Re: What is polymorphism?

"mlimber" <>
6 Jun 2006 10:12:40 -0700
Seigfried wrote:

But the specific details of C++ runtime implementation of polymorphism
wasn't the question. The question was, "The book seems wrong to me -
what do you think?"

Don't top post. It's considered rude here.

The book is right. C++ implements run-time (or dynamic) polymorphism
through the virtual function mechanism. For instance:

 #include <iostream>

 struct Foo
   virtual void Message() const { std::cout << "Foo::Message()\n"; }

 struct Bar : Foo
   virtual void Message() const { std::cout << "Bar::Message()\n"; }

 void CallMessage( const Foo& foo )
   foo.Message(); // This is the key line

 int main()
   Foo foo;
   Bar bar;
   CallMessage( foo );
   CallMessage( bar );

Note that CallMessage() accepts a reference (it could just as well
accept a pointer, but not an object). That function requests that the
object of type Foo (or a derived type thereof) print its message. Any
object in the hierarchy based at Foo can be passed into that function,
but each can respond to the "message" (i.e. function call) differently
thanks to the virtual function mechanism in C++. This is just what your
book said: Foo and Bar are different (though related) objects that
respond differently to the same message (i.e., invocation of the
virtual function Message()).

Cheers! --M

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