Question

I’m trying to implement the singleton design pattern i found in a book, i’m aware that i might not need to use the singleton here but that aside i’ve created a GamePropertiesManager and i’m getting the following compilation (or rather, linker?) error: Main.cpp|| undefined reference to GamePropertiesManager::GetInstance

I’m trying to use my singleton in my main function like so..

#include "GamePropertiesManager.hpp"

..

int main()
{
      GamePropertiesManager::Create();
      GamePropertiesManager::GetInstance()->test();
...
}

My GamePropertiesManager is defined like this in a header file:

class GamePropertiesManager
{
   public:
      static GamePropertiesManager* GetInstance();
      static void Create();
      static void Destroy();

      void test();


   protected:
      GamePropertiesManager();
      static GamePropertiesManager* _instance;

      enum GameMode{ PLAYERVSPLAYER, PLAYERVSCOM };
      GameMode _gameMode;
      std::string _player1Name, _player2Name;
      int _player1Score, _player2Score;
      std::string _matchSurvivor;
      int _gameSpeed;

};

And declared in a separate cpp file:

#include "GamePropertiesManager.hpp"
//testic
#include <iostream>

GamePropertiesManager* GamePropertiesManager::_instance = NULL;

void GamePropertiesManager::Create()
{
   if (!_instance) //instance not yet created
      _instance = new GamePropertiesManager();
}

void GamePropertiesManager::Destroy()
{
   delete _instance;
   _instance = 0;
}

GamePropertiesManager::GamePropertiesManager() :
   _gameMode(PLAYERVSCOM), _player1Name("Player 1"), _player2Name("Player 2"),
      _player1Score(0), _player2Score(0), _matchSurvivor("NONE"), _gameSpeed(1)
{

}

void GamePropertiesManager::test()
{
   std::cout << "test success!" << std::endl;
}

If someone could look over it an explain what i’m doing wrong i’d appreciate it very much!

Answer 1

You’ve declared a function static GamePropertiesManager* GetInstance(); but not defined it. Actually, your GamePropertiesManager::Create() is doing most of the work. If I were you, I’d get rid of it and replace it with:

GamePropertiesManager* GamePropertiesManager::GetInstance()
{
    if (!_instance) //instance not yet created
        _instance = new GamePropertiesManager();

    return _instance;
}