Pointer to class

[kiberg]

hi,

class A
{
 int i;
}

class B
{
 A *a;
 int number;

 B(A obj){
 a=&obj;
 number=*a.i;//ERROR:
}

}

ERROR: request for member `i' in `B:a', which is of non-aggregate type `A *'

What's the problem?

[Banfa]

Apart from missing the ; from the end of both classes you have an operator preceedence error in
[code=cpp] number= *a.i; // ERROR:[/code]

The . operator (Member access from an object) has higher preceedence than * (Dereference) so this is interpreted as

[code=cpp] number= *(a.i); // ERROR:[/code]

and since a is not a structure, class or union (it is a point to a aclass) this is not valid.

You can fix this error by adding parenthises to make the * operator evaluated first

[code=cpp] number= (*a).i;[/code]

or better still you can use the Member access from a pointer operator, ->

[code=cpp] number= a->i;[/code]

Eliminating any conflict since there is now only 1 operator (you could also use the array access operator but since it is not an array I would say this would be bad form).

When you have done this you will get another error because a->i tries to access i which is a private member of A and therefore not accessible in B.

When you have fixed that then the code will compile but will likely cause segmentation faults at some point because you point B::a at a temporary copy of an A object that exists on the stack while the constructor is running but is deleted once the constructor exits resulting in an invalid pointer.