A base class pointer can point to a derived class object, but vice versa in not true

Question

A derived class object should have information about contents of base class even then a pointer of derived class cannot point to object of base class.
But instead a pointer of base class can point to derived class. Why so ? Can u explicitly change this behaviour ,if so then how ?

Answer 1

In C/C++, you can make a pointer point to any memory location as you want, using explicit type casting. This means you can let a pointer (derived object type) point to its base object also. But whether you can do something useful with this pointer is another story.
So, in term of class' methods usage, when you declare a pointer of class X, you usually expect that you can call all the exposed methods of X thru the pointer, as the compiler have knowledge about X and will arrange the code to do so.
Now, we already know that derived object is also (contains) base object, so base object pointer (which points to derived object) can call the base methods as the memory layout of the base object part is known the compiler.
If we let the derived object pointer point to its base object, base object is surely not a derived object, so you can't call the derived methods with this pointer.
So, normally, we should not point a derived object pointer to its base object.

Answer 2

Suppose following structure :
Class A
{ member1, member2
}
Class B extends/inherits B
{ member3, member4
}

A object1=new A() ;
A objectA=new B() ; (X)
B objectB=new B() ;
B object2=new A() ;

Here pointer of class B can point to address where all members are stored but pointer of class A can not point to such address that has all members.

You can do this by explicitly tyoecasting with data loss.

Answer 3

Because derived class has all data of base class (by definition) then compiler can access to all member of derived like base but opposite is not true because derived can add members not avaible in base class.

Answer 4

Ina u mean to say..
Q) why don't we make a pointer of derived class instead of base class.
Ans) due to resource wastage. Is that right ?

Answer 5

Ina I asked the same question in different words.