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, which I didn't actually post before as I thought it was just part of the other one:

error LNK2019: unresolved external symbol "private: __thiscall gameState::gameState(void)" (??0gameState@@AAE@XZ) referenced in function "void __cdecl `dynamic initializer for 'private: static class gameState * gameState::state''(void)" (??__E?state@gameState@@0PAV1@A@@YAXXZ)

any good tip on how to fix that one as well?

Thanks both of you, its fixed :D

Answer 1

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 gameStates constructor, not only a declaration.

Answer 2

Add definition for your constructor to fix second linker error.

private:
gameState()
{
}