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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Ranges 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
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

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)