【C++】【学习笔记】【020】友元

快来打我* 2022-08-13 19:44 148阅读 0赞

1、有些时候，一个完全无关的类由于某些特殊的原因需要访问某个protected成员，甚至某个private成员，这就引入了友元的概念；

2、友元关系是类之间的一种特殊关系，这种关系不仅允许友元类访问对方的public方法和属性，还允许友元访问对方的protected和private方法和属性；

3、声明一个友元关系的语法很简单，只要在类声明里的某个地方加上friend class \*\*就行了，这条语句可以放在任何地方，放在public、protected、private段落里都可以；

**样例**：友元使用方法

#include <iostream>
    #include <string>
    
    // 友元用法示例
    class Lovers
    {
    public:
        Lovers(std::string theName);
        void kiss(Lovers *lover);
        void ask(Lovers *lover, std::string something);
    protected:
        std::string name;
        friend class Others; // 交友不慎
    };
    
    Lovers::Lovers(std::string theName)
    {
        name = theName;
    }
    
    void Lovers::kiss(Lovers *lover)
    {
        std::cout << name << " Lovers::kiss() " << lover->name << std::endl;
    }
    
    void Lovers::ask(Lovers *lover, std::string something)
    {
        std::cout << name << " Lovers::ask() " << lover->name << something << std::endl;
    }
    
    class Boyfriend : public Lovers
    {
    public:
        Boyfriend(std::string theName);
    };
    
    Boyfriend::Boyfriend(std::string theName) : Lovers(theName)
    {
    }
    
    class Girlfriend : public Lovers
    {
    public:
        Girlfriend(std::string theName);
    };
    
    Girlfriend::Girlfriend(std::string theName) : Lovers(theName)
    {
    }
    
    class Others
    {
    public:
        Others(std::string theName);
        void kiss2(Lovers *lover);
    protected:
        std::string name;
    };
    
    Others::Others(std::string theName)
    {
        name = theName;
    }
    
    void Others::kiss2(Lovers *lover)
    {
        std::cout << name << " Others::kiss2() " << lover->name << std::endl;
    }
    
    int main()
    {
        Boyfriend boyfriend("BOY");
        Girlfriend girlfriend("GIRL");
        Others others("OTHER");
    
        girlfriend.kiss(&boyfriend);
        girlfriend.ask(&boyfriend, " Wash clothes.");
    
        std::cout << "OTHER is coming." << std::endl;
        others.kiss2(&girlfriend);
    
        return 0;
    }

输出：
    GIRL Lovers::kiss() BOY
    GIRL Lovers::ask() BOY Wash clothes.
    OTHER is coming.
    OTHER Others::kiss2() GIRL