C++ : 友元

友元函数
友元成员函数

#include <iostream>
using namespace std;
class A;//当用到友元成员函数时，需注意友元声明与友元定义之间的互相依赖。这是类A的声明
class B{
public:
    void dis(int x, A &t);//该函数是类A的友元函数
};
class A{
public:
    friend void B::dis(int x, A &t);//声明B中的dis函数为友元成员函数
    friend void dis2(A &t);//全局友元函数
private:
    int x;
};
void B::dis(int x, A &t){
    t.x = x;
    cout << t.x << endl;
}
void dis2(A &t){
    cout << t.x << endl;
}

int main(){
    class A a; class B b;
    b.dis(1,a);
    dis2(a);
    return 0;
}

运算符重载函数作为类友元函数

#include <iostream>
using namespace std;
class Complex
{
public:
    Complex(){real=0;imag=0;}
    Complex(double t,double i){real=t;imag=i;}
    friend Complex operator + (Complex &c1, Complex &c2);//重载运算符函数作为友元函数
    void display();
private:
    double real; double imag;
};
Complex operator + (Complex &c1, Complex &c2)
{ return Complex(c1.real+c2.real, c1.imag+c2.imag); }
void Complex::display()
{ cout<<"("<<real<<","<<imag<<"i)"<<endl; }
int main()
{
    Complex c1(3,4),c2(5,-10),c3;
    c3 = c1 + c2;//运算符+用于复数运算
    cout<<"c1=";c1.display();
    cout<<"c2=";c2.display();
    cout<<"c1+c2=";c3.display();
    return 0;
}

友元类

#include <iostream>
using namespace std;

class A
{
public:
    friend class C;//友元类的声明
private:
    int data;
};
class C//C是A的友元类，类C的所有成员函数都是类A的友元函数，可以访问类A的私有成员和保护成员
{
public:
    void set_show(int x, A &a) { a.data = x; cout << a.data << endl; }
};

int main(void)
{
    class A a;
    class C c;
    c.set_show(1, a);
    return 0;
}