Re: virtual operator +

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 11 Sep 2007 10:52:19 -0400

Message-ID:

<fc6a32$cfe$1@news.datemas.de>

Hunk wrote:

I ws wondering if there is a way to implement operator+ in case of
virtual classes.
Here's the problem. I have to have a base string class from which two
classes (normal char string and a hash string class ) are derived. The
two derived classes are template classes specifying the sizes. The
base class is a non-template class so that it can be used generically
in the interface classes. the design would look like

class Base_string {
         };

template<size>
class Char_string : Base_string {
                                          };

template<size>
class Hash_string: Base_string{
                                       };
So that in the interface class of the application he can use just the
generic Base_string to access the functions and doesnt have to know
whether its a Char or hash string
The issue is in implementing the operator+ . Since all the methods are
virtual in the base class and it should call the desired methods
polymorphically, operator+ is a challenge as it returns a Base_string
object
So if I have something like
Char_string<24> char_string1("Hello");
Char_string<24> char_string2("world");
Char_string<24> char_result;
Base_string* base_a = &char_string1;
Base_string* base_b = &char_string2;
Base_string* base_r = &char_result;

i wouldnt be able to do
*base_r = *base_a + *base_b; as the operator+ would return a
Base_object?
Any soultions to the above issue is most welcome

Don't think much of overriding the operator+. Let it live in the base
class, and let it return the Base_string. Overload the _assignment_
operator in each of the derived classes:

    template<size>
    class Char_string : Base_string {
        Char_string& operator=(Base_string const&) {
            // do what's needed
            return *this;
        }
    };

    template<size>
    class Hash_string: Base_string {

        Hash_string& operator=(Base_string const&) {
            // do what's needed
            return *this;
        }
    };

That way you can assign the result of the operator+ to the correct
object. And the proper operator= function will be called. Do in it
what you have to.

V
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