foo.cc: In function 'int main()':
foo.cc:11:13: error: 'ostream_iterator' was not declared in this scope
  copy(a,a+n,ostream_iterator<int>(cout," "));
             ^~~~~~~~~~~~~~~~
foo.cc:11:30: error: expected primary-expression before 'int'
  copy(a,a+n,ostream_iterator<int>(cout," "));
                              ^~~

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