Stl

[sattenapalli]

I am studying "Standard Template Library"from Deitle & Deitle .In that Book to define a vector he used following syntax.

#include<iostream>
using std::cout;
using std::cin;

std::vector<int> vec;

why can not we declare like this?
#include<iostream.h>

vector<int>vec;

why we can not use "cout","cin" directly rather than declaring "using std::cout,using std::cin?

explain in detail?