// In header: <boost/pool/pool_alloc.hpp>
template<typename UserAllocator, typename Mutex, unsigned NextSize, 
         unsigned MaxSize> 
class pool_allocator<void, UserAllocator, Mutex, NextSize, MaxSize> {
public:
  // types
  typedef void *       pointer;      
  typedef const void * const_pointer;
  typedef void         value_type;   
  // member classes/structs/unions
  // Nested class rebind allows for transformation from pool_allocator<T> to
  // pool_allocator<U>.
  template<typename U> 
  struct rebind {
    // types
    typedef pool_allocator< U, UserAllocator, Mutex, NextSize, MaxSize > other;
  };
};