Question

I am trying to make a board game, since every move has to be valid, so I am making a copy of board and make a move so I can verify if that move is valid or not.

First I initialize all the positions on the board to be 0 (iterate through the board and set every p to 0

 pair<int, int> p(y, x); 
 board_[p] = 0;

This is the copy board method

void Board::copy(Board & gb) {
for (int y = MIN_Y; y <= MAX_Y; ++y) {
    for (int x = MIN_X; x <= MAX_X; ++x) {
        pair<int, int> p(y, x); 
        if (gb.board_.at(p) != 0) {
            board_[p] = new Pieces(*gb.board_.at(p));  // **where I am confused**
        } else {
            board_[p] = 0;
        }   
    }
}
}

My container in Board is:

map<pair<int,int>, Pieces*> board_;

Now in a play method, I make a copy of the board

unsigned int play(Board & b){
   b.copy(*this);
}

My question: both

board_[p] = new Pieces(*gb.board_.at(p)); //Pieces is a class I defined

and

board_[p] = gb.board_.at(p);

compile without any errors or warnings. Which one should I use though?

Answer 1

Either may be correct, but you probably want the first one. The first will copy each of the Pieces over to the new board – this is known as a deep copy. The second will only copy the pointers to each Pieces over, so both boards point at the same set of Pieces – this is a shallow copy.

However, there is a bigger issue here. You are defining a copy function, but C++ gives us a language feature for doing this – copy constructors. You should instead define a function like so:

Board::Board(const Board& other_board) {
  // Copy everything from other_board to this board
}

And you would use it like so:

Board board;
Board newBoard(b);