On Mon, 05 Sep 2011 22:05:56 -0400, Dan Olson <zans.is.for.c...@yahoo.com>
wrote:
Jonathan M Davis <jmdavisp...@gmx.com> writes:
On Tuesday, September 06, 2011 02:12:11 Andrej Mitrovic wrote:
On 9/6/11, Jonathan M Davis <jmdavisp...@gmx.com> wrote:
> Why would you
> ever try and use an object that had been cleared?
TDPL, that's why. Things might have changed but how would someone new
to D know that?
I really don't pay much attention to clear, since I rarely use it, and
I don't
know why you'd ever actually want to try and use an object that you
cleared. I
do remember discussions about making it clear out the vtable, since you
_want_
it to blow up after it's been cleared rather than have an invalid
object work
on some level. Rereading the relevant section in TDPL though, it does
look
like it describes a different situation than seems to have been
ultimately
settled on. So, I guess that I don't know exactly what the situation
currently
is or what it's supposed to be. But I wouldn't have expected clear to
result
in a valid object. But clearing out the vtable at least makes it blow up
nicely instead of doing who knows what with memory when you try and call
functions on an invalid object. So, if the ultimate goal is just to
make it
memory safe, then both clearing out the vtable and the situation that
TDPL
describes do the trick, though deleting the vtable actually helps tell
you
that you've used an object when you shouldn't, whereas TDPL's
description
results in using the object succeeding, which probably isn't a good
thing.
- Jonathan M Davis
Thanks for the clearification.
It was based on TDPL 6.3.5 that I thought the idea of clear() was to
clean up but leave the thing in an initialized state. And because
clear() on dynamic arrays and other types seems to be be intended to
leave you with a valid and initialized object. Or is that not true?
There is a large issue with leaving a class in a "valid state". A class's
valid state only occurs after a constructor is called. The same is not
true for structs.
So we have problems:
1. what if the class has no default constructor? What to use as
parameters?
2. if it does have a default constructor, what if that default constructor
is costly, or allocates more resources? Didn't we just designate it for
destruction?
The point of clear is to call the *destructor*, not the constructor, and
not deallocate the memory. We could have left the object data in its init
state (i.e. before any ctor is called), but that would leave the false
impression that it's valid. By clearing out the vtable, we guarantee it's
not valid and that any usage will create a huge noticeable error.
Basically, if you clear an object, or an array, or anything, you are
saying "I no longer need this, GC, collect this when you can" you
shouldn't use it again unless you reinitialize it. All dtors are called,
and it's invariant may be invalid. The only thing that's left is a
placeholder to ensure you cannot corrupt anything else.
However, you can reassign it to a newly initialized struct or object, and
you can continue to use it after that.
-Steve
