[section:skeleton_and_content Separating structure from content]

When communicating data types over MPI that are not fundamental to MPI
(such as strings, lists, and user-defined data types), Boost.MPI must
first serialize these data types into a buffer and then communicate
them; the receiver then copies the results into a buffer before
deserializing into an object on the other end. For some data types,
this overhead can be eliminated by using [classref
boost::mpi::is_mpi_datatype `is_mpi_datatype`]. However,
variable-length data types such as strings and lists cannot be MPI
data types. 

Boost.MPI supports a second technique for improving performance by
separating the structure of these variable-length data structures from
the content stored in the data structures. This feature is only
beneficial when the shape of the data structure remains the same but
the content of the data structure will need to be communicated several
times. For instance, in a finite element analysis the structure of the
mesh may be fixed at the beginning of computation but the various
variables on the cells of the mesh (temperature, stress, etc.) will be
communicated many times within the iterative analysis process. In this
case, Boost.MPI allows one to first send the "skeleton" of the mesh
once, then transmit the "content" multiple times. Since the content
need not contain any information about the structure of the data type,
it can be transmitted without creating separate communication buffers.

To illustrate the use of skeletons and content, we will take a
somewhat more limited example wherein a master process generates
random number sequences into a list and transmits them to several
slave processes. The length of the list will be fixed at program
startup, so the content of the list (i.e., the current sequence of
numbers) can be transmitted efficiently. The complete example is
available in `example/random_content.cpp`. We being with the master
process (rank 0), which builds a list, communicates its structure via
a [funcref boost::mpi::skeleton `skeleton`], then repeatedly
generates random number sequences to be broadcast to the slave
processes via [classref boost::mpi::content `content`]:

  
    // Generate the list and broadcast its structure
    std::list<int> l(list_len);
    broadcast(world, mpi::skeleton(l), 0);

    // Generate content several times and broadcast out that content
    mpi::content c = mpi::get_content(l);
    for (int i = 0; i < iterations; ++i) {
      // Generate new random values
      std::generate(l.begin(), l.end(), &random);

      // Broadcast the new content of l
      broadcast(world, c, 0);
    }

    // Notify the slaves that we're done by sending all zeroes
    std::fill(l.begin(), l.end(), 0);
    broadcast(world, c, 0);


The slave processes have a very similar structure to the master. They
receive (via the [funcref boost::mpi::broadcast
`broadcast()`] call) the skeleton of the data structure, then use it
to build their own lists of integers. In each iteration, they receive
via another `broadcast()` the new content in the data structure and
compute some property of the data:


    // Receive the content and build up our own list
    std::list<int> l;
    broadcast(world, mpi::skeleton(l), 0);

    mpi::content c = mpi::get_content(l);
    int i = 0;
    do {
      broadcast(world, c, 0);

      if (std::find_if
           (l.begin(), l.end(),
            std::bind1st(std::not_equal_to<int>(), 0)) == l.end())
        break;

      // Compute some property of the data.

      ++i;
    } while (true);


The skeletons and content of any Serializable data type can be
transmitted either via the [memberref
boost::mpi::communicator::send `send`] and [memberref
boost::mpi::communicator::recv `recv`] members of the
[classref boost::mpi::communicator `communicator`] class
(for point-to-point communicators) or broadcast via the [funcref
boost::mpi::broadcast `broadcast()`] collective. When
separating a data structure into a skeleton and content, be careful
not to modify the data structure (either on the sender side or the
receiver side) without transmitting the skeleton again. Boost.MPI can
not detect these accidental modifications to the data structure, which
will likely result in incorrect data being transmitted or unstable
programs. 

[endsect:skeleton_and_content]