std::execution::seq, std::execution::par, std::execution::par_unseq, std::execution::unseq

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

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Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <execution>
inline constexpr std::execution::sequenced_policy seq { /* unspecified */ };		(since C++17)
inline constexpr std::execution::parallel_policy par { /* unspecified */ };		(since C++17)
inline constexpr std::execution::parallel_unsequenced_policy par_unseq { /* unspecified */ };		(since C++17)
inline constexpr std::execution::unsequenced_policy unseq { /* unspecified */ };		(since C++20)

The execution policy types

• std::execution::sequenced_policy,
• std::execution::parallel_policy,
• std::execution::parallel_unsequenced_policy, and
• std::execution::unsequenced_policy

have the following respective instances:

• std::execution::seq,
• std::execution::par,
• std::execution::par_unseq, and
• std::execution::unseq.

These instances are used to specify the execution policy of parallel algorithms, i.e., the kinds of parallelism allowed.

Additional execution policies may be provided by a standard library implementation (possible future additions may include std::parallel::cuda and std::parallel::opencl).

Example

Run this code

#include <algorithm>
#include <chrono>
#include <cstdint>
#include <iostream>
#include <random>
#include <vector>
 
#ifdef PARALLEL
#include <execution>
    namespace execution = std::execution;
#else
    enum class execution { seq, unseq, par_unseq, par };
#endif
 
void measure([[maybe_unused]] auto policy, std::vector<std::uint64_t> v)
{
    const auto start = std::chrono::steady_clock::now();
#ifdef PARALLEL
    std::sort(policy, v.begin(), v.end());
#else
    std::sort(v.begin(), v.end());
#endif
    const auto finish = std::chrono::steady_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(finish - start)
              << '\n';
};
 
int main()
{
    std::vector<std::uint64_t> v(1'000'000);
    std::mt19937 gen {std::random_device{}()};
    std::ranges::generate(v, gen);
 
    measure(execution::seq, v);
    measure(execution::unseq, v);
    measure(execution::par_unseq, v);
    measure(execution::par, v);
}

Possible output:

// online GNU/gcc compiler (PARALLEL macro is not defined)
81ms
80ms
79ms
78ms
 
// with g++ -std=c++23 -O3 ./test.cpp -ltbb -DPARALLEL
165ms
163ms
30ms
27ms

See also

sequenced_policyparallel_policyparallel_unsequenced_policyunsequenced_policy (C++17)(C++17)(C++17)(C++20)

execution policy types (class)