本博客参考自https://thispointer.com/c11-tutorial/

C++多线程的在某些场景需要实现异步操作，由std::futer 和std::asych, std::packaged_task, std::promise搭配使用。

目录

1.std::future

2.std::promise

3.std::asych

4.std::packaged_task

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1.std::future

std::future是一个类模板，它的模板类可访问“未来”的值，从而实现异步访问。

创建语法：

与std::futer 和std::asych, std::packaged_task, std::promise搭配使用对应的创建语法如下

std::future<int> futureObj = promiseObj.get_future(); std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data"); std::future<std::string> result = task.get_future();

调用语法：

获取future结果有三种方式：get、wait、wait_for，其中get等待异步操作结束并返回结果，wait只是等待异步操作完成，没有返回值，wait_for是超时等待返回结果（参考自https://blog.csdn.net/daaikuaichuan/article/details/81173303）

std::cout<<futureObj.get()<<std::endl; //在get到值之前程序会一直等待

 

2.std::promise

创建语法：

std::promise<int> promiseObj;

调用语法：

promObj->set_value(35);

完整代码：

#include <iostream> #include <thread> #include <future> void initiazer(std::promise<int> * promObj) { std::cout<<"Inside Thread"<<std::endl; promObj->set_value(35); } int main() { std::promise<int> promiseObj; std::future<int> futureObj = promiseObj.get_future(); std::thread th(initiazer, &promiseObj); std::cout<<futureObj.get()<<std::endl; th.join(); return 0; }

3.std::asych

创建语法：

std::asych直接在创建时就与future变量关联，并且创建对应的线程。与普通的多线程不同的是它可以通过future得到该线程函数的结果。

std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");

 

完整代码：

#include <iostream> #include <string> #include <chrono> #include <thread> #include <future> using namespace std::chrono; std::string fetchDataFromDB(std::string recvdData) { // Make sure that function takes 5 seconds to complete std::this_thread::sleep_for(seconds(5)); //Do stuff like creating DB Connection and fetching Data return "DB_" + recvdData; } std::string fetchDataFromFile(std::string recvdData) { // Make sure that function takes 5 seconds to complete std::this_thread::sleep_for(seconds(5)); //Do stuff like fetching Data File return "File_" + recvdData; } int main() { // Get Start Time system_clock::time_point start = system_clock::now(); std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data"); //Fetch Data from File std::string fileData = fetchDataFromFile("Data"); //Fetch Data from DB // Will block till data is available in future<std::string> object. std::string dbData = resultFromDB.get(); // Get End Time auto end = system_clock::now(); auto diff = duration_cast < std::chrono::seconds > (end - start).count(); std::cout << "Total Time Taken = " << diff << " Seconds" << std::endl; //Combine The Data std::string data = dbData + " :: " + fileData; //Printing the combined Data std::cout << "Data = " << data << std::endl; return 0; }

4.std::packaged_task

创建语法：

输入模板是函数原型std::packaged_task<std::string (std::string)> task(getDataFromDB);初始化输入的是函数getDataFromDB

std::packaged_task<std::string (std::string)> task(getDataFromDB);

调用语法：

这时packaged_task对象就相当于普通的回调函数，传入thread对象实现多线程，与普通的多线程不同的是它可以通过future得到该线程函数的结果。

// Fetch the associated future<> from packaged_task<> std::future<std::string> result = task.get_future(); // Pass the packaged_task to thread to run asynchronously std::thread th(std::move(task), "Arg");

完整代码：

#include <iostream> #include <thread> #include <future> #include <string> // Fetch some data from DB std::string getDataFromDB( std::string token) { // Do some stuff to fetch the data std::string data = "Data fetched from DB by Filter :: " + token; return data; } int main() { // Create a packaged_task<> that encapsulated the callback i.e. a function std::packaged_task<std::string (std::string)> task(getDataFromDB); // Fetch the associated future<> from packaged_task<> std::future<std::string> result = task.get_future(); // Pass the packaged_task to thread to run asynchronously std::thread th(std::move(task), "Arg"); // Join the thread. Its blocking and returns when thread is finished. th.join(); // Fetch the result of packaged_task<> i.e. value returned by getDataFromDB() std::string data = result.get(); std::cout << data << std::endl; return 0; }