C++: static member functions and variables- redefinition of static variable?

Question

I was trying to incorporate the Singleton design pattern into my code, but I started getting a weird error: ``` main.obj : error LNK2005: "private: static class gameState * gameState::state" (?state@gameState@@0PAV1@A) already defined in gameState.obj ``` If you're not familiar with the singleton pattern, it is basically used to enforce only 1 instance of a certain object in the entire program. Here is the relevant code: gameState.h: ``` class gameState { public: static gameState* Instance() {return state;} . . . private: gameState(); static gameState* state; }; gameState* gameState::state = new gameState(); ``` and right now I'm just using the instance of that object in the main.cpp file: ``` gameState *currState = gameState::Instance(); . . . for_each(currState->getHumanPieces().begin(),currState->getHumanPieces().end(), drawPieces); ``` It would seem I am trying to redefine gameState::state, but can't figure out why... help anyone? that solved that, but one error still remains, whi…

Accepted Answer

You need to put the definition of the static `gameState*` into exactly one source file, i.e. this line:

```
gameState* gameState::state = new gameState();
```

If you put it in a header which is included by multiple source files, each has a definition of `gameState::state` which leads to errors at link-time.

For the follow-up problem, use Vadakkumpadaths advice: you need to provide a definition for `gameState`s constructor, not only a declaration.

C++: static member functions and variables- redefinition of static variable?

Solution

Problem