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Update documentation for oneTBB 2021.10.0 (#1147)
* Update documentation for oneTBB 2021.10.0 Signed-off-by: Olga Malysheva <[email protected]>
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| Original file line number | Diff line number | Diff line change |
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| .. _std_invoke: | ||
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| Invoke a Callable Object | ||
| ========================== | ||
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| Starting from C++17, the requirements for callable objects passed to algorithms or Flow Graph nodes are relaxed. It allows using additional types of bodies. | ||
| Previously, the body of the algorithm or Flow Graph node needed to be a Function Object (see `C++ Standard Function Object <https://en.cppreference.com/w/cpp/utility/functional>`_) and provide an | ||
| ``operator()`` that accepts input parameters. | ||
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| Now the body needs to meet the more relaxed requirements of being Callable (see `C++ Standard Callable <https://en.cppreference.com/w/cpp/named_req/Callable>`_) that covers three types of objects: | ||
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| * **Function Objects that provide operator(arg1, arg2, ...)**, which accepts the input parameters | ||
| * **Pointers to member functions** that you can use as the body of the algorithm or the Flow Graph node | ||
| * **Pointers to member objects** work as the body of the algorithm or parallel construct | ||
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| You can use it not only for a Flow Graph but also for algorithms. See the example below: | ||
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| .. code:: | ||
| // The class models oneTBB Range | ||
| class StrideRange { | ||
| public: | ||
| StrideRange(int* s, std::size_t sz, std::size_t str) | ||
| : start(s), size(sz), stride(str) {} | ||
| // A copy constructor | ||
| StrideRange(const StrideRange&) = default; | ||
| // A splitting constructor | ||
| StrideRange(StrideRange& other, oneapi::tbb::split) | ||
| : start(other.start), size(other.size / 2) | ||
| { | ||
| other.size -= size; | ||
| other.start += size; | ||
| } | ||
| ~StrideRange() = default; | ||
| // Indicate if the range is empty | ||
| bool empty() const { | ||
| return size == 0; | ||
| } | ||
| // Indicate if the range can be divided | ||
| bool is_divisible() const { | ||
| return size >= stride; | ||
| } | ||
| void iterate() const { | ||
| for (std::size_t i = 0; i < size; i += stride) { | ||
| // Performed an action for each element of the range, | ||
| // implement the code based on your requirements | ||
| } | ||
| } | ||
| private: | ||
| int* start; | ||
| std::size_t size; | ||
| std::size_t stride; | ||
| }; | ||
| Where: | ||
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| * The ``StrideRange`` class models oneTBB range that should be iterated with a specified stride during its initial construction. | ||
| * The ``stride`` value is stored in a private field within the range. Therefore, the class provides the member function ``iterate() const`` that implements a loop with the specified stride. | ||
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| ``range.iterate()`` | ||
| ******************* | ||
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| Before C++17, to utilize a range in a parallel algorithm, such as ``parallel_for``, it was required to provide a ``Function Object`` as the algorithm's body. This Function Object defined the operations to be executed on each iteration of the range: | ||
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| .. code:: | ||
| int main() { | ||
| std::size_t array_size = 1000; | ||
| int* array_to_iterate = new int[array_size]; | ||
| StrideRange range(array_to_iterate, array_size, /* stride = */ 2); | ||
| // Define a lambda function as the body of the parallel_for loop | ||
| auto pfor_body = [] (const StrideRange& range) { | ||
| range.iterate(); | ||
| }; | ||
| // Perform parallel iteration | ||
| oneapi::tbb::parallel_for(range, pfor_body); | ||
| delete[] array_to_iterate; | ||
| } | ||
| An additional lambda function ``pfor_body`` was also required. This lambda function invoked the ``rage.iterate()`` function. | ||
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| Now with C++17, you can directly utilize a pointer to ``range.iterate()`` as the body of the algorithm: | ||
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| .. code:: | ||
| int main() { | ||
| std::size_t array_size = 1000; | ||
| int* array_to_iterate = new int[array_size]; | ||
| // Performs the iteration over the array elements with the specified stride | ||
| StrideRange range(array_to_iterate, array_size, /* stride = */ 2); | ||
| // Parallelize the iteration over the range object | ||
| oneapi::tbb::parallel_for(range, &StrideRange::iterate); | ||
| delete[] array_to_iterate; | ||
| } | ||
| ``std::invoke`` | ||
