bad_cast exception thrown when doing downcasting from Base class object by reference to derived class object

Question

I still wonder why the following gives std::bad_cast exception

#include <typeinfo>
class A {virtual void fun() {}};
class B : public A {};
int main() {
    try { 
       A a; 
       B b = dynamic_cast<B&>(a);
    } catch (std::bad_cast& e) {
       std::cerr << e.what() << '\n';
    }
}

class A is a polymorphic class class B is publicly derived from A properly doing a downcast from base class A object through reference, dynamic_cast does accept a reference argument but still why this exception?

Did you expect the cast operator to return a reference to a valid B object? Where could it find one? — n. m. could be an AI, Apr 09 '17 at 09:47
`A a` is not a `B`, so the cast failure is the right behavior. — Sergey Kalinichenko, Apr 09 '17 at 09:49
if you don't want an exception, cast it to a pointer instead of a reference, then check the return value against nullptr to see if the cast succeeded. — xaxxon, Apr 09 '17 at 10:43

songyuanyao · Answer 1 · 2017-04-09T12:07:50.670

4

This is what dynamic_cast supposed to do. a is an object of A in fact, converting it to the derived class B would fail. On the other hand, if you convert something with type of reference or pointer to the base class, but refers to an object of B in fact, then it will work fine. e.g.

B b1;
A& ra = b1;
B b2 = dynamic_cast<B&>(ra);

edited Apr 09 '17 at 12:07

answered Apr 09 '17 at 09:47

songyuanyao

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score 3 · Answer 2 · answered Apr 09 '17 at 10:46

What you've basically said is "I have an animal, but is it a lion?"

And the language needs to be able to say either "yes it is a lion" or "no it is not a lion".

When you cast to a pointer type, it can say "no" by returning nullptr to you or the actual pointer to a lion if "yes"

However, there's no such thing as a null reference. Because of this, when you cast to a reference, it needs a different mechanism to inform you that your animal isn't a lion. It throws a bad cast exception in that case.

bad_cast exception thrown when doing downcasting from Base class object by reference to derived class object

2 Answers2