/in/foo.c:1:2: error: unknown type name 'using'
  using namespace std;
  ^~~~~
/in/foo.c:1:18: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
  using namespace std;
                  ^~~
/in/foo.c:2:21: fatal error: iostream: No such file or directory
 # include <iostream>
                     ^
compilation terminated.