[Modern C++]現代C++中優雅的異步操做

背景

現代C++提供了一些很好的特性可讓咱們實現優雅、高效的異步操做，今天向你們簡要地介紹一下。

std::async

std::async(std::launch::async | std::launch::deferred, f, args...)

• f爲須要異步執行的函數，

• args爲f函數的參數，

• 第一個參數可選，在Visiual C++中默認爲std::launch::async,即建立的future對象在建立完成後當即異步執行，當第一個參數爲std::launch::deferred時表示future對象延遲執行。

樣例

代碼

#include <iostream>
#include <future>
#include <thread>
#include <chrono>
using namespace std;
int main(){
    auto future1 = async([](int &&x)->int{
        auto ret = 0;
        for(auto i = 0; i < x; i++){
            this_thread::sleep_for(chrono::seconds(1));
            ret += i;
            cout << "Task 1 calculating to "  << x - 1 << " and now the turn is " << i << " and the value is " << ret << endl;
        }
        return ret;
    },10);    //future1 當即異步執行
    auto future2 = async([](int &&x)->int{
        auto ret = 1;
        for(auto i = 1; i < x + 1; i++){
            this_thread::sleep_for(chrono::seconds(2));
            ret *= i;
            cout << "Task 2 calculating to "  << x << " and now the turn is " << i << " and the value is " << ret << endl;
        }
        return ret;
    },5);     //future2 當即異步執行
    std::cout << "waiting...\n";
    cout << "Result of task 1: " << future1.get() << endl;
    cout << "Result of task 2: " << future2.get() << endl;
}

輸出

c:\Users\yixian\workspace\test>async
waiting...
Task 1 calculating to 9 and now the turn is 0 and the value is 0
Task 1 calculating to 9 and now the turn is 1 and the value is 1
Task 2 calculating to 5 and now the turn is 1 and the value is 1
Task 2 calculating to 5 and now the turn is 2 and the value is 2
Task 2 calculating to 5 and now the turn is 3 and the value is 6Task 1 calculating to 9 and now the turn is 2 and the value is 3
Task 1 calculating to 9 and now the turn is 3 and the value is 6
Task 1 calculating to 9 and now the turn is 4 and the value is 10
Task 1 calculating to 9 and now the turn is 5 and the value is 15
Task 1 calculating to 9 and now the turn is 6 and the value is 21
Task 1 calculating to 9 and now the turn is 7 and the value is 28
Task 1 calculating to 9 and now the turn is 8 and the value is 36
Task 1 calculating to 9 and now the turn is 9 and the value is 45

Task 2 calculating to 5 and now the turn is 4 and the value is 24
Task 2 calculating to 5 and now the turn is 5 and the value is 120
Result of task 1: 45

指定參數爲std::launch::deffered

#include <iostream>
#include <future>
#include <thread>
#include <chrono>
using namespace std;
int main(){
    auto future1 = async(launch::deferred,[](int &&x)->int{
        auto ret = 0;
        for(auto i = 0; i < x; i++){
            this_thread::sleep_for(chrono::seconds(1));
            ret += i;
            cout << "Task 1 calculating to "  << x - 1 << " and now the turn is " << i << " and the value is " << ret << endl;
        }
        return ret;
    },10);    //future1 並不會當即異步執行
    auto future2 = async(launch::deferred,[](int &&x)->int{
        auto ret = 1;
        for(auto i = 1; i < x + 1; i++){
            this_thread::sleep_for(chrono::seconds(2));
            ret *= i;
            cout << "Task 2 calculating to "  << x << " and now the turn is " << i << " and the value is " << ret << endl;
        }
        return ret;
    },5);    //future2 並不會當即異步執行
    std::cout << "waiting...\n";
    cout << "Result of task 1: " << future1.get() << endl; //future1在此時執行
    cout << "Result of task 2: " << future2.get() << endl; //futute2在此時執行
}

輸出

c:\Users\yixian\workspace\test>async
waiting...
Task 1 calculating to 9 and now the turn is 0 and the value is 0
Task 1 calculating to 9 and now the turn is 1 and the value is 1
Task 1 calculating to 9 and now the turn is 2 and the value is 3
Task 1 calculating to 9 and now the turn is 3 and the value is 6
Task 1 calculating to 9 and now the turn is 4 and the value is 10
Task 1 calculating to 9 and now the turn is 5 and the value is 15
Task 1 calculating to 9 and now the turn is 6 and the value is 21
Task 1 calculating to 9 and now the turn is 7 and the value is 28
Task 1 calculating to 9 and now the turn is 8 and the value is 36
Task 1 calculating to 9 and now the turn is 9 and the value is 45
Result of task 1: 45
Task 2 calculating to 5 and now the turn is 1 and the value is 1
Task 2 calculating to 5 and now the turn is 2 and the value is 2
Task 2 calculating to 5 and now the turn is 3 and the value is 6
Task 2 calculating to 5 and now the turn is 4 and the value is 24
Task 2 calculating to 5 and now the turn is 5 and the value is 120
Result of task 2: 120