved external symbol "private: static class

Question

error LNK2001: unresolved external symbol "private: static class irrklang::ISoundEngine * GameEngine::Sound::_soundDevice" (?_soundDevice@Sound@GameEngine@@0PAVISoundEngine@irrklang@@A)

I cannot figure out why i am receiving this error. I believe i am initializing correctly. Can anyone lend a hand?

sound.h

class Sound
{
private:
    static irrklang::ISoundEngine* _soundDevice;
public:
    Sound();
    ~Sound();

    //getter and setter for _soundDevice
    irrklang::ISoundEngine* getSoundDevice() { return _soundDevice; }
//  void setSoundDevice(irrklang::ISoundEngine* value) { _soundDevice = value; }
    static bool initialise();
    static void shutdown();

sound.cpp

namespace GameEngine
{
Sound::Sound() { }
Sound::~Sound() { }

bool Sound::initialise()
{
    //initialise the sound engine
    _soundDevice = irrklang::createIrrKlangDevice();

    if (!_soundDevice)
    {
        std::cerr << "Error creating sound device" << std::endl;
        return false;
    }

}

void Sound::shutdown()
{
    _soundDevice->drop();
}

and where i use the sound device

GameEngine::Sound* sound = new GameEngine::Sound();

namespace GameEngine
{
bool Game::initialise()
{
    ///
    /// non-related code removed
    ///

    //initialise the sound engine
    if (!Sound::initialise())
        return false;

Any help would be greatly appreciated

Answer 1

Put this into sound.cpp:

irrklang::ISoundEngine* Sound::_soundDevice;

NOTE: You might want to initialize it as well, for example:

irrklang::ISoundEngine* Sound::_soundDevice = 0;

static, but non-const data members should be defined outside of the class definition and inside the namespace enclosing the class. The usual practice is to define it in the translation unit (*.cpp) because it is considered to be an implementation detail. Only static and const integral types can be declared and defined at the same time (inside class definition):

class Example {
public:
  static const long x = 101;
};

in this case you don't need to add x definition because it is already defined inside the class definition. However, in your case it is necessary. Extract from section 9.4.2 of the C++ Standard:

The definition for a static data member shall appear in a namespace scope enclosing the member’s class definition.

Answer 2

Eventually, the answer given by @Alexander resolved a similar issue in my own code, but not without a few trials. For the benefit of the next visitor, when he says "Put this into sound.cpp," to be perfectly clear, this is in addition to what is already present in sound.h.