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: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-03-13 17:03:34

AMDG

Ryan McConnehey wrote:
> 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?

The difference is that when you use direct construction,
the pointer that you pass to the constructor is of type
Derived*, but when you use auto_ptr, the constructor
argument is of type IDerived.  For enable_shared_from_this
to work, the static type of the pointer passed to the shared_ptr
constructor must inherit from enable_shared_from_this.

In Christ,
Steven Watanabe