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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Dangling pointer?
Date: 1996/04/16
Message-ID: <Dpz2BK.L4y@world.std.com>#1/1
X-Deja-AN: 147841383
references: <4l0r36$jte@dewey.csun.edu>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-04-16T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <4l0r36$jte@dewey.csun.edu>, chen <kc44097@csun.edu> wrote:
>
>    What is "dangling pointer",can someone give me a defination and example?
>Please e-mail me kc44097@huey.csun.edu

A dangling pointer is a pointer that points to something that no longer
exists, or is invalid in some way.  For example:

    X: Some_Pointer := new Whatever;
    Y: Some_Pointer := X;
    ...
    Free(Y);
    ... -- Now, X is a dangling pointer, and you better not say X.all.

You can also create dangling pointers by using the 'Unchecked_Access
attribute to create a pointer to a local variable, and save that pointer
in a global.  When the procedure is left, the global contains a dangling
pointer.

In general, in Ada, dereferencing dangling errors is not detected -- you
won't usually get a sensible error message, but your program may go
haywire.

According to the RM, it's not a bug to create dangling pointers.  It's a
bug to deference them.

- Bob