C++笔记 友元函数，友元类，运算符重载

1 友元函数
在友元函数中可以访问私有属性

class A{    friend void modify_i(A *p,int a);private:    int i;public:    A(int i){        this->i = i;    }    void myprint(){        cout << i << endl;    }};//友元函数的实现，在友元函数中可以访问私有的属性void modify_i(A *p,int a){    p->i = a;}void main(){    A *a = new A(10);    a->myprint();    modify_i(a,20);    a->myprint();    system("pause");}

void main(){    A *a = new A(10);    a->myprint();    modify_i(a,20);    a->myprint();    system("pause");}

通过在类中定义友元函数，然后在外部实现，通过访问这个实现的友元函数，就可以访问类中的私有属性。

打印结果

1020

2 友元类

class A{    //友元类    friend  class B;private:    int i;};class B{public:    //B这个友元类可以访问A类的任何成员    void accessAny(){        a.i = 30;    }private:    A a;};

3 运算符重载

class Point{public:    int x;    int y;public :    Point(int x = 0,int y = 0){        this->x = x;        this->y = y;    }    void myprint(){        cout << x << ","<<y<< endl;    }};//重载+号Point operator+(Point &p1,Point &p2){    Point tmp(p1.x + p2.x, p1.y + p2.y);    return tmp;}

void main(){    Point p1(10,20);    Point p2(20,10);    Point p3 = p1 + p2;    p3.myprint();    system("pause");}

打印结果

30,30

4 成员函数 运算符重载

class Point{public:    int x;    int y;public:    Point(int x = 0, int y = 0){        this->x = x;        this->y = y;    }    Point operator+(Point &p2){        Point tmp(this->x+p2.x,this->y+p2.y);        return tmp;    }    void myprint(){        cout << x << "," << y << endl;    }};

void main(){        Point p1(10,20);        Point p2(20,10);        Point p3 = p1 + p2;        //p1.operator+(p2); 运算符重载，本质还是函数的调用        p3.myprint();        system("pause");    }

打印结果

30,30

5 当属性私有时 通过友元函数完成运算符重载

class Point{    friend Point operator+(Point &p1, Point &p2);private:    int x;    int y;public:    Point(int x = 0, int y = 0){        this->x = x;        this->y = y;    }    Point operator+(Point &p2){        Point tmp(this->x + p2.x, this->y + p2.y);        return tmp;    }    void myprint(){        cout << x << "," << y << endl;    }};Point operator+(Point &p1,Point &p2){    //此处可以访问p.x    Point tmp(p1.x + p2.x, p1.y + p2.y);    return tmp;}

void main(){    Point p1(10, 20);    Point p2(20, 10);    Point p3 = p1 + p2;    p3.myprint();    system("pause");}

打印结果

30,30