Re: [Boost-users] scoped_ptr faq

From: Daniel Krügler (dsp_at_[hidden])
Date: 2005-11-24 04:10:41

Bruno Martínez wrote:
> On Wed, 23 Nov 2005 08:34:26 -0200, Peter Dimov <pdimov_at_[hidden]> wrote:
> 
> 
>>Bruno Martínez wrote:
>>
>>>Hi.
>>>
>>>I had the same question that is answered there, but the answer didn't
>>>fit my problem.  I'm using scoped_ptr<node> inside node itself, so
>>>node is incomplete at the point of use.  That forbids using auto_ptr,
>>>IIUC.
>>
>>You can use a scoped_ptr member with an incomplete type, as long as you
>>include out of line default constructor (if you need one) and destructor.
> 
> 
> 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 and the main difference between std::auto_ptr and 
boost::scoped_ptr in this point is that the standard says, that "no 
diagnose is required", while boost will diagnose that due a safety belt 
in boost::checked_delete.

> 
> 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....

Greetings from Bremen,

Daniel