[section:user_data_types User-defined data types]

The inclusion of `boost/serialization/string.hpp` in the previous
examples is very important: it makes values of type `std::string`
serializable, so that they can be be transmitted using Boost.MPI. In
general, built-in C++ types (`int`s, `float`s, characters, etc.) can
be transmitted over MPI directly, while user-defined and
library-defined types will need to first be serialized (packed) into a
format that is amenable to transmission. Boost.MPI relies on the
_Serialization_ library to serialize and deserialize data types. 

For types defined by the standard library (such as `std::string` or
`std::vector`) and some types in Boost (such as `boost::variant`), the
_Serialization_ library already contains all of the required
serialization code. In these cases, you need only include the
appropriate header from the `boost/serialization` directory. 

[def _gps_position_ [link gps_position `gps_position`]]
For types that do not already have a serialization header, you will
first need to implement serialization code before the types can be
transmitted using Boost.MPI. Consider a simple class _gps_position_
that contains members `degrees`, `minutes`, and `seconds`. This class
is made serializable by making it a friend of
`boost::serialization::access` and introducing the templated
`serialize()` function, as follows:[#gps_position]

  class gps_position
  {
  private:
      friend class boost::serialization::access;

      template<class Archive>
      void serialize(Archive & ar, const unsigned int version)
      {
          ar & degrees;
          ar & minutes;
          ar & seconds;
      }

      int degrees;
      int minutes;
      float seconds;
  public:
      gps_position(){};
      gps_position(int d, int m, float s) :
          degrees(d), minutes(m), seconds(s)
      {}
  };

Complete information about making types serializable is beyond the
scope of this tutorial. For more information, please see the
_Serialization_ library tutorial from which the above example was
extracted. One important side benefit of making types serializable for
Boost.MPI is that they become serializable for any other usage, such
as storing the objects to disk and manipulated them in XML.


Some serializable types, like _gps_position_ above, have a fixed
amount of data stored at fixed offsets and are fully defined by
the values of their data member (most POD with no pointers are a good example).
When this is the case, Boost.MPI can optimize their serialization and 
transmission by avoiding extraneous copy operations. 
To enable this optimization, users must specialize the type trait [classref
boost::mpi::is_mpi_datatype `is_mpi_datatype`], e.g.:

  namespace boost { namespace mpi {
    template <>
    struct is_mpi_datatype<gps_position> : mpl::true_ { };
  } }

For non-template types we have defined a macro to simplify declaring a type 
as an MPI datatype

  BOOST_IS_MPI_DATATYPE(gps_position)

For composite traits, the specialization of [classref
boost::mpi::is_mpi_datatype `is_mpi_datatype`] may depend on
`is_mpi_datatype` itself. For instance, a `boost::array` object is
fixed only when the type of the parameter it stores is fixed:

  namespace boost { namespace mpi {
    template <typename T, std::size_t N>
    struct is_mpi_datatype<array<T, N> > 
      : public is_mpi_datatype<T> { };
  } }
  
The redundant copy elimination optimization can only be applied when
the shape of the data type is completely fixed. Variable-length types
(e.g., strings, linked lists) and types that store pointers cannot use
the optimization, but Boost.MPI will be unable to detect this error at
compile time. Attempting to perform this optimization when it is not
correct will likely result in segmentation faults and other strange
program behavior.

Boost.MPI can transmit any user-defined data type from one process to
another. Built-in types can be transmitted without any extra effort;
library-defined types require the inclusion of a serialization header;
and user-defined types will require the addition of serialization
code. Fixed data types can be optimized for transmission using the
[classref boost::mpi::is_mpi_datatype `is_mpi_datatype`]
type trait.

[endsect:user_data_types]