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：C++11中std::future的具体使用方法_C 语言_脚本之家

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 <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 returnif (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):bar.get();foo.get(); // 注：注释掉此句，则不会输出'**********'std::cout << '\n';return 0;}///// reference: https://en.cppreference.com/w/cpp/thread/asyncstd::mutex m;struct X {int testxx;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;x.testxx = 8;// Calls (&x)->foo(42, "Hello") with default policy:// may print "Hello 42" concurrently or defer executionauto a1 = std::async(&X::foo, x, 42, "Hello");// Calls x.bar("world!") with deferred policy// prints "world!" when a2.get() or a2.wait() is calledauto a2 = std::async(std::launch::deferred, &X::bar, x, "world!");// Calls X()(43); with async policy// prints "43" concurrentlyauto a3 = std::async(std::launch::async, X(), 43);std::this_thread::sleep_for(std::chrono::seconds(2));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 completestd::this_thread::sleep_for(seconds(5));//Do stuff like creating DB Connection and fetching Datareturn "DB_" + recvdData;};auto fetchDataFromFile = [](std::string recvdData) {// Make sure that function takes 5 seconds to completestd::this_thread::sleep_for(seconds(5));//Do stuff like fetching Data Filereturn "File_" + recvdData;};// Get Start Timesystem_clock::time_point start = system_clock::now();std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");//Fetch Data from Filestd::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 Timeauto 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 Datastd::string data = dbData + " :: " + fileData;//Printing the combined Datastd::cout << "Data = " << data << std::endl;return 0;}} // namespace future_