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## Class communicator

boost::mpi::communicator — A communicator that permits communication and synchronization among a set of processes.

## Synopsis

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

    class communicator {
    public:
      // construct/copy/destruct
      communicator();
      communicator(const MPI_Comm &, comm_create_kind);
      communicator(const communicator &, const boost::mpi::group &);

      // public member functions
      int rank() const;
      int size() const;
      boost::mpi::group group() const;
      template<typename T> void send(int, int, const T &) const;
      template<typename T> void send(int, int, const skeleton_proxy< T > &) const;
      template<typename T> void send(int, int, const T *, int) const;
      void send(int, int) const;
      template<typename T> status recv(int, int, T &) const;
      template<typename T>
        status recv(int, int, const skeleton_proxy< T > &) const;
      template<typename T> status recv(int, int, skeleton_proxy< T > &) const;
      template<typename T> status recv(int, int, T *, int) const;
      status recv(int, int) const;
      template<typename T> request isend(int, int, const T &) const;
      template<typename T>
        request isend(int, int, const skeleton_proxy< T > &) const;
      template<typename T> request isend(int, int, const T *, int) const;
      request isend(int, int) const;
      template<typename T> request irecv(int, int, T &) const;
      template<typename T> request irecv(int, int, T *, int) const;
      request irecv(int, int) const;
      status probe(int = any_source, int = any_tag) const;
      optional< status > iprobe(int = any_source, int = any_tag) const;
      void barrier() const;
      operator bool() const;
      operator MPI_Comm() const;
      communicator split(int) const;
      communicator split(int, int) const;
      optional< intercommunicator > as_intercommunicator() const;
      optional< graph_communicator > as_graph_communicator() const;
      bool has_cartesian_topology() const;
      void abort(int) const;
    };

## Description

The `communicator` class abstracts a set of communicating processes in MPI. All of the processes that belong to a certain communicator can determine the size of the communicator, their rank within the communicator, and communicate with any other processes in the communicator.

### `communicator` public construct/copy/destruct

1.     communicator();

   Build a new Boost.MPI communicator for `MPI_COMM_WORLD`.

   Constructs a Boost.MPI communicator that attaches to `MPI_COMM_WORLD`. This is the equivalent of constructing with ``(MPI_COMM_WORLD, comm_attach).

2.     communicator(const MPI_Comm & comm, comm_create_kind kind);

   Build a new Boost.MPI communicator based on the MPI communicator `comm`.

   `comm` may be any valid MPI communicator. If `comm` is MPI_COMM_NULL, an empty communicator (that cannot be used for communication) is created and the `kind` parameter is ignored. Otherwise, the `kind` parameters determines how the Boost.MPI communicator will be related to `comm:`

   - If `kind` is `comm_duplicate`, duplicate `comm` to create a new communicator. This new communicator will be freed when the Boost.MPI communicator (and all copies of it) is destroyed. This option is only permitted if `comm` is a valid MPI intracommunicator or if the underlying MPI implementation supports MPI 2.0 (which supports duplication of intercommunicators).

   - If `kind` is `comm_take_ownership`, take ownership of `comm`. It will be freed automatically when all of the Boost.MPI communicators go out of scope. This option must not be used when `comm` is MPI_COMM_WORLD.

   - If `kind` is `comm_attach`, this Boost.MPI communicator will reference the existing MPI communicator `comm` but will not free `comm` when the Boost.MPI communicator goes out of scope. This option should only be used when the communicator is managed by the user or MPI library (e.g., MPI_COMM_WORLD).

3.     communicator(const communicator & comm, const boost::mpi::group & subgroup);

   Build a new Boost.MPI communicator based on a subgroup of another MPI communicator.

   This routine will construct a new communicator containing all of the processes from communicator `comm` that are listed within the group `subgroup`. Equivalent to `MPI_Comm_create`.

