Passing parameters to start a scene

cocos2d-x C++
5

I think you should create a Constant.cpp…
Or may be like this:
Constant.h

class Constant {
public:
    static int level;
};

HelloWorld.cpp

#include"Constant.h"
void HelloWorld::startLevelEasy()
{
      Constant::level=1;
      CCDirector::sharedDirector()->replaceScene(GameScene::scene());
}
int Constant::level = 1;

But I think you had better to create a Constant.cpp file.