Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Header boost/pointer_to_other.hpp

The pointer to other utility provides a way, given a source pointer type, to obtain a pointer of the same type to another pointee type. The utility is defined in boost/pointer_to_other.hpp.

Rationale

When building pointer independent classes, like memory managers, allocators, or containers, there is often a need to define pointers generically, so that if a template parameter represents a pointer (for example, a raw or smart pointer to an int), we can define another pointer of the same type to another pointee (a raw or smart pointer to a float.)

template <class IntPtr> class FloatPointerHolder { // Let's define a pointer to a float typedef typename boost::pointer_to_other <IntPtr, float>::type float_ptr_t; float_ptr_t float_ptr; };

Synopsis

namespace boost { template<class T, class U> struct pointer_to_other; template<class T, class U, template <class> class Sp> struct pointer_to_other< Sp<T>, U > { typedef Sp<U> type; }; template<class T, class T2, class U, template <class, class> class Sp> struct pointer_to_other< Sp<T, T2>, U > { typedef Sp<U, T2> type; }; template<class T, class T2, class T3, class U, template <class, class, class> class Sp> struct pointer_to_other< Sp<T, T2, T3>, U > { typedef Sp<U, T2, T3> type; }; template<class T, class U> struct pointer_to_other< T*, U > { typedef U* type; }; } // namespace boost

If these definitions are not correct for a specific smart pointer, we can define a specialization of pointer_to_other.

Example

// Let's define a memory allocator that can // work with raw and smart pointers #include <boost/pointer_to_other.hpp> template <class VoidPtr> class memory_allocator { // Predefine a memory_block struct block; // Define a pointer to a memory_block from a void pointer // If VoidPtr is void *, block_ptr_t is block* // If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block> typedef typename boost::pointer_to_other <VoidPtr, block>::type block_ptr_t; struct block { std::size_t size; block_ptr_t next_block; }; block_ptr_t free_blocks; };