TDPL: Manual invocation of destructor

[Lutger]

Steven Schveighoffer wrote:

> On Mon, 09 Aug 2010 08:28:38 -0400, Andrej Mitrovic
> <andrej.mitrovich at gmail.com> wrote:
> 
>> It's rather perplexing, isn't it? It states in TDPL:
>>
>> "After you invoke clear, the object is still alive and well, but its
>> destructor has been called and the object is now carrying its
>> default-constructed stated. During the next garbage collection, the
>> destructor is called again, because the garbage collector has no idea in
>> what state you have left the object."
> 
> This seems totally wrong, what if an object has no default constructor?
> The spec used to say (maybe it still does) that a destructor is guaranteed
> to only ever be called once.

The spec still does, it is not updated since it describes delete, not clear. 

If you omit the default constructor, no constructor will be called. Also not for 
the base classes even if they have a default constructor. This looks like a bug. 

Confusingly, if an object has a default constructor but is constructed from 
anything else, clear will still call the default constructor. I reckon it is 
also surprising if you later insert a previously omitted default constructor 
that the behavior can change a lot, especially when base classes are involved. 
 
> I honestly thought the point of clear was to simply leave the memory in
> place as a kind of "zombie" object until the GC could collect it, to avoid
> having the block get recycled into a new object, and then use an old
> reference to it (via delete).  I didn't know someone would ever
> purposefully use it.  What is the point of calling clear then, if clear
> doesn't get rid of the object and leave it uninitialized?
> 
>> But in what situation would you want to manipulate an object that was
>> already cleared and ready for garbage collection?
> 
> None.  That's the point.  clear is saying "I don't want to use this object
> any more".  The runtime (I thought) was just being conservative and
> leaving the memory in place until it could verify there were no other
> pointers to it.
> 
> I'm starting to climb the fence into the "leave delete in the language"
> camp...
> 
> -Steve