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.