| **************** | ||
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| ``std::invoke`` is a function template that provides a syntax for invoking different types of callable objects with a set of arguments. | ||
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| oneTBB implementation uses the C++ standard function ``std::invoke(&StrideRange::iterate, range)`` to execute the body. It is the equivalent of ``range.iterate()``. | ||
| Therefore, it allows you to invoke a callable object, such as a function object, with the provided arguments. | ||
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| .. tip:: Refer to `C++ Standard <https://en.cppreference.com/w/cpp/utility/functional/invoke>`_ to learn more about ``std::invoke``. | ||
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| Example | ||
| ^^^^^^^^ | ||
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| Consider a specific scenario with ``function_node`` within a Flow Graph. | ||
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| In the example below, a ``function_node`` takes an object as an input to read a member object of that input and proceed it to the next node in the graph: | ||
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| .. code:: | ||
| struct Object { | ||
| int number; | ||
| }; | ||
| int main() { | ||
| using namespace oneapi::tbb::flow; | ||
| // Lambda function to read the member object of the input Object | ||
| auto number_reader = [] (const Object& obj) { | ||
| return obj.number; | ||
| }; | ||
| // Lambda function to process the received integer | ||
| auto number_processor = [] (int i) { /* processing integer */ }; | ||
| graph g; | ||
| // Function node that takes an Object as input and produces an integer | ||
| function_node<Object, int> func1(g, unlimited, number_reader); | ||
| // Function node that takes an integer as input and processes it | ||
| function_node<int, int> func2(g, unlimited, number_processor); | ||
| // Connect the function nodes | ||
| make_edge(func1, func2); | ||
| // Provide produced input to the graph | ||
| func1.try_put(Object{1}); | ||
| // Wait for the graph to complete | ||
| g.wait_for_all(); | ||
| } | ||
| Before C++17, the ``function_node`` in the Flow Graph required the body to be a Function Object. A lambda function was required to extract the number from the Object. | ||
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| With C++17, you can use ``std::invoke`` with a pointer to the member number directly as the body. | ||
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| You can update the previous example as follows: | ||
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| .. code:: | ||
| struct Object { | ||
| int number; | ||
| }; | ||
| int main() { | ||
| using namespace oneapi::tbb::flow; | ||
| // The processing logic for the received integer | ||
| auto number_processor = [] (int i) { /* processing integer */ }; | ||
| // Create a graph object g to hold the flow graph | ||
| graph g; | ||
| // Use a member function pointer to the number member of the Object struct as the body | ||
| function_node<Object, int> func1(g, unlimited, &Object::number); | ||
| // Use the number_processor lambda function as the body | ||
| function_node<int, int> func2(g, unlimited, number_processor); | ||
| // Connect the function nodes | ||
| make_edge(func1, func2); | ||
| // Connect the function nodes | ||
| func1.try_put(Object{1}); | ||
| // Wait for the graph to complete | ||
| g.wait_for_all(); | ||
| } | ||
| Find More | ||
| ********* | ||
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| The following APIs supports Callable object as Bodies: | ||
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| * `parallel_for <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_for_func.html>`_ | ||
| * `parallel_reduce <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_reduce_func.html>`_ | ||
| * `parallel_deterministic_reduce <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_deterministic_reduce_func.html>`_ | ||
| * `parallel_for_each <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_for_each_func.html>`_ | ||
| * `parallel_scan <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_scan_func.html>`_ | ||
| * `parallel_pipeline <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/algorithms/functions/parallel_pipeline_func.html>`_ | ||
| * `function_node <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/flow_graph/func_node_cls.html>`_ | ||
| * `multifunction_node <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/flow_graph/multifunc_node_cls.html>`_ | ||
| * `async_node <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/flow_graph/async_node_cls.html>`_ | ||
| * `sequencer_node <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/flow_graph/sequencer_node_cls.html>`_ | ||
| * `join_node with key_matching policy <https://oneapi-src.github.io/oneAPI-spec/spec/elements/oneTBB/source/flow_graph/join_node_cls.html>`_ |
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