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://blog.csdn.net/fengbingchun/article/details/104115489
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_