Re: inheritance headache....

"Alf P. Steinbach" <>
Thu, 31 Jan 2008 23:12:57 +0100

I have a construction problem that I need to fix;

currrently the following hierarchy exists;

       / \
      / \
 conreteTarget1 concreteTarget2

       abstractDescription (this class family can NOT be modified)
         / \
   concreteDesc1 concDesc1

Now somewhere, actually EVERYWHERE in this legacy code I see
statements like this;

// semi pseudo with
abstractTarget* createTarget(abstractDescription* desc)

// shortcut pseudo code for dynamic cast< > ()
   if ( desc is concreteDesc1)
     return new concreteTarget1();
   else if (desc is concreteDesc2)
     return new concreteTarget2();
     return 0;



Now I CANT add methods to abstractDescription, if I could I would have
a method;

class abstractDescription
  AbstractTarget* create(AbstractDescription* desc)


It's clear what you mean, something like

   struct AbstractDescription
       virtual std::auto_ptr<AbstractTarget> create() const = 0;

and each ConcreteDescription subclass would override as needed the
create the right concreteTarget.

BUT I cant do that as the AbstractDesciption family of objects cannot
be modified in any way. Can anybody help me out?

Im trying to tease out a solution here but I always get stung with a
cast or such like. anybody out there have any ideas?

E.g. install factory functions in a std::map using typeinfo (from typeid
operator) as key. Then given an AbstractDescription, apply typid, look
up pointer in map, call resulting factory function. Only tricky thing
is to get that map properly initialized.

Cheers, & hth.,

- Alf

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