关于本章
使用C++ thread support library进行asynchronous programming。
cppreference Thread support library # Futures
The standard library provides facilities to obtain values that are returned and to catch exceptions that are thrown by asynchronous tasks (i.e. functions launched in separate threads). These values are communicated in a shared state, in which the asynchronous task may write its return value or store an exception, and which may be examined, waited for, and otherwise manipulated by other threads that hold instances of std::future or std::shared_future that reference that shared state.
NOTE:
上面这段话是对C++ asynchronous programming的总结，在 std::future 中，有着类似的、更加详细的说明。
Mode: asynchronous task and std::future
从cppreference Thread support library # Futures 中的内容可以看出，C++为asynchronous programming提供了非常统一、抽象的使用模式:
1 从下面的内容可知，C++支持多种方式来实现asynchronous task
2 std::future 是获取这些asynchronous task的return value的统一方式，显然 std::future 是C++ asynchronous programming的核心所在
3 std::promise、std::future都是操作shared state的facility。
Asynchronous task
在 cppreference Thread support library # Futures 中这样介绍asynchronous task:
The standard library provides facilities to obtain values that are returned and to catch exceptions that are thrown by asynchronous tasks (i.e. functions launched in separate threads).
在 cppreference std::future 中，这样介绍asynchronous task:
The class template
std::futureprovides a mechanism to access the result of asynchronous operations:
- An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a
std::futureobject to the creator of that asynchronous operation.
从上面的描述可以看出，C++支持如下方式的Asynchronous task:
在 cppreference std::future 中，给出了包含上述三种 asynchronous task 的example。