std::experimental::future<T>::operator=
来自cppreference.com
                    
                                        
                    < cpp | experimental | future
                    
                                                            
                    |   std::experimental::future<T>& operator=( std::experimental::future<T>&& other ) noexcept;  | 
(1) | |
|   std::experimental::future<T>& operator=( const std::experimental::future<T>& other ) = delete;  | 
(2) | |
赋值另一 future 对象的内容。
1) 释放任何共享状态并移动赋值 other 给 *this 。赋值后 other.valid() == false 而 this->valid() 将生成与 other.valid() 在赋值前相同的值。
2) std::experimental::future 不可复制赋值 (CopyAssignable) 。
参数
| other | - |  将传递状态给 *this 的 std::experimental::future
 | 
返回值
*this