   Parameters: |
   `comm` | An MPI communicator.
   `subgroup` | A subgroup of the MPI communicator, `comm`, for which we will construct a new communicator.

### `communicator` public member functions

1.     int rank() const;

   Determine the rank of the executing process in a communicator.

   This routine is equivalent to `MPI_Comm_rank`.

   Returns: | The rank of the process in the communicator, which will be a value in [0, size())

2.     int size() const;

   Determine the number of processes in a communicator.

   This routine is equivalent to `MPI_Comm_size`.

   Returns: | The number of processes in the communicator.

3.     boost::mpi::group group() const;

   This routine constructs a new group whose members are the processes within this communicator. Equivalent to calling `MPI_Comm_group`.

4.     template<typename T> void send(int dest, int tag, const T & value) const;

   Send data to another process.

   This routine executes a potentially blocking send with tag `tag` to the process with rank `dest`. It can be received by the destination process with a matching `recv` call.

   The given `value` must be suitable for transmission over MPI. There are several classes of types that meet these requirements:

   - Types with mappings to MPI data types: If `is_mpi_datatype<T>` is convertible to `mpl::true_`, then `value` will be transmitted using the MPI data type `get_mpi_datatype<T>()`. All primitive C++ data types that have MPI equivalents, e.g., `int`, `float`, `char`, `double`, etc., have built-in mappings to MPI data types. You may turn a Serializable type with fixed structure into an MPI data type by specializing ``is_mpi_datatype`` for your type.

   - Serializable types: Any type that provides the `serialize()` functionality required by the Boost.Serialization library can be transmitted and received.

   - Packed archives and skeletons: Data that has been packed into an ``mpi::packed_oarchive`` or the skeletons of data that have been backed into an ``mpi::packed_skeleton_oarchive`` can be transmitted, but will be received as ``mpi::packed_iarchive`` and ``mpi::packed_skeleton_iarchive``, respectively, to allow the values (or skeletons) to be extracted by the destination process.

   - Content: Content associated with a previously-transmitted skeleton can be transmitted by `send` and received by `recv`. The receiving process may only receive content into the content of a value that has been constructed with the matching skeleton.

   For types that have mappings to an MPI data type (including the concent of a type), an invocation of this routine will result in a single MPI_Send call. For variable-length data, e.g., serialized types and packed archives, two messages will be sent via MPI_Send: one containing the length of the data and the second containing the data itself. Note that the transmission mode for variable-length data is an implementation detail that is subject to change.

   Parameters: |
   `dest` | The rank of the remote process to which the data will be sent.
   `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
   `value` | The value that will be transmitted to the receiver. The type `T` of this value must meet the aforementioned criteria for transmission.

5.     template<typename T>
         void send(int dest, int tag, const skeleton_proxy< T > & proxy) const;

   Send the skeleton of an object.

   This routine executes a potentially blocking send with tag `tag` to the process with rank `dest`. It can be received by the destination process with a matching `recv` call. This variation on `send` will be used when a send of a skeleton is explicitly requested via code such as:

           comm.send(dest, tag, skeleton(object));

   The semantics of this routine are equivalent to that of sending a ``packed_skeleton_oarchive`` storing the skeleton of the `object`.

   Parameters: |
   `dest` | The rank of the remote process to which the skeleton will be sent.
   `proxy` | The ``skeleton_proxy`` containing a reference to the object whose skeleton will be transmitted.
   `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.

6.     template<typename T>
         void send(int dest, int tag, const T * values, int n) const;

   Send an array of values to another process.

   This routine executes a potentially blocking send of an array of data with tag `tag` to the process with rank `dest`. It can be received by the destination process with a matching array `recv` call.

   If `T` is an MPI datatype, an invocation of this routine will be mapped to a single call to MPI_Send, using the datatype `get_mpi_datatype<T>()`.

