foo.cc: In function 'int main()':
foo.cc:4:25: error: 'scanf' was not declared in this scope
     scanf("%d%d", &a, &b);
                         ^
foo.cc:5:25: error: 'printf' was not declared in this scope
     printf("%d\n", a + b);
                         ^

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