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From: haskell@janus.Berkeley.EDU (Paul Haskell)
Newsgroups: comp.lang.c++
Subject: QUESTION:  calling appropriate copy constructor
Message-ID: <38098@ucbvax.BERKELEY.EDU>
Date: 12 Aug 90 07:51:31 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: haskell@janus.Berkeley.EDU (Paul Haskell)
Distribution: usa
Organization: University of California, Berkeley
Lines: 46

Does anyone have useful insight on the following question?

Here is our (simplified) situation...

One of our applications has a base class "video_obj" and
derived classes "bitmap", "graphics",  and "text".
We have a list of pointers to video_obj called "list_elems"; the actual
pointed-to objects may be of any of the derived types.

We would like to be able to make copies of the list's members.
To make a copy, currently we find the list entry we wish to duplicate
with something like:

    int j = 0;
    while((j < list_length)&&(win_id_to_match != list_elems[j].win_id))
    {
      j++;
    }
    if (win_id_to_match == list_elems[j].win_id)
    {
       ...  ...
    }

(So, our method of deciding which list element to copy has nothing to do
with the actual type of the list element.)

Presumably for "... ..." we have to call either
  "new_pointer = new bitmap(list_elems[j])",
  "new_pointer = new graphics(list_elems[j])", or
  "new_pointer = new text(list_elems[j])"
depending on the derived type of "list_elems[j]".

So, here is the question:   Is there a cleaner way of deciding which
constructor to call other than
1)  adding a virtual function "return_type()" to video_obj and its
    derived classes that returns the actual type of the object
2)  switch()'ing on list_elems[j].return_type()  ?

The problem with this method is that when new derived classes are
designed, the designer has to remember to add the virtual return_type()
function with a unique return-value AND to modify the switch() to
contain another "new derived_class(list_elems[j])" statement.

Thanks very much for any suggestions.

Paul