View Compilation Error

0_0_21104262_26030.cpp: In function 'int main()':
0_0_21104262_26030.cpp:16:28: error: 'stdin' was not declared in this scope
  freopen("input.txt", "r", stdin);
                            ^
0_0_21104262_26030.cpp:16:33: error: 'freopen' was not declared in this scope
  freopen("input.txt", "r", stdin);
                                 ^