   Parameters: |
   `dest` | The process rank of the remote process to which the data will be sent.
   `n` | The number of values stored in the array. The destination process must call receive with at least this many elements to correctly receive the message.
   `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
   `values` | The array of values that will be transmitted to the receiver. The type `T` of these values must be mapped to an MPI data type.

7.     void send(int dest, int tag) const;

   Send a message to another process without any data.

   This routine executes a potentially blocking send of a message to another process. The message contains no extra data, and can therefore only be received by a matching call to `recv()`.

   Parameters: |
   `dest` | The process rank of the remote process to which the message will be sent.
   `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.

8.     template<typename T> status recv(int source, int tag, T & value) const;

   Receive data from a remote process.

   This routine blocks until it receives a message from the process `source` with the given `tag`. The type `T` of the `value` must be suitable for transmission over MPI, which includes serializable types, types that can be mapped to MPI data types (including most built-in C++ types), packed MPI archives, skeletons, and content associated with skeletons; see the documentation of `send` for a complete description.

   Parameters: |
   `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
   `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
   `value` | Will contain the value of the message after a successful receive. The type of this value must match the value transmitted by the sender, unless the sender transmitted a packed archive or skeleton: in these cases, the sender transmits a ``packed_oarchive`` or ``packed_skeleton_oarchive`` and the destination receives a ``packed_iarchive`` or ``packed_skeleton_iarchive``, respectively.
   Returns: | Information about the received message.

9.     template<typename T>
         status recv(int source, int tag, const skeleton_proxy< T > & proxy) const;

   Receive a skeleton from a remote process.

   This routine blocks until it receives a message from the process `source` with the given `tag` containing a skeleton.

   Parameters: |
   `proxy` | The ``skeleton_proxy`` containing a reference to the object that will be reshaped to match the received skeleton.
   `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
   `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
   Returns: | Information about the received message.

10.     template<typename T>
          status recv(int source, int tag, skeleton_proxy< T > & proxy) const;

    Receive a skeleton from a remote process.

    This routine blocks until it receives a message from the process `source` with the given `tag` containing a skeleton.

    Parameters: |
    `proxy` | The ``skeleton_proxy`` containing a reference to the object that will be reshaped to match the received skeleton.
    `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    Returns: | Information about the received message.

11.     template<typename T> status recv(int source, int tag, T * values, int n) const;

    Receive an array of values from a remote process.

    This routine blocks until it receives an array of values from the process `source` with the given `tag`. If the type `T` is

    Parameters: |
    `n` | The number of values that can be stored into the `values` array. This shall not be smaller than the number of elements transmitted by the sender.
    `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    `values` | Will contain the values in the message after a successful receive. The type of these elements must match the type of the elements transmitted by the sender.
    Returns: | Information about the received message.
    Throws: | std::range_error if the message to be received contains more than `n` values.

12.     status recv(int source, int tag) const;

    Receive a message from a remote process without any data.

    This routine blocks until it receives a message from the process `source` with the given `tag`.

    Parameters: |
    `source` | The process that will be sending the message. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    Returns: | Information about the received message.

13.     template<typename T> request isend(int dest, int tag, const T & value) const;

    Send a message to a remote process without blocking.

    The `isend` method is functionality identical to the `send` method and transmits data in the same way, except that `isend` will not block while waiting for the data to be transmitted. Instead, a request object will be immediately returned, allowing one to query the status of the communication or wait until it has completed.

    Parameters: |
    `dest` | The rank of the remote process to which the data will be sent.
    `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
    `value` | The value that will be transmitted to the receiver. The type `T` of this value must meet the aforementioned criteria for transmission.
    Returns: | a `request` object that describes this communication.

14.     template<typename T>
          request isend(int dest, int tag, const skeleton_proxy< T > & proxy) const;

    Send the skeleton of an object without blocking.

    This routine is functionally identical to the `send` method for ``skeleton_proxy`` objects except that `isend` will not block while waiting for the data to be transmitted. Instead, a request object will be immediately returned, allowing one to query the status of the communication or wait until it has completed.

