std::future

1. future

• 1. future
  • 1.1. future相关结构
  • 1.2. async相关结构
  • 1.3. packaged_task 相关结构
  • 1.4. promise相关结构
  • 1.5. 参考资料

相关demo详见

promise-demo
packaged_task-demo
furture-demo

1.1. future相关结构

Member functions

fun	desc	pub/priv
(constructor)	constructs the future object	(public member function)
(destructor)	destructs the future object	(public member function)
operator=	moves the future object	(public member function)
share	transfers the shared state from *this to a shared_future and returns it	(public member function)

Getting the result

fun	desc	pub/priv
get	returns the result	(public member function)
valid	checks if the future has a shared state	(public member function)
wait	waits for the result to become available	(public member function)
wait_for	waits for the result, returns if it is not available for the specified timeout duration	(public member function)
wait_until	waits for the result, returns if it is not available until specified time point has been reached	(public member function)

1.2. async相关结构

简单快捷，简易版std::thread
std::async 并不总会开启新的线程来执行任务，你可以指定 std::launch::async 来强制开启新线程
如果 std::async 返回值 std::future 被存放在一个临时对象中，那么std::async会立马阻塞，因为临时对象在返回后立马被析构了。
std::async( std::launch::async, sleep, 5 ); // 临时对象被析构，阻塞 5s
auto f1 = std::async( std::launch::async, sleep, 5 );如果不調用f1.get(),则std::async可能都不执行

async( Function&& f, Args&&... args );
//policy: std::launch::async,std::launch::deferred,立刻异步执行或调用时再触发异步执行
async( std::launch policy, Function&& f, Args&&... args );

int asyncTime() {
    auto sleep = [](int s) { std::this_thread::sleep_for(std::chrono::seconds(s)); };

    auto start = now();
    std::async(std::launch::async, sleep, 2); // 临时对象被析构，阻塞 2s
    std::cout << "cost:" << sub(start, now()) << std::endl;//cost 2s
    start = now();
    std::async(std::launch::async, sleep, 2); // 临时对象被析构，阻塞 2s
    std::cout << "cost:" << sub(start, now()) << std::endl;//cost 2s
    
    start = now();
    auto f1 = std::async(std::launch::async, sleep, 2);
    std::cout << "cost:" << sub(start, now()) << std::endl;//cost 0s
    start = now();
    auto f2 = std::async(std::launch::async, sleep, 2);
    std::cout << "cost:" << sub(start, now()) << std::endl;//cost 0s
    f2.get();
    std::cout << "cost:" << sub(start, now()) << std::endl;//cost 2s
    return 0;
}

1.3. packaged_task 相关结构

配合 std::thread，可以取代 async
实际上就是std::futrue的封装函数，没啥特殊函数，参数是指向一个函数
可以通过 packaged_task对象直接调用关联函数(支持operator())。

    auto task = [](int i) { 
    std::this_thread::sleep_for(std::chrono::seconds(5)); return i+100; 
    };

    std::packaged_task< int(int) > package{ task };
    std::future<int> f = package.get_future();
    package(1);//直接调用
    std::cout << f.get() << "\n";

    std::packaged_task< int(int) > package2{ task };
    std::future<int> f = package.get_future();
    std::thread t { std::move(package2), 5 };//线程执行
    std::cout << f.get() << std::endl; // 阻塞，直到线程 t 结束
    t.join();

packaged_task

|Member functions

fun	desc	pub/priv
(constructor)	constructs the task object	(public member function)
(destructor)	destructs the task object	(public member function)
operator=	moves the task object	(public member function)
valid	checks if the task object has a valid function	(public member function)
swap	swaps two task objects	(public member function)
get_future	returns a std::future associated with the promised result	(public member function)
operator()	executes the function	(public member function)
make_ready_at_thread_exit	executes the function ensuring that the result is ready only once the current thread exits	(public member function)
reset	resets the state abandoning any stored results of previous executions	(public member function)
std::swap(std::packaged_task)(C++11)	specializes the std::swap algorithm	(function template)
std::uses_allocatorstd::packaged_task	specializes the std::uses_allocator type trait	(class template specialization)

1.4. promise相关结构

更像一个线程间同步的封装类，一个调用set_value设置变量，一个调用get_future().get()获取变量

    auto task = [](std::future<int> i) {
        std::cout << i.get() << std::flush; // 阻塞，直到 p.set_value() 被调用
    };

    std::promise<int> p;
    std::thread t{ task, p.get_future() };

    std::this_thread::sleep_for(std::chrono::seconds(5));
    p.set_value(5);

    t.join();

promise

Member functions

fun	desc	pub/priv
(constructor)	constructs the promise object	(public member function)
(destructor)	destructs the promise object	(public member function)
operator=	assigns the shared state	(public member function)
swap	swaps two promise objects	(public member function)

Getting the result

fun	desc	pub/priv
get_future	returns a future associated with the promised result	(public member function)
set_value	sets the result to specific value	(public member function)
set_value_at_thread_exit	sets the result to specific value while delivering the notification only at thread exit	(public member function)
set_exception	sets the result to indicate an exception	(public member function)
set_exception_at_thread_exit	sets the result to indicate an exception while delivering the notification only at thread exit	(public member function)
std::swap(std::promise)	specializes the std::swap algorithm	(function template)
std::uses_allocatorstd::promise	specializes the std::uses_allocator type trait	(class template specialization)

1.5. 参考资料

1. C++11并发指南系列
2. cppreference