foo.cc: In member function 'bool prtcmp::operator()(const char*, const char*) const':
foo.cc:17:22: error: 'strcmp' was not declared in this scope
   return strcmp(s1,s2)<0;
                      ^
foo.cc: In function 'int main()':
foo.cc:40:21: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&flag_2);
                     ^
foo.cc:48:17: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
  system("pause");
                 ^

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