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From: bobatk@microsoft.UUCP (Bob ATKINSON)
Newsgroups: comp.std.c++
Subject: Re: the most dissatisfying part of c++
Message-ID: <56236@microsoft.UUCP>
Date: 1 Aug 90 19:13:52 GMT
References: <27634@netnews.upenn.edu>
Reply-To: bobatk@microsoft.UUCP (Bob ATKINSON)
Organization: Microsoft Corp., Redmond WA
Lines: 138

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.
>}


This is a not-infrequent idiom.  Another example of its use is
creating a "copy" method for collections (what's the return type?)
What one would like to write is

	class Collection {
	public:
		virtual ??? Copy();
	};

which will then be overridden in derived classes to do the 
appropriate copy operation.  Replacing the question marks with 
"Collection*" is not acceptable, for then clients would loose
static type information, as in


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

	class Set : public Collection {
	public:
		virtual Collection* Copy();	// override for implementation
	}

	//...

	Set* pset, pset2 = ...;

	pset = pset2->Copy();	// illegal!


An approach to solving this problem involves using _non-virtual_ 
member functions.


	class Collection {
	protected:
		virtual Collection* _Copy();	// The real copy function
	public:
		Collection* Copy() { return _Copy(); }

	};

	class Set : public Collection {
		virtual Collection *_Copy();	// override implemenation
	public:
		Set* Copy() { return (Set *)_Copy(); }
	}

	//...

	Set* pset, pset2 = ...;

	pset = pset2->Copy();	// legal now


In the context of the example Limsoon presented, we would write:

	class a {
	protected:
		virtual void	_test()	{ /* do the real work here }
	public:
			a&	test()  { _test(); return *this; }
	};

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

	main()
		{
		a A;
		b B;
		B = B.test();	// legal
		}


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.


	Bob Atkinson
	Microsoft	

[Disclaimer: I am but one solitary opinionated C++ programmer]