C++11中std::future的具体使用方法

#include "future.hpp"
#include <iostream>
#include <future>
#include <chrono>
#include <utility>
#include <thread>
 
namespace future_ {
 
///////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/future/future/
int test_future_1()
{
{ // constructor/get/operator=
 auto get_value = []() { return 10; };
 std::future<int> foo; // default-constructed
 std::future<int> bar = std::async(get_value); // move-constructed
 
 int x = bar.get();
 std::cout << "value: " << x << '\n'; // 10
 
 //int x2 = bar.get(); // crash, 对于每个future的共享状态，get函数最多仅被调用一次
 //std::cout << "value: " << x2 << '\n';
 
 std::future<int> foo2(std::async(get_value));
 std::cout << "value: " << foo2.get() << '\n'; // 10
}
 
{ // share
 std::future<int> fut = std::async([]() { return 10; });
 std::shared_future<int> shfut = fut.share();
 
 //std::cout << "value: " << fut.get() << '\n'; // crash, 执行完fut.share()后，fut对象将变得无效
 std::cout << "fut valid: " << fut.valid() << '\n';// 0
 
 // shared futures can be accessed multiple times:
 std::cout << "value: " << shfut.get() << '\n'; // 10
 std::cout << "its double: " << shfut.get() * 2 << '\n'; // 20, 对于std::shared_future对象，get函数可以被多次访问
}
 
{ // valid
 std::future<int> foo, bar;
 foo = std::async([]() { return 10; });
 bar = std::move(foo);
 
 if (foo.valid()) std::cout << "foo's value: " << foo.get() << '\n';
 else std::cout << "foo is not valid\n"; // foo is not valid
 
 if (bar.valid()) std::cout << "bar's value: " << bar.get() << '\n'; // 10
 else std::cout << "bar is not valid\n";
}
 
{ // wait
 auto is_prime = [](int x) {
 for (int i = 2; i < x; ++i) if (x%i == 0) return false;
 return true;
 };
 
 // call function asynchronously:
 std::future<bool> fut = std::async(is_prime, 194232491);
 
 std::cout << "checking...\n";
 fut.wait();
 
 std::cout << "\n194232491 ";
 if (fut.get()) // guaranteed to be ready (and not block) after wait returns
 std::cout << "is prime.\n";
 else
 std::cout << "is not prime.\n";
}
 
{ // wait_for
 auto is_prime = [](int x) {
 for (int i = 2; i < x; ++i) if (x%i == 0) return false;
 return true;
 };
 
 // call function asynchronously:
 std::future<bool> fut = std::async(is_prime, 700020007);
 
 // do something while waiting for function to set future:
 std::cout << "checking, please wait";
 std::chrono::milliseconds span(100);
 while (fut.wait_for(span) == std::future_status::timeout) // 可能多次调用std::future::wait_for函数
 std::cout << '.';
 
 bool x = fut.get(); // retrieve return value
 std::cout << "\n700020007 " << (x ? "is" : "is not") << " prime.\n";
}
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://en.cppreference.com/w/cpp/thread/future
int test_future_2()
{
 // future from a packaged_task
 std::packaged_task<int()> task([] { return 7; }); // wrap the function
 std::future<int> f1 = task.get_future(); // get a future
 std::thread t(std::move(task)); // launch on a thread
 
 // future from an async()
 std::future<int> f2 = std::async(std::launch::async, [] { return 8; });
 
#ifdef _MSC_VER
 // future from a promise
 std::promise<int> p;
 std::future<int> f3 = p.get_future();
 std::thread([&p] { p.set_value_at_thread_exit(9); }).detach(); // gcc 4.9 don't support this function
#endif
 
 std::cout << "Waiting..." << std::flush;
 f1.wait();
 f2.wait();
#ifdef _MSC_VER
 f3.wait();
#endif
 std::cout << "Done!\nResults are: " << f1.get() << ' ' << f2.get() << ' '
#ifdef _MSC_VER
 << f3.get()
#endif
 << '\n';
 t.join();
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://thispointer.com/c11-multithreading-part-8-stdfuture-stdpromise-and-returning-values-from-thread/
void initiazer(std::promise<int> * promObj)
{
 std::cout << "Inside Thread" << std::endl;
 promObj->set_value(35);
}
 
int test_future_3()
{
 std::promise<int> promiseObj;
 std::future<int> futureObj = promiseObj.get_future();
 std::thread th(initiazer, &promiseObj);
 std::cout << "value: " << futureObj.get() << std::endl;
 th.join();
 
 // If std::promise object is destroyed before setting the value the calling get() function on associated std::future object will throw exception.
 // A part from this, if you want your thread to return multiple values at different point of time then
 // just pass multiple std::promise objects in thread and fetch multiple return values from thier associated multiple std::future objects.
 
 return 0;
}
 
} // namespace future_