Re: Polymorphism and vectors

[chris jefferson <caj at cs dot york dot ac dot uk>]

Anakreon wrote:

> I have a little problem with polymorphism and vectors.
This mailing list is for the development OF libstdc++, not WITH, so you 
shouldn't really be mailing here unless you think you've found a bug in 
the library. gcc-help (or a newsgroup, such as comp.lang.c++) would be 
more approriate. However, I'll answer your question anyway :)

> ------------------------------------------------------------------------
>
> ...
>
>  vector<base*> vect;
> ...
>  for (vector<base*>::iterator it = vect.begin();
> 	   it != vect.end(); it++) {
> 	cout << typeid(*it).name() << endl;
>  }
>
>  
Here you are asking for the type of a pointer to base, and so thats what 
you get!
If you want the type of the thing the pointer points to, you have to do 
typeid(**it).name(), which then gets you what you would expect. This is 
broadly  the right way to do what you are trying to do (although  
getting one of the various reference-counted pointers will make your 
life much easier in terms of not having to manually delete things as and 
when you eject them from the vector).

> ...
>  vector<base> v2;
>  
>...

>  for (vector<base>::iterator it = v2.begin();
> 	   it != v2.end(); it++) {
> 	cout << typeid(*it).name() << endl;
>  }
>  

Here you have a problem with something known as slicing. Basically the 
vector will convert all the objects to bases when they are inserted, by 
ignoring the non-base part. This is bad! :)

Chris