[Boost-users] enable_shared_from_this()

Subject: [Boost-users] enable_shared_from_this()
From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2013-02-04 12:09:29

Can someone please explain why this code:

struct Foo : boost::enable_shared_from_this<Foo> {};
Foo* p_foo = new Foo;
boost::shared_ptr<Foo> ptr_foo(p_foo->shared_from_this());

is throwing?

         msvcr100d.dll!_CxxThrowException(void * pExceptionObject, const
_s__ThrowInfo * pThrowInfo)  Line 157	C++
         boost::throw_exception<boost::bad_weak_ptr>(const
boost::bad_weak_ptr & e)  Line 61 + 0x4d bytes	C++
         boost::detail::shared_count::shared_count(const
boost::detail::weak_count & r)  Line 421 + 0x23 bytes	C++
         boost::shared_ptr<`anonymous namespace'::Foo>::shared_ptr<`anonymous
namespace'::Foo><`anonymous namespace'::Foo>(const
boost::weak_ptr<`anonymous namespace'::Foo> & r)  Line 216	C++
         boost::enable_shared_from_this<`anonymous
namespace'::Foo>::shared_from_this()  Line 49 + 0xf bytes	C++
>	`anonymous namespace'::test_method()  Line 41 + 0xf bytes	C++

Since Foo embeds a weak_ptr, why can't it new the pi_ on the spot?
(I'm using Boost 1.44).

Thanks, --DD

PS: On a related note, why doesn't boost::shared_ptr<Foo>(p_foo)
(where p_foo is a raw Foo*) implicitly apply the aliasing Ctor, to
share the same px, to avoid the classic bug of having several
different pn on the same px, since p_foo derives from
enable_shared_from_this()? Initially i thought that's what
sp_enable_shared_from_this() and pe->_internal_accept_owner() were
meant to do, but obviously that's not the case.