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From: newsuser@LTH.Se (LTH network news server)
Newsgroups: comp.lang.c++
Subject: Smart pointers in C++ 2.0
Message-ID: <1989May3.162803.28414@LTH.Se>
Date: 3 May 89 15:28:02 GMT
Reply-To: dag@Control.LTH.Se (Dag Bruck)
Organization: Dept. of Automatic Control, Lund Inst. of Technology, Sweden
Lines: 58


Now that C++ 2.0 is about to make it, it seems natural to look at some new
possibilities.  One new feature is overloading of operator -> (),
which can be used for implementing ``smart pointers'' [1].

Using InterViews [2], I often create Painter objects.  For the sake of
this discussion, assume I want to create a smart pointer to Painter
which checks for nil pointers.  Today, my code looks like this:

	class Painter;
	...
	Painter* output = new Painter;
	output->Rect(0, 0, 1, 1);

With C++ 2.0 I can create a smart pointer type, like this:

	class Painter;

	class PainterPtr {
	private:
	    Painter* p;

	public:
	    PainterPtr()
		{ p = 0; }
	    PainterPtr& operator = (Painter* q)
		{ p = q; return *this; }
	    Painter* operator -> ()
		{ if (p == 0) error(); else return p; }
	};
	...
	PainterPtr output = new Painter;
	output->Rect(0, 0, 1, 1);

This is great, I think.  I my crystal ball, I can see a number of
applications of smart pointers (including less trivial uses).  The
object-oriented jargon is probably (limited) ``delegation.''

Unfortunately, all my existing code, including 30000 lines of
InterViews must be changed so ``Painter*'' is replaced by
``PainterPtr.''  My question is now:  is there a way to get the same
``smart'' functionality I outlined above, without having to change all
my existing programs?  Any other comments are welcome, of course (Is
it really useful? Can it be done in some other way? Why don't you use LISP?).

[1] Bjarne Stroustrup: ``The Evolution of C++: 1985 to 1987,'' Proc.
    USENIX C++ Workshop, Santa FE, NM, 1987.

[2] Linton, Vlissides and Calder: ``Composing User Interfaces with
    InterViews,'' IEEE Computer, February 1989.


Dag Michael Bruck
-- 
Department of Automatic Control		Internet:  dag@control.lth.se
Lund Institute of Technology
P. O. Box 118				Phone:	+46 46-108779
S-221 00 Lund, SWEDEN			Fax:    +46 46-138118