Re: [boost] [smart_ptr] enable_shared_from_this / sp_accept_owner / virtual dtor

From: Daniel Frey (d.frey_at_[hidden])
Date: 2008-05-05 13:46:58

On Mon, 2008-05-05 at 11:13 -0400, Frank Mori Hess wrote:
> What is an ADL protector?  I'm not familiar with that idea.  I see you added a 
> namespace and a using statement, but what problem does it solve?

Generally, an ADL protector prevents derivation from
boost::enable_shared_from_this to bring in all other functions from
namespace boost. Example (try it with boost 1.35.0):

#include <boost/enable_shared_from_this.hpp>
#include <boost/utility.hpp>

namespace A
{
  struct X : boost::enable_shared_from_this< X > {};

  template< typename T > void next( const T& ) {}
}

template< typename T > void prior( const T& ) {}

int main()
{
  A::X x;

  next( x );
  prior( x );
}

That is the reason why it might be a good idea to allow/require an ADL
protector for enable_shared_from_this.

In the example implementation I showed it also solves another problem:
sp_accept_owner is called with a boost::shared_ptr as the first
argument. Given that you have a class X which is *not* derived from
enable_shared_from_this, it would pick up enable_shared_from_this'
sp_accept_owner if it's not protected by an ADL barrier (another common
name for ADL protector), since it's signature is a better match than the
default sp_accept_owner(...). By adding the ADL protector, the
sp_accept_owner inside the protector is only considered if the pointer
passed as the second argument has an accessible base class from the same
namespace (enable_shared_from_this_impl) - which can only be
enable_shared_from_this<U>.

Regards, Daniel