.. _class_join_node_extension:
Type-specified message keys for join_node
=========================================
.. note::
To enable this feature, define the ``TBB_PREVIEW_FLOW_GRAPH_FEATURES`` macro to 1.
.. contents::
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Description
***********
The extension allows a key matching ``join_node`` to obtain keys via functions associated with
its input types. The extension simplifies the existing approach by removing the need to
provide a function object for each input port of ``join_node``.
API
***
Header
------
.. code:: cpp
#include "oneapi/tbb/flow_graph.h"
Syntax
------
The extension adds a special constructor to the ``join_node`` interface when the
``key_matching<typename K, class KHash=tbb_hash_compare>`` policy is
used. The constructor has the following signature:
.. code:: cpp
join_node( graph &g )
When constructed this way, a ``join_node`` calls the
``key_from_message`` function for each incoming message to obtain the key associated
with it. The default implementation of ``key_from_message`` is the following
.. code:: cpp
namespace oneapi {
namespace tbb {
namespace flow {
template <typename K, typename T>
K key_from_message( const T &t ) {
return t.key();
}
}
}
}
``T`` is one of the user-provided types in ``OutputTuple`` and is
used to construct the ``join_node``, and ``K`` is the key type
of the node.
By default, the ``key()`` method defined in the message class will be called.
Alternatively, the user can define its own ``key_from_message`` function in the
same namespace with the message type. This function will be found via C++ argument-dependent
lookup and used in place of the default implementation.
See Also
********
`join_node Specification <https://spec.oneapi.com/versions/latest/elements/oneTBB/source/flow_graph/join_node_cls.html>`_
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