在友元函数定义时。是静态的，因此有以下特点：

所以写友元函数的原则：类的友元函数，传入的参数，只能是其本身的指针。
运行时：类的友元函数还是遵循多态的，所以，友元函数最好还是写在基类中，这样它就可以形成多态了。

总结：编译期，友元函数具有静态特性。运行期，多态

class Base {
public:
    typedef std::shared_ptr<Base> ptr;
friend void visit(Base::ptr m);
protected:
    int b = 0;
private:
    int age = 0;
};
class Derive : public Base {
typedef std::shared_ptr<Derive> ptr;
friend void visitD(Base::ptr);
private:
    int n = 0;
};

派生类的友元函数，不能通过基类指针访问基类的protected成员变量(友元函数不具备继承，传递的特性，同时具备单向性)

fdsa.png

图片.png

class Base {
public:
    typedef std::shared_ptr<Base> ptr;
friend void visit(std::shared_ptr<Derive> m);
protected:
    int b = 1;
private:
    int age = 2;
};

class Derive : public Base {
public:
    typedef std::shared_ptr<Derive> ptr;
friend void visitD(Derive::ptr);
private:
    int n = 3;
};

基类的友元函数，不能通过派生类的指针访问基类的protected 成员

图片.png

class Shape {
public:
    friend bool PointInShape(const Point<double>& cur, std::shared_ptr<const Shape> s);
    friend bool PointOnShape(const Point<double>& cur, std::shared_ptr<const Shape> s);

    enum ShapeType {
        M,
        V,
        CONN,
        Poly,
        UNKNON
    };

    typedef std::shared_ptr<Shape> ptr;
    Shape(std::string& line)
    :m_level(0) {
        static uint64_t s_key = 0;
        m_key = s_key;
        s_key++;
        std::stringstream ss;
        ss << line;
        double x;
        double y;
        while(ss >> x && ss >> y) {
            m_points.push_back(Point<double>(x,y));
        }
    };

    virtual ~Shape(){};

    // plot 内联 ---- 
    void plot() const {
        size_t pNum = m_points.size();
        std::vector<double> x(pNum + 1);
        std::vector<double> y(pNum + 1);
        for(size_t i = 0; i < pNum; ++i) {
            x[i] = (m_points[i].getX());
            y[i] = (m_points[i].getY());
        }
        x[pNum] = (m_points[0].getX());
        y[pNum] = (m_points[0].getY());
        std::string c = getColor();
        plt::plot(x, y, c);
    };

    size_t size() { return m_points.size();};    
    int getLevel() { return m_level;};
    uint64_t getKey() const {return m_key;};


    const Point<double>& operator[] (std::size_t position) const{
        return m_points[position];
    }; 

    std::string ToString() const;
protected:
    virtual const std::string getColor() const = 0;
    std::vector<Point<double>> m_points;
    int m_level = 0;
    uint64_t m_key = 0;
};

为什么PointInShape报错。

图片.png