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From: bytor@ctt.bellcore.com (Ross Huitt)
Newsgroups: comp.object,comp.lang.pascal
Subject: Re: Shouldn't debuggers be able to debug dynamic objects?
Message-ID: <1991Jan28.151241.25449@bellcore.bellcore.com>
Date: 28 Jan 91 15:12:41 GMT
References: <1991Jan24.152256.24273@resam.dk> <1991Jan25.145947.16431@bellcore.bellcore.com> <1991Jan28.100213.1822@resam.dk>
Sender: usenet@bellcore.bellcore.com (Poster of News)
Distribution: comp
Organization: Computer Technology Transfer, Bellcore
Lines: 39

In article <1991Jan28.100213.1822@resam.dk> andrew@resam.dk (Leif Andrew Rump) writes:
>In <1991Jan25.145947.16431@bellcore.bellcore.com> bytor@ctt.bellcore.com (Ross Huitt) writes:
>
>>When I _know_ which derived class a base class ptr is pointing to
>>and I want to inspect or display the object as a derived class, a cast
>>is usually sufficient to get the job done. 
>
>But, but, but, ... Why can't the debugger that has _all_ the information
>do this job! The main reason for me to write this letter was that one
>of my programmes used a dynamic list of objects and if I was going to
>use the debugger to show me the content of for example the first three
>objects in a list a had to remember what type each object was and change
>casts accordingly - then (as I wrote in my original letter) I surely
>sill make mistakes and then the debugger only caused havoc!!!
>
>Leif Andrew
>
>UUCP: andrew@resam.dk, phone: +45 32 32 51 54                \
>> > Read oe as: o <backspace> / (slash) and OE as O <backspace> / (slash) < <

The debugger does not necessarily have _all_ of the information for
objects that are dyanmically allocated.  If a derived object is later
referenced by a base pointer the only piece of information I can
think of that a debugger could use to determine the compile-time type of
would be the pointer to the virtual function table.  If the classes
involved don't have a virtuals then even this won't work.  (I know
there are other cases, too, so don't bother flaming me.) To accomplish
what you want would require support from the compiler such as embedding
a class signature in each instance of a class.  The debugger would then
read this signature to determine the class of the object regardless
of the class of the pointer.  This level of meta-class support would
be very welcome especially if we programmers had access to it.  Its
one aspect of Smalltalk that I miss very much in C++. BTW, there
are programming techniques that you can use to determine the classes of your
objects at run time.  You would still have to use casts in the debugger,
but at least you could execute a member function to determine the
class of the object before you used a cast. 

	-bytor@ctt.bellcore.com