C++11中std::async的使用详解
C++11中的std::async是个模板函数。std::async异步调用函数,在某个时候以Args作为参数(可变长参数)调用Fn,无需等待Fn执行完成就可返回,返回结果是个std::future对象。Fn返回的值可通过std::future对象的get成员函数获取。一旦完成Fn的执行,共享状态将包含Fn返回的值并ready。
std::async有两个版本:
1.无需显示指定启动策略,自动选择,因此启动策略是不确定的,可能是std::launch::async,也可能是std::launch::deferred,或者是两者的任意组合,取决于它们的系统和特定库实现。
2.允许调用者选择特定的启动策略。
std::async的启动策略类型是个枚举类enum class launch,包括:
1. std::launch::async:异步,启动一个新的线程调用Fn,该函数由新线程异步调用,并且将其返回值与共享状态的访问点同步。
2. std::launch::deferred:延迟,在访问共享状态时该函数才被调用。对Fn的调用将推迟到返回的std::future的共享状态被访问时(使用std::future的wait或get函数)。
参数Fn:可以为函数指针、成员指针、任何类型的可移动构造的函数对象(即类定义了operator()的对象)。Fn的返回值或异常存储在共享状态中以供异步的std::future对象检索。
参数Args:传递给Fn调用的参数,它们的类型应是可移动构造的。
返回值:当Fn执行结束时,共享状态的std::future对象准备就绪。std::future的成员函数get检索的值是Fn返回的值。当启动策略采用std::launch::async时,即使从不访问其共享状态,返回的std::future也会链接到被创建线程的末尾。在这种情况下,std::future的析构函数与Fn的返回同步。
std::future介绍参考:https://www.jb51.net/article/179229.htm
详细用法见下面的测试代码,下面是从其他文章中copy的测试代码,部分作了调整,详细内容介绍可以参考对应的reference:
#include "future.hpp" #include <iostream> #include <future> #include <chrono> #include <utility> #include <thread> #include <functional> #include <memory> #include <exception> #include <numeric> #include <vector> #include <cmath> #include <string> #include <mutex> namespace future_ { /////////////////////////////////////////////////////////// // reference: http://www.cplusplus.com/reference/future/async/ int test_async_1() { auto is_prime = [](int x) { std::cout << "Calculating. Please, wait...\n"; for (int i = 2; i < x; ++i) if (x%i == 0) return false; return true; }; // call is_prime(313222313) asynchronously: std::future<bool> fut = std::async(is_prime, 313222313); std::cout << "Checking whether 313222313 is prime.\n"; // ... bool ret = fut.get(); // waits for is_prime to return if (ret) std::cout << "It is prime!\n"; else std::cout << "It is not prime.\n"; return 0; } /////////////////////////////////////////////////////////// // reference: http://www.cplusplus.com/reference/future/launch/ int test_async_2() { auto print_ten = [](char c, int ms) { for (int i = 0; i < 10; ++i) { std::this_thread::sleep_for(std::chrono::milliseconds(ms)); std::cout << c; } }; std::cout << "with launch::async:\n"; std::future<void> foo = std::async(std::launch::async, print_ten, '*', 100); std::future<void> bar = std::async(std::launch::async, print_ten, '@', 200); // async "get" (wait for foo and bar to be ready): foo.get(); // 注:注释掉此句,也会输出'*' bar.get(); std::cout << "\n\n"; std::cout << "with launch::deferred:\n"; foo = std::async(std::launch::deferred, print_ten, '*', 100); bar = std::async(std::launch::deferred, print_ten, '@', 200); // deferred "get" (perform the actual calls): foo.get(); // 注:注释掉此句,则不会输出'**********' bar.get(); std::cout << '\n'; return 0; } /////////////////////////////////////////////////////////// // reference: https://en.cppreference.com/w/cpp/thread/async std::mutex m; struct X { void foo(int i, const std::string& str) { std::lock_guard<std::mutex> lk(m); std::cout << str << ' ' << i << '\n'; } void bar(const std::string& str) { std::lock_guard<std::mutex> lk(m); std::cout << str << '\n'; } int operator()(int i) { std::lock_guard<std::mutex> lk(m); std::cout << i << '\n'; return i + 10; } }; template <typename RandomIt> int parallel_sum(RandomIt beg, RandomIt end) { auto len = end - beg; if (len < 1000) return std::accumulate(beg, end, 0); RandomIt mid = beg + len / 2; auto handle = std::async(std::launch::async, parallel_sum<RandomIt>, mid, end); int sum = parallel_sum(beg, mid); return sum + handle.get(); } int test_async_3() { std::vector<int> v(10000, 1); std::cout << "The sum is " << parallel_sum(v.begin(), v.end()) << '\n'; X x; // Calls (&x)->foo(42, "Hello") with default policy: // may print "Hello 42" concurrently or defer execution auto a1 = std::async(&X::foo, &x, 42, "Hello"); // Calls x.bar("world!") with deferred policy // prints "world!" when a2.get() or a2.wait() is called auto a2 = std::async(std::launch::deferred, &X::bar, x, "world!"); // Calls X()(43); with async policy // prints "43" concurrently auto a3 = std::async(std::launch::async, X(), 43); a2.wait(); // prints "world!" std::cout << a3.get() << '\n'; // prints "53" return 0; } // if a1 is not done at this point, destructor of a1 prints "Hello 42" here /////////////////////////////////////////////////////////// // reference: https://thispointer.com/c11-multithreading-part-9-stdasync-tutorial-example/ int test_async_4() { using namespace std::chrono; auto 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; }; auto 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; }; // 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; } } // namespace future_
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