What is the most suitable way to return base object to up-casted derived class?

Question

struct B {};  // B contains data members
struct D : B {};  // D doesn't contain ANY data member

B g_b;  // global object
D& fun ()  // want to return by reference ONLY
{
  return <???>(g_b);  // how ???
}

[Note: I want to avoid overloading constructor (or assignment) such as D(const B&).]

Mat · Answer 1 · 2011-05-17T17:22:10.790

5

What you're trying to do is illegal. g_b is not a D.

edited May 17 '11 at 17:22

answered May 17 '11 at 17:09

Mat

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`g_b` you mean, but yes. It's an instance of `B` only, not `D`. There's no legal way to refer to it via a `D&`, because `D&` means "reference to an instance of D". Which it isn't :-) – Steve Jessop May 17 '11 at 17:21

score 2 · Answer 2 · edited May 23 '17 at 12:19

2

No suitable cast. That is in fact undefined behavior.

For detail, see this topic: Downcasting a base type

Note : the term is downcast when you cast base to derived class; and the term upcast is used when you cast derived to base class.

edited May 23 '17 at 12:19

Community

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answered May 17 '11 at 17:11

Nawaz

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+1, notifying for upcast and downcast however, what I meant in the question was that, I am just receiving by base class ; the object is of known type (which is derived). That's why used upcasting. – iammilind Jun 10 '11 at 03:45

score 1 · Accepted Answer · answered May 17 '11 at 17:16

1

That is undefined behavior.
You can use dynamic_cast for performing safe down casting of Base class pointer/reference to derived class pointer/reference. It returns a null in case of pointers or throws an exception in case of references.

answered May 17 '11 at 17:16

Alok Save

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What is the most suitable way to return base object to up-casted derived class?

3 Answers3