Question

I’m creating a program containing Cells, and for that I have a Cell class and a CellManager class. The cells are organized in a two dimensional array, and the Cell class manager has two int member variables, xgrid and ygrid that reflect the size of the array.

For some reason I just cannot figure out, these member variables change during the course of program execution. Can anyone see why this is happening, or maybe point me in a direction of where to look.

The classes and functions used look like this:

class Cell
{
    public:
        Cell(int x, int y);
}

---------------------------------

class CellManager
{
     public:
         CellManager(int xg, int yg)

         void registercell(Cell* cell, int x, int y);
         int getxgrid() {return xgrid;}
         int getygrid() {return ygrid;}

     private:
         int xgrid;
         int ygrid;         
         Cell *cells[40][25];

}

-----------------------

and CellManagers functions:

CellManager::CellManager(int xg, int yg)
{
    CellManager::xgrid = xg;
    CellManager::ygrid = yg;
}

void CellManager::registercell(Cell *cell, int x, int y)
{
    cells[x][y] = cell;
}

and here is the main function:

int main ()
{
    const int XGRID = 40;
    const int YGRID = 25;

    CellManager *CellsMgr = new CellManager(XGRID, YGRID);

    std::cout << CellsMgr->getxgrid() << std::endl; // PRINTS 40 
    std::cout << CellsMgr->getygrid() << std::endl; // PRINTS 25

    //create the cells and register them with CellManager
    for(int i = 1; i <= XGRID; i++) {

        for(int j = 1; j <= YGRID; j++) {

            Cell* cell = new Cell(i, j);
            CellsMgr->registercell(cell, i, j);
        }
    }

    std::cout << CellsMgr->getxgrid() << std::endl; // PRINTS A RANDOM LARGE INT, EX. 7763680 !!
    std::cout << CellsMgr->getygrid() << std::endl; // PRINTS 1, ALWAYS !!

So, I initialize a CellMgr, and set xgrid and ygrid through the constructor. Then I create a bunch of Cells and register them with the CellMgr. After this, the two member variables of CellMgr have changed, anyone know how this can happen?

Answer 1

Arrays are zero-indexed, but you’re using them as if they are indexed from 1. As a result, your array indexing will be overwriting cells, and writing off the end of the array, which is undefined behaviour. Overwriting random other variables is certainly possible.