Question

First of all I am not proficient programing, so please be lenient. 🙂
I was curious what causes the error called “Stack overflow”. I am using Visual C++ 2010 Express.

struct elem
{
    BITMAP * colltile;
    elem * next;
};

/* put some code here */

int collision_map (unsigned int poz_x, unsigned int poz_y)
{
    elem * wsk = this->where_the_head_of_list_is;
    int x,y;
    x = poz_x%64; //coord x on tile (0-63px)
    y = poz_y%64; //coord y on tile (0-63px)
    poz_x /= 64;  //preparing poz_x and poz_y to point on a tile on a grid
    poz_y /= 64;  //integers do not have to be floored

    //for (int j=(poz_y*(this->size_x)+poz_x); j>0; j--) //normally works... but
    for (int j=0; j<1000; j++) //this version is not
    {
        if ((!(wsk = wsk->next)) ||
            ((poz_x+1) > this->size_x) ||
            ((poz_y+1) > this->size_y))
        {   //should check if there is no new pointer or just out of map
            return -1;
        }
    }
    return getpixel(wsk->colltile, x, y); 
}

Why is the condition not working when j reaches the value of length of the list?

Answer 1

Things you can/have to do:

• Adding debug output will help you find the error. Add some std::cerr << wsk << std::endl in your code and check if/when it becomes zero.
• Possibly the pointer is zero at the beginning of your function at elem* wsk= .... If this is the case wsk->next will access a null pointer. You have to check if the pointer is valid.
• Since you have pointer members in your class (as indicated by this->where_the_head_of_list_is) you need to implement copy constructor, assignment operator and destructor, which you might not have done. See here for an explanation.