foo.cc: In function 'int main()':
foo.cc:46:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &n);
  ~~~~~^~~~~~~~~~
foo.cc:47:59: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  for(register int i = 1; i <= n; i++) scanf("%d", &a[i].h), a[i].id = i;
                                       ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~