/in/foo.cc: In function 'int main()':
/in/foo.cc:8:15: error: no matching function for call to 'isalpha(std::__cxx11::string&)'
   if(isalpha(s)!=0)
               ^
In file included from /usr/include/c++/5/cctype:42:0,
                 from /usr/include/c++/5/bits/localefwd.h:42,
                 from /usr/include/c++/5/ios:41,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from /in/foo.cc:1:
/usr/include/ctype.h:111:1: note: candidate: int isalpha(int)
 __exctype (isalpha);
 ^
/usr/include/ctype.h:111:1: note:   no known conversion for argument 1 from 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'int'
In file included from /usr/include/c++/5/bits/basic_ios.h:37:0,
                 from /usr/include/c++/5/ios:44,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/5/bits/locale_facets.h:2595:5: note: candidate: template<class _CharT> bool std::isalpha(_CharT, const std::locale&)
     isalpha(_CharT __c, const locale& __loc)
     ^
/usr/include/c++/5/bits/locale_facets.h:2595:5: note:   template argument deduction/substitution failed:
/in/foo.cc:8:15: note:   candidate expects 2 arguments, 1 provided
   if(isalpha(s)!=0)
               ^