std::basic_ios<CharT,Traits>::narrow
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   char narrow( char_type c, char dfault ) const;  | 
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Converts a current locale-specific character c to its standard equivalent. The result is converted from char_type to char if needed. If no conversion can be performed, the function returns dfault.
Effectively calls std::use_facet< std::ctype<char_type> >(getloc()).narrow(c, dfault);.
Parameters
| c | - | character to convert | 
| dfault | - | character to return if the conversion was unsuccessful | 
Return value
Character converted to its standard equivalent and then to char. dfault is returned if the conversion fails.
See also
|   widens characters  (public member function)  | |
  invokes do_narrow (public member function of std::ctype<CharT>)  | |
|   narrows a wide character to a single-byte narrow character, if possible  (function)  |