1# Parallel STL 2 3Parallel STL is an implementation of the C++ standard library algorithms with support for execution policies, 4as specified in ISO/IEC 14882:2017 standard, commonly called C++17. The implementation also supports the unsequenced 5execution policy specified in Parallelism TS version 2 and proposed for the next version of the C++ standard in the 6C++ working group paper [P1001](https://wg21.link/p1001). 7Parallel STL offers efficient support for both parallel and vectorized execution of algorithms. For sequential 8execution, it relies on an available implementation of the C++ standard library. 9 10## Prerequisites 11 12To use Parallel STL, you must have the following software installed: 13* C++ compiler with: 14 * Support for C++11 15 * Support for OpenMP* 4.0 SIMD constructs 16* Threading Building Blocks (TBB) which is available for download at https://github.com/01org/tbb/ 17 18## Known issues and limitations 19 20* `unseq` and `par_unseq` policies only have effect with compilers that support `#pragma omp simd` or `#pragma simd`. 21* Parallel and vector execution is only supported for the algorithms if random access iterators are provided, 22 while for other iterator types the execution will remain serial. 23* The following algorithms do not allow efficient SIMD execution: `includes`, `inplace_merge`, `merge`, `nth_element`, 24 `partial_sort`, `partial_sort_copy`, `set_difference`, `set_intersection`, `set_symmetric_difference`, `set_union`, 25 `sort`, `stable_partition`, `stable_sort`, `unique`. 26* The initial value type for `exclusive_scan`, `inclusive_scan`, `transform_exclusive_scan`, `transform_inclusive_scan` 27 shall be DefaultConstructible. A default constructed-instance of the initial value type shall be the identity element 28 for the specified binary operation. 29* For `max_element`, `min_element`, `minmax_element`, `partial_sort`, `partial_sort_copy`, `sort`, `stable_sort` 30 the dereferenced value type of the provided iterators shall be DefaultConstructible. 31* For `remove`, `remove_if`, `unique` the dereferenced value type of the provided iterators shall be MoveConstructible. 32* The following algorithms require additional O(n) memory space for parallel execution: `copy_if`, `inplace_merge`, 33 `partial_sort`, `partial_sort_copy`, `partition_copy`, `remove`, `remove_if`, `rotate`, `sort`, `stable_sort`, 34 `unique`, `unique_copy`. 35 36