/in/foo.cc: In function 'int main()':
/in/foo.cc:37:32: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("data.in", "r", stdin);
                                ^
/in/foo.cc:38:34: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("data.out", "w", stdout);
                                  ^