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From: warsaw@nlm.nih.gov (Barry A. Warsaw)
Newsgroups: comp.lang.c++
Subject: Curious about operator delete setting ptrs to zero...
Message-ID: <WARSAW.91Jan7113607@warsaw.nlm.nih.gov>
Date: 7 Jan 91 16:36:07 GMT
Sender: news@cme.nist.gov
Reply-To: warsaw@nlm.nih.gov
Organization: Century Computing, Inc.
Lines: 45


Having been bitten lately with multiple destruction of freestore
allocated objects, I'm curious as to why operator delete does not, by
default, set the pointer whose object is being deleted to zero? I'm
sure there must be some good reasons why this is not done by default
in C++, and I can perhaps(?) come up with some scenarios where this
might be a Bad Thing, but in general why can't delete perform this
very useful function, and allow overloading of delete to bypass when
this would cause problems due to a particular class's implementation.
Aside from the reason that the language definition would have to
change to facilitate this (would it? see below).

Seems to me it would be a useful thing since deleting a null pointer
should be harmless but deleting an already freed block can have
disasterous effects.  Just to quickly illustrate with a totally
contrived example:

#include <stream.h>
class Foo {
public:
	Foo( int val ) { data = val; }
	int value() { return( data ); }
private:
	int data;
};


main() {
	Foo* foo = new Foo( 10 );
	cout << "foo = " << foo << ", foo's value is " << foo->value() << endl;
	delete foo;
	cout << "foo = " << foo << endl;
	// foo = 0; // necessary since delete doesn't set foo=0;
	delete foo; // if delete set foo=0, this would be harmless;
};

So enlighten me as to why the current functionality. Also, can you
provide a hint as to how one might go about overloading operator
delete or writing a destructor to give a class the ability to set the
pointer to zero?  As I see it, it would currently be difficult since
`this' doesn't get passed to the destructor by reference and operator
delete's first argument is a void*.  Seems to me it would have to be a
void*& to allow setting to zero.  Comments?

-Barry