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1 # Parallel STL
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2
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3 Parallel STL is an implementation of the C++ standard library algorithms with support for execution policies,
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4 as specified in ISO/IEC 14882:2017 standard, commonly called C++17. The implementation also supports the unsequenced
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5 execution policy specified in Parallelism TS version 2 and proposed for the next version of the C++ standard in the
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6 C++ working group paper [P1001](https://wg21.link/p1001).
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7 Parallel STL offers efficient support for both parallel and vectorized execution of algorithms. For sequential
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8 execution, it relies on an available implementation of the C++ standard library.
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9
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10 ## Prerequisites
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11
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12 To use Parallel STL, you must have the following software installed:
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13 * C++ compiler with:
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14 * Support for C++11
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15 * Support for OpenMP* 4.0 SIMD constructs
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16 * Threading Building Blocks (TBB) which is available for download at https://github.com/01org/tbb/
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17
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18 ## Known issues and limitations
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19
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20 * `unseq` and `par_unseq` policies only have effect with compilers that support `#pragma omp simd` or `#pragma simd`.
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21 * Parallel and vector execution is only supported for the algorithms if random access iterators are provided,
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22 while for other iterator types the execution will remain serial.
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23 * The following algorithms do not allow efficient SIMD execution: `includes`, `inplace_merge`, `merge`, `nth_element`,
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24 `partial_sort`, `partial_sort_copy`, `set_difference`, `set_intersection`, `set_symmetric_difference`, `set_union`,
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25 `sort`, `stable_partition`, `stable_sort`, `unique`.
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26 * The initial value type for `exclusive_scan`, `inclusive_scan`, `transform_exclusive_scan`, `transform_inclusive_scan`
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27 shall be DefaultConstructible. A default constructed-instance of the initial value type shall be the identity element
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28 for the specified binary operation.
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29 * For `max_element`, `min_element`, `minmax_element`, `partial_sort`, `partial_sort_copy`, `sort`, `stable_sort`
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30 the dereferenced value type of the provided iterators shall be DefaultConstructible.
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31 * For `remove`, `remove_if`, `unique` the dereferenced value type of the provided iterators shall be MoveConstructible.
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32 * The following algorithms require additional O(n) memory space for parallel execution: `copy_if`, `inplace_merge`,
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33 `partial_sort`, `partial_sort_copy`, `partition_copy`, `remove`, `remove_if`, `rotate`, `sort`, `stable_sort`,
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34 `unique`, `unique_copy`.
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