Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!samsung!umich!vela!schemers
From: schemers@vela.acs.oakland.edu (Roland Schemers III)
Newsgroups: comp.std.c++
Subject: Re: the most dissatisfying part of c++
Message-ID: <2314@vela.acs.oakland.edu>
Date: 31 Jul 90 18:18:10 GMT
References: <27634@netnews.upenn.edu>
Reply-To: schemers@vela.acs.oakland.edu (Roland Schemers III)
Organization: Oakland University, Rochester MI
Lines: 66

In article <27634@netnews.upenn.edu> limsoon@saul.cis.upenn.edu.UUCP (Limsoon Wong) writes:
>
>the program below is a contrived one.   it cannot  be compiled.
>the reason is a type fault. however, it exhibits a very central
>characteristic of update.    something should be  done to allow
>this kinds of programs.	   			    ---limsoon.
>
>#include <stream.h>
>class a { 
>  public: 
>    a& test() { 
>	// do some modification to the state of this object,
>	// then ...
>	return *this;
>};
>class b : public a { 
>  public:
>    int b;
>};
>main() { 
>  b B; 
>  B = B.test();
>  // type violation despite the fact that 
>  // `B' and `B.test()' are the same object.
>}

I for one do not see where 'B' and 'B.test()' are the same object. The
function B.test() clearly returns a reference to an 'a' object. What
you could do is:

class b : public a { 
  public:
    int b;
    b &test() { a::test(); return *this; }
};

(Forgive me if the syntax is not correct)

I understand your point of view though, and I think it would be nice
if there was some construct in C++ to do what you want. I know some of
my classes could use it. Maybe something like:

class a { 
  public: 
    virtual& test() { 
	// do some modification to the state of this object,
	// then ...
	return *this;
};

In otherwords, make the return type 'virtual' like functions. 
In this case, the virtual means the type of the class. So when class
'b' is derived from class 'a', all virtual return types in class 'a'
will now return type 'b'.

Of course this we be very ugly to implement, and is sort of like
parametric return types. 

Actually, would templates solve this problem?

Roland
-- 
Roland J. Schemers III                              Systems Programmer 
schemers@vela.acs.oakland.edu (Ultrix)              Oakland University 
schemers@argo.acs.oakland.edu (VMS)                 Rochester, MI 48309-4401
"Get off your LEF and do something!"                (313)-370-4323