std_for_each_par.cpp

#include <chrono>
#include <vector>
#include <random>
#include <optional>
#include <iostream>
#include <execution>
#include <functional>

void calcTime(const std::function<void(void)>& func)
{
    auto start = std::chrono::system_clock::now();
    func();
    auto end = std::chrono::system_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::microseconds>(end - start).count() << std::endl;
}

int main()
{
    std::mt19937 engine;
    std::uniform_real_distribution<double> distribution(0.0, 1.0);
    for(int num : {100, 10000, 1000000}){
        std::cout << "num: " << num << std::endl;
        std::vector<double> vec(num);
        calcTime([&](){
            std::for_each(std::execution::par, vec.begin(), vec.end(), [&](double& value) { value = distribution(engine); });
        });
        
        calcTime([&](){
            for(int i = 0; i < vec.size(); ++i){
                vec[i] = distribution(engine);
            }
        });
    }
}

num: 100
88
2

num: 10000
281
373

num: 1000000
14138
27528