Can't properly transfer data from a class in C++ - c++

Question

I am trying to make a simple game
What will happen is a player will be hit by various objects that result in increase/ decrease of health
The mainGame(); is created just to see if its working properly.
On compilation i get this
(.text$_ZN5gamer7setDataEv[__ZN5gamer7setDataEv]+0x117)||undefined reference to `gamer::mainGame()'|
I tried the friend keyword but that gave me the error of undefined reference to object
class gamer
{
public:
string gName;
int gHealth;
int continueGame;
void mainGame();
void setData()
{
cout << "Enter your name " <<endl;
cin >> gName;
srand(time(0));
gHealth = 100 + (rand()%200);
cout << gName << " has health of " << gHealth << endl;
cout << "Would you like to continue ? 0 for yes" << endl;
cin >> continueGame;
if (continueGame == 0)
{
mainGame();
}
}
};
void mainGame()
{
gamer gamer;
while(gamer.gHealth >= 0 || gamer.gHealth <= 500)
{
cout << " aaaaaa" << endl;
gamer.gHealth -= 50;
cout << gamer.gHealth << endl ;
}
}

Answer 1

You have declared the mainGame function as a member of your gamer class but never actually defined it. You came close, but forgot to declare, in the definition, that it a member of the class. The below should work:
void gamer::mainGame() // This is a member function of `gamer`, so declare it as such!
{
// gamer gamer; // We don't need this reference, as the member function will have...
while(gHealth >= 0 || gHealth <= 500) // ... an implied "this" object when called
{
cout << " aaaaaa" << endl;
gHealth -= 50; // Just using the member name on its own is enough to get it.
cout << gHealth << endl ;
}
}
Inside a member function, you don't need code declaring an object of that class! When the function is called (by an object of the class) the function effectively receives a pointer to the object that called it; you can explicitly access this pointer with the this keyword but, generally, you don't need to. Just using the name of a member variable in that function will reference that member variable for the object that called the function.