#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。

slope函数定义也拿出来看看吧

看到了，那个slop函数并非类Point的成员函数，你的slop函数写的有问题，double Point<T>::slope(Point<T> &a,Point<T> &b)改成T slope(Point<T> &a,Point<T> &b)试试

如果要严谨些，那得这么写

#include <cassert>

template<typename T>
class Point;

template<typename T>
double slope( const Point<T>& a, const Point<T>& b );

template<typename T>
class Point
{
public:
    Point() : x_(), y_()
    {
    }
    Point( T x, T y ) : x_(x), y_(y)
    {
    }

private:
    T x_, y_;

    friend double slope<>( const Point<T>& a, const Point<T>& b );
};

template<typename T>
double slope( const Point<T>& a, const Point<T>& b )
{
    assert( a.x_ != b.x_ );
    return (a.y_-b.y_)/(a.x_-b.x_);
}

#include <iostream>
using namespace std;

int main( void )
{
    Point<int> point1(1,1);
    Point<int> point2(2,2);
    cout << slope(point1,point2) << endl;
    return 0;
}

不要求那么严谨，可以

#include <cassert>

template<typename T>
class Point
{
public:
    Point() : x_(), y_()
    {
    }
    Point( T x, T y ) : x_(x), y_(y)
    {
    }

private:
    T x_, y_;

    template<typename U> friend double slope( const Point<U>& a, const Point<U>& b );
};

template<typename U>
double slope( const Point<U>& a, const Point<U>& b )
{
    assert( a.x_ != b.x_ );
    return (a.y_-b.y_)/(a.x_-b.x_);
}

#include <iostream>
using namespace std;

int main( void )
{
    Point<int> point1(1,1);
    Point<int> point2(2,2);
    cout << slope(point1,point2) << endl;
    return 0;
}

差别在于，第一段代码中 slope<int>是Point<int>的友元，slope<double>是Point<double>的友元，……
而第二段代码中，slope<int>、slope<double>……是Point<int>的友元，也是Point<double>的友元，……

我还是觉得你过度设计了，应该简单点儿

#include <cassert>

template<typename T>
struct Point
{
    T x, y;
   

    Point() : x(), y()
    {
    }
    Point( T x, T y ) : x(x), y(y)
    {
    }
};

template<typename T, typename U>
double slope( const T& a, const U& b )
{
    assert( a.x != b.x );
    return (a.y-b.y)/(a.x-b.x+0.0);
}

#include <iostream>
using namespace std;

int main( void )
{
    Point<int> point1(1,1);
    Point<int> point2(2,2);
    cout << slope(point1,point2) << endl;
    return 0;
}

谢谢，很详细

谢谢，确实是这儿的毛病，我友元函数知识不精所导致

谢谢