Issue with forward declaring class - c++

Question

So I am trying to forward declare a class in my C++ project and then create it in main.
So I have player_obj.cpp which contains the class, classes.h which forward declares the class, and main.cpp which uses it.
classes.h
#ifndef CLASSES_H
#define CLASSES_H
class player_class
{
public:
int x;
int y;
char sprite;
int xprevious;
int yprevious;
private:
bool active;
public:
void update_xy();
player_class(int _x, int _y, char _sprite);
void step();
void destroy();
};
#endif
main.cpp
#include <iostream>
#include "classes.h"
using namespace std;
int main()
{
player_class player_obj (5,5,'#');
cout << player_obj.x << ", " << player_obj.y << endl;
return 0;
}
and player_obj.cpp
#include <iostream>
#include <Windows.h>
using namespace std;
class player_class
{
public:
//Coordinates
int x;
int y;
//Sprite
char sprite;
//Previous coordinates
int xprevious;
int yprevious;
//Not everyone can set the activity
private:
//Active
bool active;
//Update xprevious and yprevious - Called by the step event
void update_xy()
{
xprevious = x;
yprevious = y;
}
//All functions public
public:
//Create event/Constructer
player_class(int _x, int _y, char _sprite)
{
//Set default variables
x = _x;
y = _y;
sprite = _sprite;
xprevious = x;
yprevious = y;
active = true;
}
//Step event
void step()
{
//Update old xprevious and yprevious
update_xy();
//Do other stuff here
}
//Drestroy event
void destroy()
{
active = false;
}
};
I thought that would work out all right but when I compile and run it I get:
main.cpp:(.text+0x2c): undefined reference to`player_class::player_class(int, int, char)'
I've done some research, but I can't seem to fix this issue.
I greatly appreciate any help!

Answer 1

Well you're sort of close, what you have in your header is indeed a class declaration (not a forward declaration mind you).
The problem is you never defined it. What you have in player_obj.cpp is an abomination of class redefinition, but you already have your class declared. Just include the header file and define the functions one by one and you're done!
#include "classes.h"
player_class::player_class(int _x, int _y, char _sprite)
{
//Set default variables
x = _x;
y = _y;
sprite = _sprite;
xprevious = x;
yprevious = y;
active = true;
}
// and so on
If you're serious about learning modern C++ though, a few notes:
#pragma once is the modern way of guarding header files. Don't use those #ifdef..#endif constructs.
generally speaking, don't name anything starting with underscores. Especially not parameters visible as part of your public contract.
you have class initializers for a reason, use them! You don't need half a screen of copy pasting variables in your constructors.

Answer 2

You dont want a forward declaration. You want a declaration. It is a classical case of declaring a class in a header file and defining its functions in a cpp file. Then including the header where-ever you want to use your class
You only need forward declarations when you want to use a pointer to that class as a parameter to a function or a member variable somewhere but the definition of that class is not available yet.
Note that when you forward declare a class, you cannot use this class's member variables or functions in that header
-regards
Gautam