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## Class template allocator

boost::mpi::allocator — Standard Library-compliant allocator for the MPI-2 memory allocation routines.

## Synopsis

    // In header: <boost/mpi/allocator.hpp>

    template<typename T>
    class allocator {
    public:
      // types
      typedef std::size_t    size_type;        // Holds the size of objects.
      typedef std::ptrdiff_t difference_type;  // Holds the number of elements between two pointers.
      typedef T *            pointer;          // A pointer to an object of type T.
      typedef const T *      const_pointer;    // A pointer to a constant object of type T.
      typedef T &            reference;        // A reference to an object of type T.
      typedef const T &      const_reference;  // A reference to a constant object of type T.
      typedef T              value_type;       // The type of memory allocated by this allocator.

      // member classes/structs/unions

      // Retrieve the type of an allocator similar to this allocator but for a
      // different value type.
      template<typename U>
      struct rebind {
        // types
        typedef allocator< U > other;
      };

      // construct/copy/destruct
      allocator();
      allocator(const allocator &);
      template<typename U> allocator(const allocator< U > &);
      ~allocator();

      // public member functions
      pointer address(reference) const;
      const_pointer address(const_reference) const;
      pointer allocate(size_type, allocator< void >::const_pointer = 0);
      void deallocate(pointer, size_type);
      size_type max_size() const;
      void construct(pointer, const T &);
      void destroy(pointer);
    };

## Description

This allocator provides a standard C++ interface to the `MPI_Alloc_mem` and `MPI_Free_mem` routines of MPI-2. It is intended to be used with the containers in the Standard Library (`vector`, in particular) in cases where the contents of the container will be directly transmitted via MPI. This allocator is also used internally by the library for character buffers that will be used in the transmission of data.

The `allocator` class template only provides MPI memory allocation when the underlying MPI implementation is either MPI-2 compliant or is known to provide `MPI_Alloc_mem` and `MPI_Free_mem` as extensions. When the MPI memory allocation routines are not available, `allocator` is brought in directly from namespace `std`, so that standard allocators are used throughout. The macro `BOOST_MPI_HAS_MEMORY_ALLOCATION` will be defined when the MPI-2 memory allocation facilities are available.

### `allocator` public construct/copy/destruct

1.     allocator();

   Default-construct an allocator.

2.     allocator(const allocator &);

   Copy-construct an allocator.

3.     template<typename U> allocator(const allocator< U > &);

   Copy-construct an allocator from another allocator for a different value type.

4.     ~allocator();

   Destroy an allocator.

### `allocator` public member functions

1.     pointer address(reference x) const;

   Returns the address of object `x`.

2.     const_pointer address(const_reference x) const;

   Returns the address of object `x`.

3.     pointer allocate(size_type n, allocator< void >::const_pointer = 0);

   Allocate enough memory for `n` elements of type `T`.

   Parameters: |
   `n` | The number of elements for which memory should be allocated.
   Returns: | a pointer to the newly-allocated memory

4.     void deallocate(pointer p, size_type);

   Deallocate memory referred to by the pointer `p`.

   Parameters: |
   `p` | The pointer whose memory should be deallocated. This pointer shall have been returned from the `allocate()` function and not have already been freed.

5.     size_type max_size() const;

   Returns the maximum number of elements that can be allocated with `allocate()`.

6.     void construct(pointer p, const T & val);

   Construct a copy of `val` at the location referenced by `p`.

7.     void destroy(pointer p);

   Destroy the object referenced by `p`.

### Specializations

- Class allocator<void>