Re: [Boost-users] scoped_ptr faq

From: Daniel Krügler (dsp_at_[hidden])
Date: 2005-11-28 03:57:07

Bruno Martínez wrote:
> On Thu, 24 Nov 2005 07:10:41 -0200, Daniel Krügler <dsp_at_[hidden]> wrote:
> 
> 
>>Bruno Martínez wrote:
>>
>>
>>>I know.  The problem is that I want incomplete types support *and*  
>>>release
>>>member function.
>>
>>Use std::auto_ptr and ensure that you obey Peter Dimov's
>>recommendations. I know, that the standard says, in 5.3.5
>>
>>"If the object being deleted has incomplete class type at the point of
>>deletion and the complete class has a
>>non-trivial destructor or a deallocation function, the behavior is
>>undefined."
>>
>>and in 17.4.3.6
>>
>>"if an incomplete type (3.9) is used as a template argument when
>>instantiating a template component."
>>
>>There exists no special restriction on std::auto_ptr for incomplete
>>classes
> 
> 
> I don't understand.  There is a restriction.  It's 17.4.3.6!

OK, so you have to write your own auto_ptr or move_ptr.

 From a legal point of view, you can't use auto_ptr for incomplete
classes, from a practical point of view you can do that provided that
some rules concerning d'tor definition are obeyed.

>>>I don't seem to require a destructor in this case, for vc71 or gcc3.4:
>>>
>>>#include <boost/scoped_ptr.hpp>
>>>
>>>struct node {
>>>     boost::scoped_ptr<node> next;
>>>};
>>>
>>>int main()
>>>{
>>>     node n;
>>>}
>>
>>This snippet does not proof, that no d'tor is needed, because node::next
>>is empty for the complete program run-time. Just assign a node to next,
>>and you will see...
> 
> 
> I did, but it still works:
> 
> #include <boost/scoped_ptr.hpp>
> 
> struct node {
>      boost::scoped_ptr<node> next;
>      node(node* next = 0) : next(next) {}
> };
> 
> int main()
> {
>      node n;
>      n.next.reset(new node);
> }

I wasn't carefully reading your code, obviously ;-) It works, because at 
the point of the destructor of next the hosting class node is completely 
defined. You have a special case here, not the classical pimpl problem:

struct A {
   struct Impl;
   boost::scoped_ptr<Impl> pimpl;
};

because of your next member referencing the hosting class itself.
In the A case pimpl's d'tor does not have the complete definition of 
Impl available, in contrast to your node class.

Greetings,

Daniel