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";
Date: Sat, Mar 14, 2009 04:29 AM
To: "Boost-users";
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