    The semantics of this routine are equivalent to a non-blocking send of a ``packed_skeleton_oarchive`` storing the skeleton of the `object`.

    Parameters: |
    `dest` | The rank of the remote process to which the skeleton will be sent.
    `proxy` | The ``skeleton_proxy`` containing a reference to the object whose skeleton will be transmitted.
    `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
    Returns: | a `request` object that describes this communication.

15.     template<typename T>
          request isend(int dest, int tag, const T * values, int n) const;

    Send an array of values to another process without blocking.

    This routine is functionally identical to the `send` method for arrays except that `isend` will not block while waiting for the data to be transmitted. Instead, a request object will be immediately returned, allowing one to query the status of the communication or wait until it has completed.

    Parameters: |
    `dest` | The process rank of the remote process to which the data will be sent.
    `n` | The number of values stored in the array. The destination process must call receive with at least this many elements to correctly receive the message.
    `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
    `values` | The array of values that will be transmitted to the receiver. The type `T` of these values must be mapped to an MPI data type.
    Returns: | a `request` object that describes this communication.

16.     request isend(int dest, int tag) const;

    Send a message to another process without any data without blocking.

    This routine is functionally identical to the `send` method for sends with no data, except that `isend` will not block while waiting for the message to be transmitted. Instead, a request object will be immediately returned, allowing one to query the status of the communication or wait until it has completed.

    Parameters: |
    `dest` | The process rank of the remote process to which the message will be sent.
    `tag` | The tag that will be associated with this message. Tags may be any integer between zero and an implementation-defined upper limit. This limit is accessible via `environment::max_tag()`.
    Returns: | a `request` object that describes this communication.

17.     template<typename T> request irecv(int source, int tag, T & value) const;

    Prepare to receive a message from a remote process.

    The `irecv` method is functionally identical to the `recv` method and receive data in the same way, except that `irecv` will not block while waiting for data to be transmitted. Instead, it immediately returns a request object that allows one to query the status of the receive or wait until it has completed.

    Parameters: |
    `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    `value` | Will contain the value of the message after a successful receive. The type of this value must match the value transmitted by the sender, unless the sender transmitted a packed archive or skeleton: in these cases, the sender transmits a ``packed_oarchive`` or ``packed_skeleton_oarchive`` and the destination receives a ``packed_iarchive`` or ``packed_skeleton_iarchive``, respectively.
    Returns: | a `request` object that describes this communication.

18.     template<typename T>
          request irecv(int source, int tag, T * values, int n) const;

    Initiate receipt of an array of values from a remote process.

    This routine initiates a receive operation for an array of values transmitted by process `source` with the given `tag`.

    Parameters: |
    `n` | The number of values that can be stored into the `values` array. This shall not be smaller than the number of elements transmitted by the sender.
    `source` | The process that will be sending data. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    `values` | Will contain the values in the message after a successful receive. The type of these elements must match the type of the elements transmitted by the sender.
    Returns: | a `request` object that describes this communication.

19.     request irecv(int source, int tag) const;

    Initiate receipt of a message from a remote process that carries no data.

    This routine initiates a receive operation for a message from process `source` with the given `tag` that carries no data.

    Parameters: |
    `source` | The process that will be sending the message. This will either be a process rank within the communicator or the constant `any_source`, indicating that we can receive the message from any process.
    `tag` | The tag that matches a particular kind of message sent by the source process. This may be any tag value permitted by `send`. Alternatively, the argument may be the constant `any_tag`, indicating that this receive matches a message with any tag.
    Returns: | a `request` object that describes this communication.

20.     status probe(int source = any_source, int tag = any_tag) const;

    Waits until a message is available to be received.

    This operation waits until a message matching (`source`, `tag`) is available to be received. It then returns information about that message. The functionality is equivalent to `MPI_Probe`. To check if a message is available without blocking, use `iprobe`.

