foo.cc: In function 'int main()':
foo.cc:4:7: error: 'a' was not declared in this scope
  cin>>a>>b;
       ^
foo.cc:4:10: error: 'b' was not declared in this scope
  cin>>a>>b;
          ^

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