CODEWITHSUNDEEP
c++pointersinheritancememory
VINOTH ENERGETIC
VINOTH ENERGETIC

Reputation: 1843

Why i cannot allocate a a memory using a a base class type pointing to a derived class pointer?

class a
{
    int var;
}
class b :public a
{
    int var2,var3;
}

int main()
{
    a *poi;
    poi=new b;
    //b *poi2;
    //poi2=new a;
    return 0
}

In above code,i was able to allocate a memory using a derived class type pointing to a base class pointer variable. i.e, a *poi=new b; But i was not able to allocate a memory using a a base class type pointing to a derived class pointer. i.e. b *poi=new a;

Why the following memory allocation is not possible? What is the logic behind it?

Upvotes: 1

Views: 146

Answers (3)

Raul Andres
Raul Andres

Reputation: 3806

That's not about memory allocation but about inheritance. 

Fruit* f = new Apple

works well, but

Apple* a = new Fruit

would give you a incomplete apple, or even a banana, so compiler doesn't allow you doing that.

Both pointers have same size, and you could use a unsafe static cast to store a fruit pointer into a apple pointer, but you should avoid doing that

Upvotes: 4

rajenpandit
rajenpandit

Reputation: 1361

here a *poi; poi=new b; is valid because poi has the reference of var and the object of b also has the variable var because of inheritance. which is a valid reference.

in case of b *poi2; poi2=new a; poi2 also can have reference of var2 and var3 but object of a doesn't contain var2 and var3. which leads to an invalid reference. I mean poi2->var2 is a valid statement but there is no var2 if compiler allow poi2=new a; then poi2->var2 will point to an invalid memory segment.

Upvotes: 1

Joseph Mansfield
Joseph Mansfield

Reputation: 110658

This doesn't really have anything to do with the memory allocation. It is simply that the first conversion is a valid implicit conversion (from derived class pointer to base pointer), but the second is not (from base class pointer to derived pointer):

§4.10/3 [conv.ptr] A prvalue of type “pointer to cv D”, where D is a class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class (Clause 10) of D.

The reason for this is that polymorphism represents an is-a relationship. A b is an a, but an a is not a b. Therefore, you can have an a pointer pointing at a b, because the object it's pointing at is a valid a object. However, you can't have a b pointer pointing at an a, because an a object is not necessarily a valid b object.

Upvotes: 2

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