Multiple dispatch in C++

Question

I am trying to understand what multiple dispatch is. I read a lot of various texts but I still have no idea what multiple dispatch is and what it is good for. Maybe the thing I am missing is piece of code using multiple dispatch. Please, can you write a little piece of code in C++ using multiple dispatch so that I can see it cannot be compiled/runned properly because C++ has only single dispatch? I need to see the difference. Thanks.

Answer 1

Multi-dispatch is the ability to choose which version of a function to call based on the runtime type of the arguments passed to the function call.

Here's an example that won't work right in C++ (untested):

class A { }; class B : public A { }; class C : public A { }   class Foo {   virtual void MyFn(A* arg1, A* arg2) { printf("A,A\n"); }   virtual void MyFn(B* arg1, B* arg2) { printf("B,B\n"); }   virtual void MyFn(C* arg1, B* arg2) { printf("C,B\n"); }   virtual void MyFn(B* arg1, C* arg2) { printf("B,C\n"); }   virtual void MyFn(C* arg1, C* arg2) { printf("C,C\n"); } };  void CallMyFn(A* arg1, A* arg2) {   // ideally, with multi-dispatch, at this point the correct MyFn()    // would be called, based on the RUNTIME type of arg1 and arg2   pFoo->MyFn(arg1, arg2); }  ...  A* arg1 = new B(); A* arg2 = new C(); // Using multi-dispatch this would print "B,C"... but because C++ only // uses single-dispatch it will print out "A,A" CallMyFn(arg1, arg2);