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## Function gather

boost::mpi::gather — Gather the values stored at every process into a vector at the root process.

## Synopsis

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

    template<typename T>
      void gather(const communicator & comm, const T & in_value,
                  std::vector< T > & out_values, int root);
    template<typename T>
      void gather(const communicator & comm, const T & in_value, T * out_values,
                  int root);
    template<typename T>
      void gather(const communicator & comm, const T & in_value, int root);
    template<typename T>
      void gather(const communicator & comm, const T * in_values, int n,
                  std::vector< T > & out_values, int root);
    template<typename T>
      void gather(const communicator & comm, const T * in_values, int n,
                  T * out_values, int root);
    template<typename T>
      void gather(const communicator & comm, const T * in_values, int n, int root);

## Description

`gather` is a collective algorithm that collects the values stored at each process into a vector of values at the `root` process. This vector is indexed by the process number that the value came from. The type `T` of the values may be any type that is serializable or has an associated MPI data type.

When the type `T` has an associated MPI data type, this routine invokes `MPI_Gather` to gather the values.

Parameters:

`comm` | The communicator over which the gather will occur.
`in_value` | The value to be transmitted by each process. For gathering arrays of values, `in_values` points to storage for `n*comm`.size() values.
`out_values` | A vector or pointer to storage that will be populated with the values from each process, indexed by the process ID number. If it is a vector, it will be resized accordingly. For non-root processes, this parameter may be omitted. If it is still provided, however, it will be unchanged.
`root` | The process ID number that will collect the values. This value must be the same on all processes.