foo.cc: In function 'int main()':
foo.cc:5:2: error: 'cin' was not declared in this scope
  cin >> n;
  ^~~
foo.cc:5:2: note: suggested alternative: 'main'
  cin >> n;
  ^~~
  main
foo.cc:6:2: error: 'cout' was not declared in this scope
  cout << n << endl;
  ^~~~
foo.cc:6:15: error: 'endl' was not declared in this scope
  cout << n << endl;
               ^~~~
foo.cc:6:15: note: suggested alternative: 'enum'
  cout << n << endl;
               ^~~~
               enum

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。