C++中的std::async()详解

template<class Fn, class... Args>
future<typename result_of<Fn(Args...)>::type> async(launch policy, Fn&& fn, Args&&...args);

// STLasync.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//
 
#include "pch.h"
#include <iostream>
#include <string>
#include <chrono>
#include <thread>
#include <future>
 
using namespace std::chrono;
 
std::string fetchDataFromDB(std::string recvData) {
 
	std::cout << "fetchDataFromDB start" << std::this_thread::get_id() << std::endl;
	std::this_thread::sleep_for(seconds(5));
	return "DB_" + recvData;
}
 
std::string fetchDataFromFile(std::string recvData) {
 
	std::cout << "fetchDataFromFile start" << std::this_thread::get_id() << std::endl;
	std::this_thread::sleep_for(seconds(3));
	return "File_" + recvData;
}
 
int main() {
 
	std::cout << "main start" << std::this_thread::get_id() << std::endl;
 
	//获取开始时间
	system_clock::time_point start = system_clock::now();
 
	std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");
 
	//从文件获取数据
	std::future<std::string> fileData = std::async(std::launch::deferred, fetchDataFromFile, "Data");
 
	//知道调用get函数fetchDataFromFile才开始执行
	std::string FileData = fileData.get();
	//如果fetchDataFromDB()执行没有完成，get会一直阻塞当前线程
	std::string dbData = resultFromDB.get();
	
	//获取结束时间
	auto end = system_clock::now();
 
	auto diff = duration_cast<std::chrono::seconds>(end - start).count();
	std::cout << "Total Time taken= " << diff << "Seconds" << std::endl;
 
	//组装数据
	std::string data = dbData + " :: " + FileData;
 
	//输出组装的数据
	std::cout << "Data = " << data << std::endl;
 
	return 0;
}
 

// STLasync.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//
 
#include "pch.h"
#include <iostream>
#include <string>
#include <chrono>
#include <thread>
#include <future>
 
using namespace std::chrono;
 
std::string fetchDataFromDB(std::string recvData) {
 
	std::cout << "fetchDataFromDB start" << std::this_thread::get_id() << std::endl;
	std::this_thread::sleep_for(seconds(5));
	return "DB_" + recvData;
}
 
 
int main() {
 
	std::cout << "main start" << std::this_thread::get_id() << std::endl;
 
	//获取开始时间
	system_clock::time_point start = system_clock::now();
 
	std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");
 
	std::future_status status;
	std::string dbData;
	do
	{
		status = resultFromDB.wait_for(std::chrono::seconds(1));
 
		switch (status)
		{
		case std::future_status::ready:
			std::cout << "Ready..." << std::endl;
			//获取结果
			dbData = resultFromDB.get();
			std::cout << dbData << std::endl;
			break;
		case std::future_status::timeout:
			std::cout << "timeout..." << std::endl;
			break;
		case std::future_status::deferred:
			std::cout << "deferred..." << std::endl;
			break;
		default:
			break;
		}
 
	} while (status != std::future_status::ready);
 
	
	//获取结束时间
	auto end = system_clock::now();
 
	auto diff = duration_cast<std::chrono::seconds>(end - start).count();
	std::cout << "Total Time taken= " << diff << "Seconds" << std::endl;
 
	return 0;
}