c++ - Why is this pointer needed when calling std::call_once()? -

"C ++ concurrency in action" in book "§3.3.1, when starting a thread-safe lazy launch of a class member From std :: call_once () , it gives the following example:

  #include   
, the third parameter is the parameter passing through the function  X :: open_connection () .  is given to  std :: call_once ()  when there is no input parameter for  X :: open_connection ()   this  Need Pointer?  
  std :: call_once (connection_init_flag, and x :: :: open_connection, this);   
 PS: Removing   indicator will cause C 2064 error:  
 
  Error C2064: term Do not evaluate 0 logic in action    
 
  UPDATED:  The problem is more explicitly in the book "C + + Concurrency "" When to start the same work, then  std :: async :  
 
  If the first argument ( std :: call_once ) Is an indicator for a member function, the second argument ( std :: call_once  to be third) provides the object on which the member functions (either directly or, an indicator, or  std :: ref ) is wrapped, and the remaining argument is used for member functions .  Otherwise, the second ( Std :: call_once ) and subsequent logic functions or callable object specification are passed as arguments Are the first arguments as AD.    
 
  
 Why is this indicator necessary when calling STD: call_once ()?   
 Because  open_connection  is a non-static data member, it has to be called on some, and such a similar example is,  this  (Technically, the first parameter for  this  in the non-static member functions is an inherent first parameter). )  
 This could have been applied with a different example, although in this case it would not be understandable:  
  xx; Std :: call_once (connection_init_flag, & x :: open_connection, & amp; x);