.. _parallel_for_each_semantics:
parallel_for_each Body semantics and requirements
=================================================
.. contents::
:local:
:depth: 1
Description
***********
This page clarifies `ParallelForEachBody <https://spec.oneapi.com/versions/latest/elements/oneTBB/source/named_requirements/algorithms/par_for_each_body.html>`_
named requirements for ``tbb::parallel_for_each`` algorithm specification.
.. code:: cpp
namespace oneapi {
namespace tbb {
template <typaname InputIterator, typename Body>
void parallel_for_each( InputIterator first, InputIterator last, Body body ); // overload (1)
template <typename InputIterator, typename Body>
void parallel_for_each( InputIterator first, InputIterator last, Body body, task_group_context& group ); // overload (2)
template <typename Container, typename Body>
void parallel_for_each( Container& c, Body body ); // overload (3)
template <typename Container, typename Body>
void parallel_for_each( Container& c, Body body, task_group_context& group ); // overload (4)
template <typename Container, typename Body>
void parallel_for_each( const Container& c, Body body ); // overload (5)
template <typename Container, typename Body>
void parallel_for_each( const Container& c, Body body, task_group_context& group ); // overload (6)
} // namespace tbb
} // namespace oneapi
Terms
-----
* ``iterator`` determines the type of the iterator passed into ``parallel_for_each`` algorithm (which is ``InputIterator`` for overloads `(1)` and `(2)`
and ``decltype(std::begin(c))`` for overloads `(3) - (6)`)
* ``value_type`` - the type ``typename std::iterator_traits<iterator>::value_type``
* ``reference`` - the type ``typename std::iterator_traits<iterator>::reference``.
Requirements for different iterator types
-----------------------------------------
If the ``iterator`` satisfies `Input iterator` named requirements from [input.iterators] ISO C++ Standard section and do not satisfies
`Forward iterator` named requirements from [forward.iterators] ISO C++ Standard section, ``tbb::parallel_for_each`` requires the execution
of the ``body`` with an object of type ``const value_type&`` or ``value_type&&`` to be well-formed. If both forms are well-formed, an overload with
rvalue reference will be preferred.
.. caution::
If the ``Body`` only takes non-const lvalue reference to ``value_type``, named requirements above are violated and the program can be ill-formed.
If the ``iterator`` satisfies `Forward iterator` named requirements from [forward.iterators] ISO C++ Standard section, ``tbb::parallel_for_each`` requires the execution of the ``body``
with an object of type ``reference`` to be well-formed.
Requirements for ``Body`` with ``feeder`` argument
--------------------------------------------------
Additional elements submitted into ``tbb::parallel_for_each`` through the ``feeder::add`` passes to the ``Body`` as rvalues and therefore the corresponding
execution of the ``Body`` is required to be well-formed.
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