Can I Initialize a derived class reference with a base class reference to derived class instance?

Question

I have something like the following:

class A { ... };
class B : public A { ... };

// ...

B b;
const A& aref(b);

// ...

const B& bref(aref);

and when I compile, I get:

no suitable user-defined conversion from "const A" to "const B" exists

Now, if these were pointers rather than references, I would use

bptr = dynamic_cast<B*>(aptr);

but references don't have that. What should I do? Switch to pointers? something else?

score 5 · Accepted Answer · answered Mar 07 '16 at 10:39

You can use dynamic_cast for references, they just throw an exception rather than returning nullptr on failure:

try {
    const B& bref(dynamic_cast<const B&>(aref));
}
catch (const std::bad_cast& e) {
    //handle error
}

If you absolutely know that aref is actually a B, then you can do a static_cast:

const B& bref(static_cast<const B&>(aref));

1 Answers1