C++11中多執行緒程式設計-std::async的深入講解
前言
C++11中提供了非同步執行緒介面std::async,std::async是非同步程式設計的高階封裝,相對於直接使用std::thread,std::async的優勢在於:
1、std::async會自動建立執行緒去呼叫執行緒函式,相對於低層次的std::thread,使用起來非常方便;
2、std::async返回std::future物件,通過返回的std::future物件我們可以非常方便的獲取到執行緒函式的返回結果;
3、std::async提供了執行緒的建立策略,可以指定同步或者非同步的方式去建立執行緒;
1、函式原型
C++ 11中提供如下函式原型:
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,引數f接收一個可呼叫物件(仿函式、lambda表示式、類成員函式、普通函式……),用於非同步或是同步執行。
引數policy用於指定同步執行或者非同步執行可呼叫物件,它的可選值有三種:
1)std::launch::async:非同步執行可呼叫物件;
2)std::launch::deferred:同步執行可呼叫物件;
3)std::launch::async | std::launch::deferred 可以非同步或是同步,取決於具體實現。
函式返回值:
函式返回值是std::future物件,我們可以執行get、wait、wait_for、wait_until函式獲取或者等待執行結果。
呼叫std::future物件的get函式時,如果執行的是非同步執行策略,如果非同步執行沒有結束,get函式呼叫會阻塞當前當前呼叫執行緒;如果執行的是同步執行策略,只有當呼叫get函式時才真正執行。
呼叫std::future物件的wait*函式時,可能返回三種狀態:
1)std::future_status::deferred:可呼叫物件尚未開始執行;
2)std::future_status::ready:可呼叫物件執行完畢;
3)std::future_status::timeout:可呼叫物件執行超時;
2、標頭檔案
#include <future>
3、同步或非同步讀取檔案內容
我們模擬非同步從資料庫中讀取資料和同步方式從檔案中讀取資料,從其中可以看到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; //獲取開始時間 system_clock::time_point start = system_clock::now(); std::future<std::string> resultFromDB = std::async(std::launch::async,fetchDataFromDB,"Data"); //從檔案獲取資料 std::future<std::string> fileData = std::async(std::launch::deferred,fetchDataFromFile,"Data"); //呼叫get函式fetchDataFromFile才開始執行 std::string FileData = fileData.get(); //如果fetchDataFromDB執行沒有完成,get會一直阻塞當前執行緒 std::string dbData = resultFromDB.get(); //獲取結束時間 auto end = system_clock::now(); auto diff = duration_cast<std::chrono::seconds>(end - start).count(); std::cout << "Total Time taken= " << diff << "Seconds" << std::endl; //組裝資料 std::string data = dbData + " :: " + FileData; //輸出組裝的資料 std::cout << "Data = " << data << std::endl; return 0; }
程式碼輸出:
main start140677737994048
fetchDataFromFile start140677737994048
fetchDataFromDB start140677720131328
Total Time taken= 5Seconds
Data = DB_Data :: File_Data
4、設定非同步資料讀取超時機制
有時我們不能無限制的等待非同步任務執行,可以設定超時等待時間(timeout),當超時時間到達時,可以選擇放棄等待非同步任務。
如果程式碼中,我們設定了1s的超時設定,用於檢查非同步執行緒是否執行完畢。
#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; //獲取開始時間 system_clock::time_point start = system_clock::now(); std::future<std::string> resultFromDB = std::async(std::launch::async,"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; //獲取結果 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); //獲取結束時間 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; }
程式輸出:
main start140406593357632
fetchDataFromDB start140406575482624
timeout...
timeout...
timeout...
timeout...
Ready...
DB_Data
Total Time taken= 5Seconds
5、使用std::async實現多執行緒併發
既然std::async可以實現非同步呼叫,我們很容易就可以借用它實現多執行緒併發。
#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,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; }
程式輸出如下:
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、其它注意事項
在使用時需要注意,std::future物件的析構需要等待std::async執行完畢,也就是說,如下面的程式碼並不能實現併發。原因在於std::async的返回的std::future物件無人接收,是個臨時變數,臨時變數的析構會阻塞,直至std::async非同步任務執行完成。
std::async(std::launch::async,[]{ f(); }); // temporary's dtor waits for f() std::async(std::launch::async,[]{ g(); }); // does not start until f() completes
參考材料
https://en.cppreference.com/w/cpp/thread/async
www.jb51.net/article/198761.htm
總結
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