Re: virtual operator +

 James Kanze <>
Wed, 12 Sep 2007 11:26:09 -0000
On Sep 11, 4:17 pm, Hunk <> wrote:

I ws wondering if there is a way to implement operator+ in case of
virtual classes.

In general, polymorphism and operator overloading don't work
well together; operator overloading requires a value type for
the return values. (In cases where it is necessary, the
letter/envelop idiom can be used to may a class with value
semantics act polymorphically.)

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 {

class Char_string : Base_string {

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
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

Just a general question: if I add a Char_string< 16 > and a
Hash_string< 14 >, what concrete type should the results have?
This is an important aspect of the problem. But not the only
aspect. Typically, for addition if there are N different
derived types, then you have NxN different functions to
implement, each with potentially a different return type (or
different rules for determining the return type).

I'm not sure what the difference is between a Hash_string and a
Char_string, but for just a single hierarchy, where the derived
types only differ in length, you can probably work something out
with templates, e.g.:

    template< size_t L, size_t R >
    FixedLengthString< L + R >
    operator+( FixedLengthString< L > const& lhs,
               FixedLengthString< R > const& rhs ) ;

Trying to use polymorphism here, however, will not work.

James Kanze (GABI Software)
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