Is it possible to pass derived classes by reference to a function taking base class as a parameter

Question

Say we have an abstract base class IBase with pure virtual methods (an interface).

Then we derive CFoo, CFoo2 from the base class.

And we have a function that knows how to work with IBase.

Foo(IBase *input);

The usual scenario in these cases is like this:

IBase *ptr = static_cast<IBase*>(new CFoo("abc")); Foo(ptr); delete ptr;

But pointer management is better to be avoided, so is there a way to use references in such scenario?

CFoo inst("abc"); Foo(inst);

where Foo is:

Foo(IBase &input);

Yes; polymorphism works for both pointers and references. And stop casting, please. We're not in Hollywood. — Kerrek SB
– Kerrek SB, Commented Feb 14, 2012 at 23:17
@KerrekSB Well you'd have to cast if you tried IBase& b = CFoo(); — bobobobo
– bobobobo, Commented Nov 8, 2021 at 4:18

gwiazdorrr · Accepted Answer · 2012-02-14 23:30:09Z

Yes. You don't have to upcast your objects. All references/pointers to derived types are converted implicitly to base objects references/pointers when necessary.

So:

IBase* ptr = new CFoo("abc"); // good CFoo* ptr2 = static_cast<CFoo*>(ptr); // good CFoo* ptr3 = ptr; // compile error CFoo instance("abc"); IBase& ref = instance; // good CFoo& ref2 = static_cast<CFoo&>(ref); // good CFoo& ref3 = ref; // compile error

When you have to downcast you may want to consider using dynamic_cast, if your types are polymorphic.

I found where my problem was. I was using Foo(CFoo("abc")); which was not compiling.
@Coder : In case you're not aware, the reason that's a problem is because you're passing Foo an rvalue, and rvalues cannot bind to non-const lvalue references (i.e., if Foo were Foo(IBase const& input); instead, it would have compiled).

John · Accepted Answer · 2012-02-14 23:16:39Z

You can cast an object just as you can a pointer. I remember this was common when converting char to unsigned char and various other sign changing casts in days of yore.

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