/in/foo.cc: In function 'int main()':
/in/foo.cc:5:2: error: 'cin' was not declared in this scope
  cin>>a;
  ^~~
/in/foo.cc:5:2: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~