id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
1996	"""void"" template specialisation for the pool allocators"	Armin Felke	Chris Newbold	"It might be reasonable to add a template specialisation for the allocator classes ""pool_allocator<T,..>"" and ""fast_pool_allocator<T,..>"" with T=void?

The library boost::multi_index, e.g., currently (version 1.35.0) uses the allocator<void> notation in its generic code (expecting a ""default allocator"").

In section 20.4.1 [lib.default.allocator] of the C++ standard the following specialisation is defined for class std::allocator as follows:

<code>
template <class T> class allocator;
//  specialize for  void:
template <> class allocator<void> {
public:
   typedef void*        pointer;
   typedef const void*  const_pointer;
   // reference-to-void members are impossible.
   typedef void  value_type;
   template <class U> struct rebind { typedef allocator<U> other; };
};
</code>

In order to use the pool allocators with multi_index_container and ease generic programming, I would suggest the following additions to pool_alloc.hpp:

<code>
   template<
     typename UserAllocator,
     typename Mutex,
     unsigned NextSize>
   class pool_allocator<void, UserAllocator, Mutex, NextSize> {
   public:
     typedef void*       pointer;
     typedef const void* const_pointer;
     typedef void        value_type;
     template <class U> struct rebind {
         typedef pool_allocator<U, UserAllocator, Mutex, NextSize> other;
     };
   };

   template<
     typename UserAllocator,
     typename Mutex,
     unsigned NextSize>
   class fast_pool_allocator<void, UserAllocator, Mutex, NextSize> {
   public:
     typedef void*       pointer;
     typedef const void* const_pointer;
     typedef void        value_type;
     template <class U> struct rebind {
         typedef fast_pool_allocator<U, UserAllocator, Mutex, NextSize> other;
     };
   };
</code>

Best regards
Armin Felke
"	Patches	closed	Boost 1.37.0	pool	Boost 1.35.0	Optimization	fixed