类模板的问题

#include <iostream>
#include<cassert>
using namespace std;

template <typename T>
class Point
{
    T x,y;
public:
    Point(T xi=0,T yi=0):x(xi),y(yi){}
    friend double slope(Point<T> &a,Point<T> &b);
    ~Point(){}
};

template<typename T>
double Point<T>::slope(Point<T> &a,Point<T> &b)//求个斜率
{
    assert(a.x!=b.x);
    double k=(a.y-b.y)/(a.x-b.x);
    return k;
}

int main()
{
    Point<int> point1(1,1);
    Point<int> point2(2,2);
    cout<<slope(point1,point2)<<endl;
    return 0;
}
错误:
E:\codeBlock\模板的测试\main.cpp|16|error: no 'double Point<T>::slope(Point<T>&, Point<T>&)' member function declared in class 'Point<T>'|
他说没在Point<T>中申明，不懂。是我定义错了？翻书查，没错啊。我眼拙看不出，求赐教啊。