foo.cpp: In function 'int main()':
foo.cpp:18:24: error: 'stdin' was not declared in this scope
     freopen("p.in","r",stdin);
                        ^
foo.cpp:18:29: error: 'freopen' was not declared in this scope
     freopen("p.in","r",stdin);
                             ^
foo.cpp:19:25: error: 'stdout' was not declared in this scope
     freopen("p.out","w",stdout); 
                         ^