foo.cpp: In function 'int main()':
foo.cpp:5:24: error: 'stdin' was not declared in this scope
     freopen("f.in","r",stdin);
                        ^
foo.cpp:5:29: error: 'freopen' was not declared in this scope
     freopen("f.in","r",stdin);
                             ^
foo.cpp:6:25: error: 'stdout' was not declared in this scope
     freopen("f.out","w",stdout);
                         ^