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From: ark@alice.att.com (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: problem with ** and *&
Message-ID: <20415@alice.att.com>
Date: 9 Jun 91 21:48:25 GMT
References: <MFX.91Jun8150513@kit.cs.tu-berlin.de>
Reply-To: ark@alice.UUCP ()
Distribution: comp
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 34

In article <MFX.91Jun8150513@kit.cs.tu-berlin.de> mfx@cs.tu-berlin.de (Markus Freericks) writes:

> if a 'b' can be used in the same places as an 'a' and a pointer to b can
> be used as a pointer to a; why doesn't this hold for a pointer to a
> pointer to b??

Because you could break the type system if it did.  For example:

	class A { /* stuff */ };
	class B: public A { /* other stuff */ };

	main()
	{
		A a;
		B* bp;
		A** app;

		// Suppose this were legal...
		app = &bp;

		// Then we could do this:
		*app = &a;

		// which would make bp point at "a".  Then...

		B b;
		*bp = b;	// Oops!  We've clobbered memory
	}

There are many slippery issues having to do with pointer conversions
of this kind.  Designing a workable type system is not easy.
-- 
				--Andrew Koenig
				  ark@europa.att.com