Transfer functionality for shared_ptr

From: ("Alf P. Steinbach")
Fri, 12 Oct 2007 21:19:18 GMT
If you have a shared_ptr, perhaps with a custom deleter of unknown type,
what extra functionality is required to then transfer that shared_ptr's
raw pointer and deleter to some other smart pointer?

In order to answer that question I created the small test program listed
below, where the class SharedPtr is a simplified variant of shared_ptr
(only features relevant to this problem).

The transferTo() member function seems to require, as shown here, two
member functions values() and clear() which would be very dangerous,
breaking encapsulation, if exposed to client code.

In other words, the transferTo() member function seems to be safe and
useful functionality that can only reasonably be implemented by
shared_ptr itself, and is not currently present.

I see no way to implement transferTo() using only the current public
functionality of shared_ptr.

// Example of transferTo() (a.k.a. "release") functionality for shared
// pointer.

// I had to fix up some formatting due to line-wrapping in posting.
// ...

#include <iostream>
#include <ostream>

#include <cstddef> // std::size_t
#include <utility> // std::pair
#include <stdexcept>

#include <boost/intrusive_ptr.hpp>
#include <boost/function.hpp>
#include <boost/type_traits/remove_const.hpp>

template< typename T >
void say( T const& v ) { std::cout << v << std::endl; }

namespace ext
     namespace detail
         template< typename T >
         class AbstractRefCounter
             std::size_t myRefCount;
             typedef boost::function< void (T const*) > AbstractDeleter;
             typedef std::pair< T*, AbstractDeleter > InternalValues;

             AbstractRefCounter(): myRefCount( 0 ) {}
             virtual ~AbstractRefCounter() {}
             void addRef() { ++myRefCount; }
             void release() { if( --myRefCount == 0 ) { delete this; } }
             std::size_t count() const { return myRefCount; }

             virtual InternalValues values() const = 0;
             virtual void clear() = 0;

         template< typename T >
         void intrusive_ptr_add_ref( AbstractRefCounter<T>* p )
         { p->addRef(); }

         template< typename T >
         void intrusive_ptr_release( AbstractRefCounter<T>* p )
         { p->release(); }

         template< typename T >
         void defaultDeleter( T const* p ) { delete p; }

         template< typename T, typename D >
         class DeleterInvoker
             typedef typename boost::remove_const<T>::type NcT;

             D deleter;

             DeleterInvoker( D d ): deleter( d ) {}
             void operator()( T const* p ) const
                 deleter( const_cast<NcT*>( p ) );

         template< typename T, typename D >
         class RefCounter: public AbstractRefCounter<T>
             T* myPtr;
             DeleterInvoker<T, D> myDeleter;
             typedef typename AbstractRefCounter<T>::InternalValues

             RefCounter( T* p, D d ): myPtr( p ), myDeleter( d ) {}
             ~RefCounter() { if( myPtr != 0 ) { myDeleter( myPtr ); } }

             InternalValues values() const
                 return InternalValues( myPtr, myDeleter );

             void clear() { myPtr = 0; }

     template< typename T >
     class SharedPtr
         typedef boost::intrusive_ptr< detail::AbstractRefCounter<T> >
         RefCountedPtr myPtr;
         typedef typename detail::AbstractRefCounter<T>::InternalValues

         SharedPtr(): myPtr() {}

         template< typename D >
         SharedPtr( T* p, D d )
             : myPtr( new detail::RefCounter<T, D>( p, d ) )
         bool isUnique() const
         { return (!myPtr? false : myPtr->count() == 1); }

         template< typename ValuesReceiver >
         ValuesReceiver transferTo()
             if( !isUnique() )
                 throw std::logic_error(
                     "SharedPtr::transferTo: is not unique()"
             InternalValues const values = myPtr->values();
             return ValuesReceiver( values );
} // namespace ext

template< typename T >
void destroy( T* p )
     say( "automatically deleted" );
     delete p;

enum TransferTestEnum{ testTransferSuccess, testTransferFailure };

void cppMain( TransferTestEnum whichTest )
     typedef ext::SharedPtr<int> IntPtr;

     IntPtr p( new int( 42 ), destroy<int> );
     IntPtr q;

     if( whichTest == testTransferFailure ) { q = p; }
     say( "main" );
     IntPtr::InternalValues const values =
     say( *values.first ); // 42
     delete values.first;
     say( "manually deleted" );

int main( int nArgs, char*[] )
             nArgs == 1? testTransferSuccess : testTransferFailure
         return EXIT_SUCCESS;
     catch( std::exception const& x )
         std::cerr << "!" << x.what() << std::endl;
         return EXIT_FAILURE;

<output arguments="">
manually deleted

<output arguments="whatever">
automatically deleted
!SharedPtr::transferTo: is not unique()

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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