Finds the first position of one of the elements in the function's argument starting from the beginning.
size_type find_first_of
(const basic_string& str, size_type pos = 0) const;
size_type find_first_of
(const charT* s, size_type pos,size_type n) const;
size_type find_first_of
(const charT* s, size_type pos = 0) const;
size_type find_first_of(charT c, size_type pos = 0) const;
The found position or npos if not found.