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From: martino@microsoft.UUCP (Martin O'RIORDAN)
Newsgroups: comp.lang.c++
Subject: Re: Curious about operator delete setting ptrs to zero...
Message-ID: <60361@microsoft.UUCP>
Date: 8 Jan 91 22:37:19 GMT
Article-I.D.: microsof.60361
References: <WARSAW.91Jan7113607@warsaw.nlm.nih.gov>
Reply-To: martino@microsoft.UUCP (Martin O'RIORDAN)
Organization: Microsoft Corp., Redmond WA
Lines: 47


The primary reason for not setting the pointer to NULL has to do with the
age old problem of aliases in C.  This has of course been inheritted by C++.
Consider the following code fragment :-

	void foo ()
	{
	    char * p, * q;

	    p = new char[ 25 ];
	    q = p;	// The alias

	    delete p;	// sets 'p' to NULL !!
	    delete q;	// But what about the alias ?
	}

Okay, so you can say that a clever compiler could do enough analyses to
determine that 'q' is a statically determinable alias for 'p'.  However,
typical alias problems are not so simple.  Pointers and heap are primarily
used to control object lifetimes in ways separate to the static/lexical
control flow of a program.  Pointers are often passed beyond the function
that allocated the object.  Consider :-

	extern void bar ( char * );

	void foo ()
	{
	    char * p;

	    p = new char[ 25 ];

	    bar ( p );	// The compiler doesn't know what 'bar' does with the value

	    delete p;	// What if 'bar' already deleted the space ?
	}

This case, despite being simple in form is impossible for a compiler based on
separate compilation technology to analyse for aliasing.  To achieve this kind
of thing, pointers would need to be handles which referred to the object only
through an indirect table, as in PASCAL.  The improved security for deletion
would be offset by the considerable increase in access cost for the object.

To add the facility for 'delete' to set the pointer provided to NULL, would
in effect impose a mechanism such as this on the language.  It is not a small
scale change.

	Martin O'Riordan