Re: [Boost-users] shared_ptr, auto_ptr and enable_shared_from_this confusion

Subject: Re: [Boost-users] shared_ptr, auto_ptr and enable_shared_from_this confusion
From: ÕÅÑÇö­ (kimi67300387_at_[hidden])
Date: 2009-03-14 02:52:58

shared_ptr's constructor in line 168 calls 'boost::detail::sp_enable_shared_from_this( pn, p, p );'.

'boost::shared_ptr<IDerived> c(new Derived());' is dispatched to 'sp_enable_shared_from_this' in line 86. pay attension to the second parameter, it can accept all types that inherit 'enable_shared_from_this<T>', so does 'Derived*'.

While, 'boost::shared_ptr<IDerived> c((IDerived*)new Derived());' is dispatched to 'sp_enable_shared_from_this' in line 111, bacause the static type IDerived* has nothing to do with 'enable_shared_from_this<T>' and can not be implicitly converted to 'enable_shared_from_this<T>'.

Then check out the difference between the two 'sp_enable_shared_from_this'. It is obvious that the second usage shall cause a BOOST_ASSERT failed in 'enable_shared_from_this::shared_from_this', so an exception is raised.

However, I think the root reason is not the language nor boost, but the design of class IDerived. Its root and root1 seem a little bit irrational because there is not direct relationship between IDerived and Base(IBase).



------------------ Original ------------------
From:  "Ryan McConnehey"<mccorywork_at_[hidden]>;
Date:  Sat, Mar 14, 2009 04:29 AM
To:  "Boost-users"<boost-users_at_[hidden]>; 

Subject:  [Boost-users]  shared_ptr,auto_ptr and enable_shared_from_this confusion

 
  I'm having some trouble with using shared_ptr, auto_ptr and 
enable_shared_from_this with my inheritance layout.  The following is 
how my inheritance is laid out.

// Inheritance Layout
//
// boost::enable_shared_from_this<T>
//   ^
//   |
// Base --------> IBase
//   ^
//   |
// Derived -----> IDerived

In my test code (that I've included with this email) the following two 
ways to instantiate a shared pointer is equivalent.

boost::shared_ptr<IDerived> r (new Derived());

std::auto_ptr<IDerived> k (new Derived);
boost::shared_ptr<IDerived> l (k.release());

When creating the shared_ptr, both declaration take me to line 185 of 
shared_ptr.hpp.  Instantiating the shared_ptr directly then takes me to 
line 105 of shared_ptr.hpp.  Though when instantiating by releasing the 
auto_ptr into the shared_ptr I'm taken to line 129.  It seems to me that 
I should be going to the same line of code.  This wouldn't be such a 
problem except when trying to call shared_from_this() an exception is 
thrown if my shared_ptr was created from an auto_ptr.  Is this behavior 
correct?

Ryan