Using object slicing for effective assignment

Question

Let's say I have the definitions

struct A{
    int a1 = 0;
    int a2 = 1;
};

struct B: public A{
    int b1 = 2;
    int b2 = 3;
};

And I have a B object that I'm going to call foo, and I want to update its fields inherited from A to the values in another A object that I call bar. Obviously I could just do this by assigning each field, but could object slicing be used here instead?

I tried using this code

(A) foo = bar;

but it doesn't modify the fields, which I'm guessing is because I'm somehow making a copy. So can I use object slicing to do this, and if so is it 1.) not UB and 2.) reasonable?

Do the most descriptive, easy-to-understand thing until forced to do something less descriptive. Even if it worked, what you're trying to do will have a lot of programmers looking at the code and wondering what you really mean to do. — user4581301, Jul 13 '22 at 18:17
`(A) foo` creates a temporary `A` object, it doesn't allows to modify `foo` as if it was an `A`. Why not just give `B` an `operator =(const A&)` function? — NathanOliver, Jul 13 '22 at 18:17
Fwiw, `(A&) foo = bar;` will probably do what you very-strangely seem to want. I concur with @NathanOliver, however. Just make a proper operator in the first place. — WhozCraig, Jul 13 '22 at 18:18
`static_cast(b_obj) = a_obj;` - live - https://godbolt.org/z/d3eGeb8WM — Richard Critten, Jul 13 '22 at 18:23
Please provide [mcve] in form of code! start with: https://godbolt.org/z/qhW5v5fda — Marek R, Jul 13 '22 at 18:26
@OP Maybe you didn't think this thing all the way through. If a client was able to do `(A) foo = bar;`, then the client would also expect to construct a `B` object from an `A` object. You only focused on assignment, but not construction. — PaulMcKenzie, Jul 13 '22 at 18:42

Using object slicing for effective assignment

0 Answers0