/in/foo.cc: In function 'int main()':
/in/foo.cc:7:5: error: 'cin' was not declared in this scope
     cin >> a >> b;
     ^~~
/in/foo.cc:7:5: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~