cppreference.com > C++ Strings > find_first_of find_first_of Syntax:
      #include <string>
      size_type find_first_of( const string &str, size_type index = 0 );
      size_type find_first_of( const char* str, size_type index = 0 );
      size_type find_first_of( const char* str, size_type index, size_type num );
      size_type find_first_of( char ch, size_type index = 0 );
The find_first_of() function either: returns the index of the first character within the current string that matches any character in str, beginning the search at index, string::npos if nothing is found, returns the index of the first character within the current string that matches any character in str, beginning the search at index and searching at most num characters, string::npos if nothing is found, or returns the index of the first occurrence of ch in the current string, starting the search at index, string::npos if nothing is found. Related topics: find find_first_not_of find_last_not_of find_last_of rfind