Thread: Dynamic memory reallocation

Hi.

I know that this may be achieved by using STL or MFC, but I have a project for school that must be written without these libraries.

I must create a simple program that deals with graphic objects- points, shapes and 3d figures. A shape object could be deffined by any number of points. So I have a pointer of type

Code:

Point **Elements;

My constructor gets three arguments:

Code:

Shape::Shape(Point& p1, Point& p2, char* _Name)
{
	num=2;
	Elements = new Point*[2];
	Elements[0]=&p1;
	Elements[1]=&p2;
	Name = new char[strlen(_Name)+1];
	strcpy(Name,_Name);
}

Now I have a method which must add a point to the elements of the shape:

Code:

void Shape::AddPoint(Point* p)
{
	Point **tmp_list;
	tmp_list = new Point*[++num];
	int i;
	for(i=0;i<num-1;i++)
		tmp_list[i]=Elements[i];
	delete[] Elements;
	Elements = new Point*[num];
	for(i=0;i<num-1;i++)
		Elements[i]=tmp_list[i];
	Elements[num-1] = new Point(*p);
}

My question is: Is this the proper way of adding a new point pointer to the elements or I just can do this:

Code:

void Shape::AddPoint(Point* p)
{
	Point **tmp_list;
	tmp_list = new Point*[++num];
	int i;
	for(i=0;i<num-1;i++)
		tmp_list[i]=Elements[i];
                tmp_list[num-1]=p;
	delete[] Elements;
                Elements=tmp_list;
}[/

And one more: How do the "delete[] " operator knows how many elements are there in the array?

Originally Posted by Gravedigga

And one more: How do the "delete[] " operator knows how many elements are there in the array?

Have you ever wondered where the memory comes from?? Why don't pointers and memory regions overlap??
whenever you allocate memory, the info on the allocated region is stored wherelse, with region start and end, which includes size. So when calling delete, the operators acesses that info and knows how many memory is there.

Look at a small example

Code:

struc block{
    void* addr; //adress of first byte
    unsigned int sz;//size of region allocated from the first byte
} memory_available[100];

void *get_me_memory(unsigned int size){
    int free_spot_index;
    //iterate in memory_available and find a clear spot
    memory_available[free_spot_index].sz = size;
    return memory_available.addr;
}
bool dump_it(void *p){
    int i;
    for(i=0;i<100;i++){
        if(memory_available[i] == p)
            //do object cleaning, blablabla, whatever
            //the size is stored in memory_available.sz
            return true;
    }
    return false;
}

The user only need to manipulate 2 functions. Any adicional info is stored way from the programmers knowledge, or acess.