C++11中多线程编程-std::async的深入讲解
前言
C++11中提供了异步线程接口std::async,std::async是异步编程的高级封装,相对于直接使用std::thread,std::async的优势在于:
1、std::async会自动创建线程去调用线程函数,相对于低层次的std::thread,使用起来非常方便;
2、std::async返回std::future对象,通过返回的std::future对象我们可以非常方便的获取到线程函数的返回结果;
3、std::async提供了线程的创建策略,可以指定同步或者异步的方式去创建线程;
1、函数原型
C++ 11中提供如下函数原型:
template< class Function, class... Args> std::future<std::result_of_t<std::decay_t<Function>(std::decay_t<Args>...)>> async( Function&& f, Args&&... args );
template< class Function, class... Args > std::future<std::result_of_t<std::decay_t<Function>(std::decay_t<Args>...)>> async( std::launch policy, Function&& f, Args&&... args );
其中,参数f接收一个可调用对象(仿函数、lambda表达式、类成员函数、普通函数……),用于异步或是同步执行。
参数policy用于指定同步执行或者异步执行可调用对象,它的可选值有三种:
1)std::launch::async:异步执行可调用对象;
2)std::launch::deferred:同步执行可调用对象;
3)std::launch::async | std::launch::deferred 可以异步或是同步,取决于具体实现。
函数返回值:
函数返回值是std::future对象,我们可以执行get、wait、wait_for、wait_until函数获取或者等待执行结果。
调用std::future对象的get函数时,如果执行的是异步执行策略,如果异步执行没有结束,get函数调用会阻塞当前当前调用线程;如果执行的是同步执行策略,只有当调用get函数时才真正执行。
调用std::future对象的wait*函数时,可能返回三种状态:
1)std::future_status::deferred:可调用对象尚未开始执行;
2)std::future_status::ready:可调用对象执行完毕;
3)std::future_status::timeout:可调用对象执行超时;
2、头文件
#include <future>
3、同步或异步读取文件内容
我们模拟异步从数据库中读取数据和同步方式从文件中读取数据,从其中可以看到std::async的使用方法。
#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"); //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; }
代码输出:
main start140677737994048
fetchDataFromFile start140677737994048
fetchDataFromDB start140677720131328
Total Time taken= 5Seconds
Data = DB_Data :: File_Data
4、设置异步数据读取超时机制
有时我们不能无限制的等待异步任务执行,可以设置超时等待时间(timeout),当超时时间到达时,可以选择放弃等待异步任务。
如果代码中,我们设置了1s的超时设置,用于检查异步线程是否执行完毕。
#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; }
程序输出:
main start140406593357632
fetchDataFromDB start140406575482624
timeout...
timeout...
timeout...
timeout...
Ready...
DB_Data
Total Time taken= 5Seconds
5、使用std::async实现多线程并发
既然std::async可以实现异步调用,我们很容易就可以借用它实现多线程并发。
#include <iostream> #include <vector> #include <algorithm> #include <numeric> #include <future> #include <string> #include <mutex> #include <thread> template <typename RandomIt> int parallel_sum(RandomIt beg, RandomIt end) { std::cout << "thread id:" << std::this_thread::get_id() << std::endl; auto len = end - beg; if (len < 1000) return std::accumulate(beg, end, 0); RandomIt mid = beg + len/2; auto handle_me = std::async(std::launch::async, parallel_sum<RandomIt>, mid, end); auto handle_bm = std::async(std::launch::async, parallel_sum<RandomIt>, beg, mid); // int sum = parallel_sum(beg, mid); return handle_bm.get() + handle_me.get(); } int main() { std::vector<int> v(10000, 1); std::cout << "The sum is " << parallel_sum(v.begin(), v.end()) << std::endl; }
程序输出如下:
The sum is thread id:140594794530624
thread id:140594776655616
thread id:140594768262912
thread id:140594759870208
thread id:140594672297728
thread id:140594680690432
thread id:140594663905024
thread id:140594655512320
thread id:140594647119616
thread id:140594638726912
thread id:140594269644544
thread id:140594630334208
thread id:140594278037248
thread id:140594252859136
thread id:140594261251840
thread id:140594252859136
thread id:140594236073728
thread id:140594252859136
thread id:140594261251840
thread id:140594630334208
thread id:140594244466432
thread id:140594252859136
thread id:140594227681024
thread id:140594261251840
thread id:140593875384064
thread id:140593850205952
thread id:140593858598656
thread id:140593866991360
thread id:140594647119616
thread id:140594269644544
thread id:140594672297728
10000
6、其它注意事项
在使用时需要注意,std::future对象的析构需要等待std::async执行完毕,也就是说,如下面的代码并不能实现并发。原因在于std::async的返回的std::future对象无人接收,是个临时变量,临时变量的析构会阻塞,直至std::async异步任务执行完成。
std::async(std::launch::async, []{ f(); }); // temporary's dtor waits for f() std::async(std::launch::async, []{ g(); }); // does not start until f() completes
参考材料
https://en.cppreference.com/w/cpp/thread/async
www.jb51.net/article/198761.htm
总结
到此这篇关于C++11中多线程编程-std::async深入讲解的文章就介绍到这了,更多相关C++11多线程编程-std::async内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们!