标准库标头＜execution＞ (C++17)学习

此头文件是算法库的一部分。本篇介绍策略类型的一些应用示例：

类
is_execution_policy (C++17)	测试一个类是否表示某种执行策略 (类模板)
在命名空间 `std::execution` 定义
sequenced_policyparallel_policyparallel_unsequenced_policyunsequenced_policy (C++17)(C++17)(C++17)(C++20)	执行策略类型 (类)
常量
在命名空间 `std::execution` 定义
seqparpar_unsequnseq (C++17)(C++17)(C++17)(C++20)	全局执行策略对象 (常量)

首先看execution在VS2022里的定义

namespace execution {_EXPORT_STD class sequenced_policy {// indicates support for only sequential execution, and requests termination on exceptionspublic:using _Standard_execution_policy   = int;static constexpr bool _Parallelize = false;static constexpr bool _Ivdep       = false;};_EXPORT_STD inline constexpr sequenced_policy seq{/* unspecified */};_EXPORT_STD class parallel_policy {// indicates support by element access functions for parallel execution with parallel forward progress// guarantees, and requests termination on exceptionspublic:using _Standard_execution_policy   = int;static constexpr bool _Parallelize = true;static constexpr bool _Ivdep       = true;};_EXPORT_STD inline constexpr parallel_policy par{/* unspecified */};_EXPORT_STD class parallel_unsequenced_policy {// indicates support by element access functions for parallel execution with weakly parallel forward progress// guarantees, and requests termination on exceptions//// (at this time, equivalent to parallel_policy)public:using _Standard_execution_policy   = int;static constexpr bool _Parallelize = true;static constexpr bool _Ivdep       = true;};_EXPORT_STD inline constexpr parallel_unsequenced_policy par_unseq{/* unspecified */};#if _HAS_CXX20_EXPORT_STD class unsequenced_policy {// indicates support by element access functions for weakly parallel forward progress guarantees, and for// executing interleaved on the same thread, and requests termination on exceptions//// (at this time, equivalent to sequenced_policy except for for_each(_n), destroy(_n),// uninitialized_default_construct(_n), and uninitialized_value_construct(_n))public:using _Standard_execution_policy   = int;static constexpr bool _Parallelize = false;static constexpr bool _Ivdep       = true;};_EXPORT_STD inline constexpr unsequenced_policy unseq{/* unspecified */};
#endif // _HAS_CXX20} // namespace execution

示例代码：

#include <algorithm>
#include <chrono>
#include <cstdint>
#include <iostream>
#include <random>
#include <vector>
#include <ctime>
#include <execution>
#include <string>//https://zh.cppreference.com/w/cpp/algorithm/execution_policy_tag
//https://blog.csdn.net/CUBE_lotus/article/details/137159423#ifdef PARALLEL
#include <execution>
namespace execution = std::execution;
#else
enum class execution { seq, unseq, par_unseq, par };
#endifvoid measure([[maybe_unused]] auto policy, std::vector<std::uint64_t> v)
{const auto start = std::chrono::steady_clock::now();
#ifdef PARALLELstd::sort(policy, v.begin(), v.end());
#elsestd::sort(v.begin(), v.end());
#endifconst auto finish = std::chrono::steady_clock::now();std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(finish - start)<< '\n';
};/*** 辅助计时类*/
class Timer {std::string str;clock_t     start;public:Timer(const std::string& str) : str(str) {start = clock();}~Timer() {clock_t end = clock();std::cout << str << " => " << (end - start) / 1000.0 << '\n';}
};/*** 串行执行策略* class sequenced_policy;*/
void test_sequenced_policy(std::vector<int> arr) {Timer timer("std::execution::seq");std::sort(std::execution::seq, arr.begin(), arr.end());
}/*** 并行执行策略* class parallel_policy;*/
void test_parallel_policy(std::vector<int> arr) {Timer timer("std::execution::par");std::sort(std::execution::par, arr.begin(), arr.end());
}/*** 并行无序执行策略* class parallel_unsequenced_policy;*/
void test_parallel_unsequenced_policy(std::vector<int> arr) {Timer timer("std::execution::par_unseq");std::sort(std::execution::par_unseq, arr.begin(), arr.end());
}/*** 无序执行策略* class unsequenced_policy;*/
void test_unsequenced_policy(std::vector<int> arr) {
#if __cpp_lib_execution >= 201902LTimer timer("std::execution::unseq");std::sort(std::execution::unseq, arr.begin(), arr.end());
#endif
}int main()
{std::vector<std::uint64_t> v(1'000'000);std::mt19937 gen{ std::random_device{}() };std::ranges::generate(v, gen);//C++ 20 标准measure(execution::seq, v);measure(execution::unseq, v);measure(execution::par_unseq, v);measure(execution::par, v);std::cout << "__cplusplus=============================" << __cplusplus << "\n";std::vector<int> arr;for (int i = 0; i < 3.0 * 100'000'00; i += 1) {arr.push_back(rand());}test_sequenced_policy(arr);test_parallel_policy(arr);test_parallel_unsequenced_policy(arr);test_unsequenced_policy(arr);std::cout << "hello world\n";
}

运行结果：