Question

(This question should probably be answered with a reference to Stroustrup.)

It seems extremely useful to be able to request a pointer to the most derived class, as in the following:

class Base { ... };
class DerivedA { ... };
class DerivedB { ... };
class Processor
{
  public:
  void Do(Base* b) {...}
  void Do(DerivedA* d) {...}
  void Do(DerivedB* d) {...}
};

list<Base*> things;
Processor p;
for(list<Base*>::iterator i=things.begin(), e=things.end(); i!=e; ++i)
{
    p.Do(CAST_TO_MOST_DERIVED_CLASS(*i));
}

But this mechanism isn’t provided in c++. Why?

Update, Motivating Example:

Suppose instead of having Base and Derived and Processor, you have:

class Fruit
class Apple : public Fruit
class Orange: public Fruit

class Eater
{
   void Eat(Fruit* f)  { ... }
   void Eat(Apple* f)  { Wash(f); ... }
   void Eat(Orange* f) { Peel(f); ... }
};

Eater me;
for each Fruit* f in Fruits
    me.Eat(f);

But this is tricky to do in C++, requiring creative solutions like the visitor pattern. The question, then, is: Why is this tricky to do in C++, when something like “CAST_TO_MOST_DERIVED” would make it much simpler?

Update: Wikipedia Knows All

I think Pontus Gagge has a good answer. Add to it this bit from the Wikipedia entry on Multiple Dispatch:

“Stroustrup mentions that he liked the concept of Multi-methods in The Design and Evolution of C++ and considered implementing it in C++ but claims to have been unable to find an efficient sample implementation (comparable to virtual functions) and resolve some possible type ambiguity problems. He goes on to state that although the feature would still be nice to have, that it can be approximately implemented using double dispatch or a type based lookup table as outlined in the C/C++ example above so is a low priority feature for future language revisions.”

For background, you can read a little summary about Multi-Methods, which would be better than a call like the one I mention, because they’d just work.

Answer 1

What you are suggesting would be equivalent to a switch on the runtime type, calling one of the overloaded functions. As others have indicated, you should work with your inheritance hierarchy, and not against it: use virtuals in your class hierarchy instead of dispatching outside it.

That said, something like this could be useful for double dispatch, especially if you also have a hierarchy of Processors. But how would the compiler implement it?

First, you’d have to extract what you call ‘the most overloaded type’ at runtime. It can be done, but how would you deal with e.g. multiple inheritance and templates? Every feature in a language must interact well with other features — and C++ has a great number of features!

Second, for your code example to work, you’d have to get the correct static overload based on the runtime type (which C++ does not allow as it is designed). Would you like this to follow the compile time lookup rules, especially with multiple parameters? Would you like this runtime dispatch to consider also the runtime type of your Processor hierarchy, and what overloads they have added? How much logic would you like the compiler to add automatically into your runtime dispatcher? How would you deal with invalid runtime types? Would users of the feature be aware of the cost and complexity of what looks like a simple cast and function call?

In all, I´d say the feature would be complex to implement, prone to errors both in implementation and usage, and useful only in rare cases.