Using 'this' to pass a class

[MrProgrammer]

Hello,

I have the following two relevant classes in this problem (see below).
When I run this, I get the following erros:
1. Player.cpp:18:5: error: 'World' does not name a type
World w;
^

2. Player.cpp:20:26: error: 'World' has not been declared
Player(int x, int y, World w) {

(and a few more but these are probably caused by the top two).
I don't understand what the problem is. I am trying to pass ''this'' (world object) to the Player object. But if I pass it as ''this'', it will be a copy and not a reference in the first place right? Should I pass it as a reference (if I want to edit the World object from the Player class).
Also, why does it show the top errors? :-s

Thank you.

World.cpp

#include <iostream>
#include "Player.cpp"
#include "Tile.cpp"

using namespace std;

class World {
private:

public:
    Tile *t[9][9];
    Player p;
    int counter;

    World() {
        
    }
    
    World() : p(2, 2, this) {
        counter=0;
        initWorld();
        printWorld();
    }

    void initWorld() {
       //init
    }

    void printWorld() {
       //print
    }

};

Player.cpp

#include <iostream>
#include <sstream>

using namespace std;

class Player {
public:
    int x;
    int y;
    World w;
    
    Player(int x, int y, World w) {
        this->x = x;
        this->w=w;
        this->y = y;
    }
    
    void moveUp() {
        y+=1;
    }
    
    void moveDown() {
        y-=1;
    }
    
    void moveRight() {
        x+=1;
    }
    
    void moveLeft() {
        x-=1;
    }
};

[Zhuge]

This can't work, because any Player contains a World, and each World contains a Player, which each contains a World, and so on. Somewhere you're going to want to use a pointer or something.

[Disch]

For starters... you should never be including cpp files. You includes headers, not source files. See this:

http://www.cplusplus.com/forum/articles/10627/

Secondly, Zhuge is correct. If a Player owns a World, then a World cannot own a Player because then you'll have an infinite number of worlds/players.

It makes more sense for the World to own the Player... and the Player simply has a pointer to the World which owns it.

[MrProgrammer]

Thanks for your comments.

I now understand.

If I want the Player object to have a pointer to the world which owns it, I have to make another class that passed the World object (reference) inside Player right? Because I won't be able to do so with 2 (World, Player) classes?

[Disch]

You do not need a third class. The World class can give its owned Player a pointer to itself (that's what the this pointer is for):

// player.h

class World;  // <- forward declare the World class

class Player {
public:
    int x;
    int y;
    World* w; // <- pointer
    
    Player(int x, int y, World* w) {  // <- pointer
        this->x = x;
        this->w=w;
        this->y = y;
    }
//... 

// world.h

#include "player.h"
class Tile;

class World {
private:

public:
    Tile *t[9][9];
    Player p;
    int counter;

    World()
       : p( player_x, player_y, this )  // <- give the owned player a pointer to 'this'
    {  
    }

[MrProgrammer]

Thank you that worked! I didn't know you had to 'forward declare the class'.

[Disch]

http://www.cplusplus.com/forum/articles/10627/

See that article for forward declaring / including / etc details.