C++ Class Entanglement

Question

I'm trying to use two classes' variables as to have access from class A's variables to class B's and vice versa. However, I can't figure a possible solution. It always ends up in either a loop, or the following error:

error: invalid use of non-static data member  

Here's the code sample:

Player.h:

  #ifndef _PLAYER_H_
  #define _PLAYER_H_

#include "Segment/Dynamic_Segment.h"

class Attributes_P;

class Attributes_P : public Attributes_DS{
  protected:
  int inv_mcols, inv_mrows;

  public:
  Attributes_P();
  void controls( int MKEY_UP, int MKEY_RIGHT, int MKEY_DOWN, int MKEY_LEFT );
  void inventory( int inv_mcols, int inv_mrows );
};

class Player : public Dynamic_Segment{
  protected:
  int   **inv;

  public:

  int   MKEY_UP, MKEY_RIGHT, MKEY_DOWN, MKEY_LEFT;

  public:

  Player();
  Attributes_P set;
  friend class Core;
  friend class Attributes_P;

};
#endif

Player.cpp:

#include "Segment/Player.h"

Attributes_P::Attributes_P(){};

Player::Player() : Dynamic_Segment(){
  set.inv_mcols = 0;
  set.inv_mrows = 0;
}

void Attributes_P::inventory( int inv_mcols, int inv_mrows ) {
  this->inv_mcols = inv_mcols;
  this->inv_mrows = inv_mrows;
  Player::inv = new int*[this->inv_mcols]; //<--- Error here
  for( int i = 0; i < this->inv_mrows; i++ ) {
    Player::inv[i] = new int[this->inv_mcols]; //<--- Error here
  }
}

void Attributes_P::controls( int MKEY_UP, int MKEY_RIGHT, int MKEY_DOWN, int MKEY_LEFT ) {
  Player::MKEY_UP = MKEY_UP; //<--- Error here
  Player::MKEY_RIGHT = MKEY_RIGHT; //<--- Error here
  Player::MKEY_DOWN = MKEY_DOWN; //<--- Error here
  Player::MKEY_LEFT = MKEY_LEFT; //<--- Error here
}

Have been banging my head against a wall for some time now... Any ideas would be appreciated!

Answer 1

The members

Player::MKEY_UP
Player::MKEY_RIGHT
Player::MKEY_DOWN
Player::MKEY_LEFT

aren't static, so you can only access them through an object of type Player, not through the class instance.

Consider you create 2 player objects, p1 and p2. When you call Attributes_P::controls, which of the two object's members should you change? p1 or p2?

You can either declare those members as static if you want them to be shared between Player objects, or pass a specific Player object as parameter and access its members directly. This is part of the logic and the choice depends on how you want the program to work.

Answer 2

You can not access the attributes MKEY_UP, MKEY_RIGHT, MKEY_DOWN, MKEY_LEFT & inv as they are private.

Make them private and write getter/setter!

Answer 3

I think your data is probably too closely intertwined, and maybe should just be one class. One purpose of classes is to encapsulate the data, so you don't go fiddling with others' privates. Also there are other issues with what you are doing using new and arrays, etc. However...

In Attributes_P::inventory you are trying to modify inv, which is declared as a member of Player. But Attributes_P doesn't know which player you are referring to. You need to either

1. pass a Player instance into the inventory function, or
2. initialise Attributes_P with a reference to the player.

Option 1:

void Attributes_P::inventory(int inv_mcols, int inv_mrows, Player& player) {
    player.inv = ...
}

Option 2:

class Player; // needed so compiler understands next few lines refering to Player

class Attributes_P {
    Player& m_player;
public:
    Attributes_P(Player& player) : m_player(player) {
    }
};

class Player {
    Attributes_P m_attributes;
public:
    Player() : m_attributes(*this) { // pass self to Attributes_P constructor
    }
}

void Attributes_P::inventory(int inv_mcols, int inv_mrows) {
    m_player.inv = ...
}