    Parameters: |
    `source` | Determine if there is a message available from this rank. If `any_source`, then the message returned may come from any source.
    `tag` | Determine if there is a message available with the given tag. If `any_tag`, then the message returned may have any tag.
    Returns: | Returns information about the first message that matches the given criteria.

21.     optional< status > iprobe(int source = any_source, int tag = any_tag) const;

    Determine if a message is available to be received.

    This operation determines if a message matching (`source`, `tag`) is available to be received. If so, it returns information about that message; otherwise, it returns immediately with an empty optional. The functionality is equivalent to `MPI_Iprobe`. To wait until a message is available, use `wait`.

    Parameters: |
    `source` | Determine if there is a message available from this rank. If `any_source`, then the message returned may come from any source.
    `tag` | Determine if there is a message available with the given tag. If `any_tag`, then the message returned may have any tag.
    Returns: | If a matching message is available, returns information about that message. Otherwise, returns an empty `boost::optional`.

22.     void barrier() const;

    Wait for all processes within a communicator to reach the barrier.

    This routine is a collective operation that blocks each process until all processes have entered it, then releases all of the processes "simultaneously". It is equivalent to `MPI_Barrier`.

23.     operator bool() const;

    Determine if this communicator is valid for communication.

    Evaluates `true` in a boolean context if this communicator is valid for communication, i.e., does not represent MPI_COMM_NULL. Otherwise, evaluates `false`.

24.     operator MPI_Comm() const;

    Access the MPI communicator associated with a Boost.MPI communicator.

    This routine permits the implicit conversion from a Boost.MPI communicator to an MPI communicator.

    Returns: | The associated MPI communicator.

25.     communicator split(int color) const;

    Split the communicator into multiple, disjoint communicators each of which is based on a particular color. This is a collective operation that returns a new communicator that is a subgroup of `this`. This routine is functionally equivalent to `MPI_Comm_split`.

    Parameters: |
    `color` | The color of this process. All processes with the same `color` value will be placed into the same group.
    Returns: | A new communicator containing all of the processes in `this` that have the same `color`.

26.     communicator split(int color, int key) const;

    Split the communicator into multiple, disjoint communicators each of which is based on a particular color. This is a collective operation that returns a new communicator that is a subgroup of `this`. This routine is functionally equivalent to `MPI_Comm_split`.

    Parameters: |
    `color` | The color of this process. All processes with the same `color` value will be placed into the same group.
    `key` | A key value that will be used to determine the ordering of processes with the same color in the resulting communicator. If omitted, the rank of the processes in `this` will determine the ordering of processes in the resulting group.
    Returns: | A new communicator containing all of the processes in `this` that have the same `color`.

27.     optional< intercommunicator > as_intercommunicator() const;

    Determine if the communicator is in fact an intercommunicator and, if so, return that intercommunicator.

    Returns: | an `optional` containing the intercommunicator, if this communicator is in fact an intercommunicator. Otherwise, returns an empty `optional`.

28.     optional< graph_communicator > as_graph_communicator() const;

    Determine if the communicator has a graph topology and, if so, return that ``graph_communicator``. Even though the communicators have different types, they refer to the same underlying communication space and can be used interchangeably for communication.

    Returns: | an `optional` containing the graph communicator, if this communicator does in fact have a graph topology. Otherwise, returns an empty `optional`.

29.     bool has_cartesian_topology() const;

    Determines whether this communicator has a Cartesian topology.

30.     void abort(int errcode) const;

    Abort all tasks in the group of this communicator.

    Makes a "best attempt" to abort all of the tasks in the group of this communicator. Depending on the underlying MPI implementation, this may either abort the entire program (and possibly return `errcode` to the environment) or only abort some processes, allowing the others to continue. Consult the documentation for your MPI implementation. This is equivalent to a call to `MPI_Abort`

    Parameters: |
    `errcode` | The error code to return from aborted processes.
    Returns: | Will not return.