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

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; };
};


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:


   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;
     };
   };