Pointer to object in C++

Question

I saw the below code in Stackoverflow.com . I was looking for the same code, bacuse I have a doubt. If we will see the below code
CRectangle r1, *r2; r2= new CRectangle;
1. Why are we doing new CRectangle ? What actually we are trying to do?

1. One of colleague told me, when we write the code we make,
   CRectangle *r2 = 0; ,

Then we initialize with some other value or address. I really got confused. Please help me.

using namespace std; 
class CRectangle
{
    int width, height;  
public:    
    void set_values (int, int);  
    int area (void) {return (width * height);
    } 
}; 
void CRectangle::set_values (int a, int b) 
{    
    width = a;   
    height = b;
} 
int main () 
{   
    CRectangle r1, *r2;    
    r2= new CRectangle;    
    r1.set_values (1,2);  
    r2->set_values (3,4);   
    cout << "r1.area(): " << r1.area() << endl;    
    cout << "r2->area(): " << r2->area() << endl;  
    cout << "(*r2).area(): " << (*r2).area() << endl;  
    delete r2;    
    return 0; 
} 

Answer 1

CRectangle r1, *r2; r2= new CRectangle;

r1 is the object. r2 is the pointer to the object.

CRectangle *r2 = 0;

Pointer contains the address of the object. If the pointer has not initialized, it can have any value (that we don't know). So, we initialize 0 so that we know our pointer has value 0 . We can use that value to avoid double initializing or some memory error.

When we are about to assign the object to the pointer, what we usually do is_

if (r2 == 0)
    r2 = new CRectangle;
else
    std::cout << "Pointer is not empty or invalid" << std::endl;

Answer 2

In your example r1 has type of CRectangle class, but r2 has type of pointer to CRectangle class. The pointer is just address in global memory ( heap ). In order to allocate and mark memory as CRectangle object we need use new .

Please, read the details in books

Answer 3

CRectangle r1, *r2;    
r2= new CRectangle;

This code declares a CRectangle object ( r1 ), and a pointer to a CRectangle object, which doesn't yet point to anything ( r2 ). Both variables are allocated on the stack.

The second line allocates memory for a new CRectangle object (from the heap), calls any constructor for that class, and then returns the address and assigns it to r2 . So r2 then points to a CRectangle object.

Setting r2 to 0 or NULL is simply a standard way to indicate that the pointer doesn't actually point to anything.