std::reference_wrapper::operator()
From cppreference.com
                    
                                        
                    < cpp | utility | functional | reference wrapper
                    
                                                            
                    |   template< class... ArgTypes > typename std::result_of<T&(ArgTypes&&...)>::type  | 
(since C++11) | |
Calls the callable object, reference to which is stored. This function is available only of the stored reference points to a Callable object.
Contents | 
[edit] Parameters
| args | - | arguments to pass to the called function | 
[edit] Return value
The return value of the called function.
[edit] Exceptions
(none)
[edit] See also
|    accesses the stored reference   (public member function)  | |