is it possible to write A = B when B inherits from A?

Asked Dec 14 '17 at 19:17

Active Dec 14 '17 at 19:17

Viewed 47 times

I have a class Video that inherits from the class Multimedia. I know that this works:

Multimedia* multi;
Video* vid;
multi = vid;

But, I don't understand why the following code doesn't:

    Multimedia** m;
    Video** v;
    m = v;

error: invalid conversion from ‘Video**’ to ‘Multimedia**’ [-fpermissive]

asked Dec 14 '17 at 19:17

J. M. Aziz

2

Because whilst a `Video` [is a](https://en.wikipedia.org/wiki/Is-a) type of `Multimedia`, a pointer-to-`Video` is not a type of pointer-to-`Multimedia`. – Oliver Charlesworth Dec 14 '17 at 19:19
Because an item of type `Video*` is not also of type `Multimedia*`. – axiac Dec 14 '17 at 19:20
You need to cast i.e. `m = (Multimedia**) v;` – Jake Freeman Dec 14 '17 at 19:22
@user0042 The error arises from the use of double pointers. – Code-Apprentice Dec 14 '17 at 19:25
@JakeFreeman that would cause undefined behaviour if `m` is later dereferenced – M.M Dec 14 '17 at 19:54
@JakeFreeman Yeah that's what I did I used cast. But, I was wondering if there was another way to do it. I haven't seen the duplicates. – J. M. Aziz Dec 14 '17 at 19:54
@J.M.Aziz -- Do not cast. The compiler is letting you know that the types are not the same. By applying a C-style case, you're telling the compiler that you know better, and to shut-up. The compiler knows better. – PaulMcKenzie Dec 14 '17 at 20:08

is it possible to write A** = B** when B inherits from A?