std::atomic_flag::operator=
来自cppreference.com
                    
                                        
                    < cpp | atomic | atomic flag
                    
                                                            
                    |   定义于头文件  <atomic>
  | 
||
|   atomic_flag& operator=( const atomic_flag& ) = delete;  | 
(1) | (C++11 起) | 
|   atomic_flag& operator=( const atomic_flag& ) volatile = delete;  | 
(2) | (C++11 起) | 
std::atomic_flag 不可赋值,其赋值运算符被删除。