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::decay_t...)>>
 async( Function&& f, Args&&... args );
template< class Function, class... Args >
std::future(std::decay_t...)>>
 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 

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 
#include 
#include 
#include 
#include 
 
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 resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");
 
 //从文件获取数据
 std::future 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(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 
#include 
#include 
#include 
#include 
 
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 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(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 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
 
template 
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, mid, end);
 auto handle_bm = std::async(std::launch::async,
        parallel_sum, beg, mid);
 // int sum = parallel_sum(beg, mid);
 return handle_bm.get() + handle_me.get();
}
 
int main()
{
 std::vector 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

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