How to get access to a public static member variable - c++

Question

I have my header file in which i have the "selection" as you can see it's public static member .
#ifndef SHAREDDATA_H_
#define SHAREDDATA_H_
#include "cocos2d.h"
#include "GameTrinkets.h"
using namespace std;
class SharedData{
private:
//...
//Instance of the singleton
static SharedData* m_mySingleton;
public:
//Get instance of singleton
static SharedData* sharedGameManager();
static int selection;
};
#endif /* SHAREDDATA_H_ */
Where i try to get access is:
I tried setting the selection trough just the namespace as it seemed the correct way to do it
I tried by going trough the singleton instance but got unlucky
So the code where i try it
#include "GameScene.h"
#include "GameTrinkets.h"
#include "SharedData.h"
#include <time.h>
void GameScene::mainSpriteSelection(int selection){
//1
SharedData::selection=3;
//2
SharedData::sharedGameManager()->selection=selection;
}
The error i get is :
[armeabi] SharedLibrary : libcocos2dcpp.so
jni/../../Classes/GameScene.cpp:41: error: undefined reference to 'SharedData::selection'

Answer 1

In C++, when you declare a variable to be static you have to redefine it in the implementation file before you can use the variable. For example,
//in the header file.
class foo{
public:
static int bar;
};
//in the implementation file
int foo::bar // optional initialisation.
//in the main program
//header files
int main(){
foo::bar = desired_initialisation_value; //no problem
}

Answer 2

The error messages tells you that you need to define static variable selection
define it in GameScene.cpp file
int GameScene::selection = 0;

Answer 3

You have to define the static variable outside your class (in the .cpp) as
int SharedData::selection;
Minimal example:
#include <iostream>
using namespace std;
struct Foo
{
static int member;
};
int Foo::member; // definition here
int main()
{
cout << Foo::member << endl;
}
Otherwise you get a linker error.