NAME¶

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

Synopsis¶

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

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_policy
parallel_policy
parallel_unsequenced_policy
unsequenced_policy execution policy types
(C++17) (class)
(C++17)
(C++17)
(C++20)