Questions about pointers

13 Jun 2004

      While testing a few ideas with pointers, I bumped into a behavior not
very clear to me.

During the development of one of the modules I'll be using in a
project of mine, there came a point where I decided I'll simply assign
the address of a pointer variable passed as a paramater as the value
of one of an object's fields.

While writing the code for the said object's destructor, I decided to
test what would happen to the variable, whose address I've assigned to
an object's field, after I've applied Dispose() to the latter.

Here's a sample of what I was trying to do:

    program BankTest;

    var
	BankCredit : PCredit;
	BankSample : PBank;

    begin
	New(BankCredit);
	BankCredit^ := 1000;

	New(BankSample, BankCredit);
	BankSample^.Destroy;
	writeln(BankSample^);
	Dispose(BankCredit);
    end.

TBank.Create has this:

    constructor TBank.Create(amount : PCredit);
    begin
	New(Self.AccountBalance);
	Self.AccountBalance := amount;
    end;

TBank.Destroy has this:

    destructor TBank.Destroy;
    begin
	Dispose(Self.AccountBalance);
    end;

I get no errors or warnings (compiled with -O2 and -Wall), but when I
try to run the resulting example, I get a `Segmentation fault' where
writeln(BankSample^) is.

Here are my questions:

    1) Is this correct behavior according to the standards?  I know
    that doing two New()'s on the same pointer is a recipe for
    disaster, but it is unclear to me whether the same applies to this
    scenario.

    2) If this is correct (i.e., I'm doing something I shouldn't be
    doing), then why doesn't GPC catch it?  If this is already
    planned, then I guess I should just be more careful; but if it
    isn't, would it be hard to implement this?

I apologize for the long message, but all of the Pascal resources I
have aren't very clear about this particular situation.  Any clues as
to where I should look (and/or help in general) would be greatly
appreciated.

TIA.

-- 
Political Dissident                                        Neil Santos
Free Software Advocate                      Jabber: nsantos@jabber.org

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