C++友元简介

#include <iostream>
#include <cmath>
​
class Rectangle {
//一个定义为private，一个定义为protected
private:
    int height;
protected:
    int width;
public:
    Rectangle() {
        height = 0;
        width = 0;
    }
​
    Rectangle(int left, int top, int right, int bottom) {
        height = abs(bottom - top);
        width = abs(right - left);
    }
​
    int getHeight() {
        return height;
    }
​
    int getWidth() {
        return width;
    }
​
    //使用friend关键字声明为友元
    friend int computerArea(Rectangle& rectangle);
};
​
int computerArea(Rectangle& rectangle) {
    //普通函数只能通过public的方法获取
    //return rectangle.getHeight() * rectangle.getWidth();
    //除了方法，友元函数可以直接访问私有成员
    return rectangle.height * rectangle.width;
}
​
int main()
{
    Rectangle rectangle(0, 0, 100, 200);
    int area = computerArea(rectangle);
    std::cout << rectangle.getWidth() << " * " << rectangle.getHeight() << " = " << area << std::endl;
}

#include <iostream>
​
class Son;//提前声明
​
class Father {
public:
    float getPocketMoneyFriend(Son& son);
    float getPocketMoney(Son& son);
};
​
class Son {
private:
    float  pocketMoney; //零花钱
public:
    Son() {
        pocketMoney = 1100;
    }
    //如果通过儿子，只能使用这个函数
    float getSonPocketMoney() {
        //超过1000时分享零花钱
        if (pocketMoney > 1000) {
            return pocketMoney - 1000;
        }
        return 0;
    }
​
    //但是申请友元，直接访问儿子存钱罐
    friend float Father::getPocketMoneyFriend(Son& son);
};
​
//友元类
float Father::getPocketMoneyFriend(Son& son) {
    //找儿子要钱
    //return son.getSonPocketMoney();
    //直接访问存钱罐
    return son.pocketMoney;
}
​
//非友元类
float Father::getPocketMoney(Son& son) {
    //找儿子要钱
    return son.getSonPocketMoney();
}
​
int main()
{
    Father father;
    Son son;
    float result = father.getPocketMoneyFriend(son);
    std::cout << result << std::endl;
    result = father.getPocketMoney(son);
    std::cout << result << std::endl;
}

#include <iostream>
​
class Son;
​
class Father {
public:
    float getPocketMoney1(Son& son);
    float getPocketMoney2(Son& son);
};
​
class Son {
private:
    float  pocketMoney; //零花钱
public:
    Son() {
        pocketMoney = 1100;
    }
    float getSonPocketMoney() {
        //超过1000时分享零花钱
        if (pocketMoney > 1000) {
            return pocketMoney - 1000;
        }
        return 0;
    }
    friend class Father;
};
​
float Father::getPocketMoney1(Son& son) {
    //找儿子要钱
    //return son.getSonPocketMoney();
    //直接访问存钱罐
    return son.pocketMoney;
}
​
float Father::getPocketMoney2(Son& son) {
    //找儿子要钱
    //return son.getSonPocketMoney();
    //直接访问存钱罐
    return son.pocketMoney;
}
​
int main()
{
    Father father;
    Son son;
    float result = father.getPocketMoney1(son);
    std::cout << result << std::endl;
    result = father.getPocketMoney2(son);
    std::cout << result << std::endl;
}