Do I need to use dynamic_cast when calling a function that accepts the base class?

Question

I have some classes like this:

interface class IA
{
};

interface class IB
{
};

public ref class C : public IA, public IB
{
};

public ref class D
{
 void DoSomething(IA^ aaa)
 {
 }

 void Run()
 {
  C^ bob = gcnew C();
  DoSomething(dynamic_cast<IA^>(bob)); // #1
  DoSomething(bob);   // #2
 }
};

At the moment I always try to use dynamic casting when calling such a function, (the #1 above).
However it does make the code quite ugly, so I want to know if it is actually necessary.

Do you use dynamic_cast in this way? If so what is the main reason?

Answer 1

No, I would think that in C++/CLI you also don't need the dynamic cast here. Derived* implicitly converts to Base* unless there's an ambiguity w.r.t. multiple inheritance. The same it probably true for "gc-pointers". In C++ a dynamic cast -- when upcasting -- requires polymorphic classes (with at least one virtual function). I don't know how C++/CLI handles it, though. I would think every CLI class is by default polymorphic.

You may want to remove the C++ tag, by the way. ;)

Answer 2

Since we know bob is of type C^, we know at compile time it can be downcasted to IA^ safely, and so dynamic_cast is equivalent to static_cast here. Moreover, the implicit cast you propose is also safe.

dynamic_cast is only needed when upcasting from a base type to a derived.

Answer 3

In standard C++, you use dynamic_cast to walk down the hierarchy, not up. In this case, you'd use it to try and convert an IA or IB into a C:

IA^ temp = /* get a C in some way. */;
C^ tempC = dynamic_cast<C^>(temp);