C++ Sharing variable between objects.

Question

I'm having a little problem where I want two different classes to be able to share and make changes to another class object.

I've got a HumanPlayer class, a ComputerPlayer class, and a Board class. The two player classes need to be able to interact with the one Board class. I thought that I could pass a reference to the same Board object to each class but it doesn't appear to be working the way I want it to. Here's part of what's in main and I'll describe what's happening as best I can:

//main.cpp

    Board *board2 = new Board();
    board2->setBoardSize(5); 
    board2->initPits(); 

    HumanPlayer firstPlayer(*board2, *menu, menu->askForFirstTurn(), true); 

    firstPlayer.removeFromPit(3);  

    board2->showBoard();

firstPlayer.removeFromPit(3); is supposed to just set a value in an array in the board class to zero. And it does that. If I were to show the board from within the code of the FirstPlayer class, it would show the change. But when I call board2->showBoard() it's as if nothing was changed. It's still the original unchanged board. What I really want to have happen is to have my firstPlayer and secondPlayer classes work on one shared board object. I'm just not sure how to implement this properly now.

Thank you all for your help. Let me know if you need more information.

Answer 1

HumanPlayer firstPlayer(*board2, *menu, menu->askForFirstTurn(), true);

you pass the 1st parameter by a value. Remove * and add it to the declaration of firstPlayer function

Answer 2

It sounds like you are copying the board object instead of passing it by reference. The constructor of your player classes should look like this:

class Player
{
public:
    Player(Board& board) // REFERENCE TO Board
       : the_board_(board) // initialize the reference
    {
    }

protected:
   Board& the_board_;  // REFERENCE to Board
};

If you are missing the & operator anywhere (either in the constructor signature or on the class member variable) then you will make a copy of the board that will be used within your class.

Answer 3

My best guess would be that your declaration of HumanPlayer's constructor looks something like:

HumanPlayer( Board board, Menu menu, int askForTurn, int otherBoolean );

so when you call it with

HumanPlayer firstPlayer(*board2, *menu, menu->askForFirstTurn(), true);

you actually have firstPlayer initialize using a copy of board2 rather than a reference to it.

If this is indeed what your code looks like, you need to make sure that HumanPlayer's constructor is declared to receive a reference to board (and probably menu?). That would look like:

HumanPlayer( Board& board, Menu& menu, int askForTurn, int otherBoolean );

Answer 4

You are probably not using a reference all the way through. There are two things to check:

1. HumanPlayer's constructor must take the board as a reference (Board&). Check that Board& is the type of the parameter. Otherwise when you call the constructor you are implicitly making a copy of the Board so that the constructor can take the Board by value.

2. The type of HumanPlayer's member variable for the board must be Board& as well. Otherwise when you set the member variable you are implicitly copying the data from the original Board object. Keep in mind that when using a reference as a member variable that you need to use the constructor initializer list rather than assignment.