Converting boost::shared_ptr< const T > to boost::shared_ptr< T >

From: Alex Tibbles (yg-boost-users_at_[hidden])
Date: 2003-04-11 04:48:41

Apologies, news editor != text editor (cntrl-enter != autocomplete).

I have persistent objects in my appplication and their factory dishes out
only constant references (more precisely only copies of a shared_ptr< const
T >). In order to make the persistent objects writable, an underlying record
needs to be locked in the RDBMS. A friend template instantiation (a
WritablePointer< T >) controls this locking, and effectively gives non-const
access to the persistent object.
The WritablePointer's constructor takes a shared_ptr< const T >. What I want
to be able to do is like this (with non-smart pointers):

template< typename T >
class WritablePointer
{
    public:
        WritablePointer(const T* const p)
        : p_(const_cast< T* const >(p))
        {
            p_->lock();
        }
        // ...
    private:
        T *p_;
};

I have:

template< typename T >
class WritablePointer
{
    public:
        typedef typename boost::shared_ptr< T > SmartPointerType;
        typedef typename boost::shared_ptr< const T > ConstSmartPointerType;
        WritablePointer(const ConstSmartPointerType& p)
        : p_(static_cast< const SmartPointerType& >(p))
        {
            p_->lock();
        }
    private:
        SmartPointerType p_;
};

This compiles under gcc 3.2.1 but with a warning:

/usr/include/boost/shared_ptr.hpp:153: warning: invalid conversion from
`const
   T* const' to `T*'
/usr/include/boost/shared_ptr.hpp: In constructor

[I have changed the concrete type from my application to T to simplify].

What I want to do is cast away the const-ness in exactly one place - the
WritablePointer ctor. But because I get the warnings it's harder to spot
truly illicit conversions.
(I copied this technique from an article I read (on Dr. Dobb's or CUJ
probably) which used volatile and a mutex to protect shared data between
threads, and a smart pointer much like mine to provide write access.

I'd be grateful for any suggestions.

Alex