foo.cc: In function 'int main()':
foo.cc:6:2: error: 'str' was not declared in this scope
  str a;
  ^~~
foo.cc:7:9: error: 'a' was not declared in this scope
  cin >> a;
         ^

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