“使用命名空间 std;"的问题

I have a source code file in C++. Here it is:

#include <iostream>
using namespace std;

class myPoint
{
public:
    double x;
    double y;
    myPoint() {x=y=0;}
};

double distance(myPoint A, myPoint B)
{
    return (A.x - B.x);
}

int main()
{
    myPoint A, B;
    A.x=5; A.y=5;
    B.x=3; B.y=2;
    cout << distance(A, B) << endl;
    return 0;
}

我的编译器(Microsoft Visual Studio C++ 2012)给了我以下错误: