Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!inria!irisa!boissier
From: boissier@irisa.fr (franck boissiere)
Newsgroups: comp.std.c++
Subject: Re: the most dissatisfying part of c++
Message-ID: <1990Aug2.061225.22629@irisa.fr>
Date: 2 Aug 90 06:12:25 GMT
References: <56236@microsoft.UUCP>
Sender: news@irisa.fr
Organization: IRISA, Rennes (FR)
Lines: 46

From article <56236@microsoft.UUCP>, by bobatk@microsoft.UUCP (Bob ATKINSON):
> 
> Yes, this is tedious for the class writer, but it makes client's job
> a lot easier.  I personally agree, though, that it would be 
> worthwhile investigating making the following constructions legal:
> 
> 	class Collection {
> 	public:
> 		virtual	Collection *	Copy();
> 	};
> 
> 	class Set : public Collection {
> 	public:
> 		virtual Set *		Copy();	   // illegal today
> 	};
> 
> 
> At present, this gives an illegal virtual return type error.
> 
> The change to the existing rules is to allow overidings of virtual
> functions to return a type to which a standard conversion may be
> applied to obtain the inherited return type.  ("standard conversion"
> may be too general; perhaps only derived-to-base standard conversions
> should be applicable).  The appropriate member function signature would 
> be chosen according to the static type of the pointer through which
> it is invoked.

As you said this is a tedious task for the programmer and I would propose
a construct such as:

       class Collection {
       public:
               virtual WHATEVER_CONCSTRUCT_MEANING_THE_CLASS_OF_RECEIVER *    Copy();
       };

With such an approach you still get type checking and you don't have to
rewrite the same line for each class in the subclass hierarchy. You would gain
a lot and could then write truly generic behaviors. This would be a good
starting point for reusable classes. The same idea could be extended for
arguments if needed.

--
Franck BOISSIERE                        boissier@irisa.irisa.fr
Prototyping Lab Manager                 boissier@ccettix.UUCP
C.C.E.T.T.   B.P. 59                    boissier%irisa.irisa.fr@uunet.uu.net
35512 CESSON SEVIGNE CEDEX  FRANCE