初学C++遇到的引用头文件问题

#include <iostream>
using namespace std;
void main()
{
int i = 0, sum = 0;
while (i <= 10)
{
sum += i;
i++;
}
cout << "sum=" << sum << endl;
}

#include <iostream>
using std::cout;
using std::endl;
void main()
{
int i = 0, sum = 0;
while (i <= 10)
{
sum += i;
i++;
}
cout << "sum=" << sum << endl;
}

#include <iostream>
void main()
{
int i = 0, sum = 0;
while (i <= 10)
{
sum += i;
i++;
}
std::cout << "sum=" << sum << std::endl;
}