C++ :友元类

友元类的概念和使用

(1)将类A声明为B中的friend class后，则A中所有成员函数都成为类B的友元函数了
(2)代码实战：友元类的定义和使用友元类是单向的
(3)友元类是单向的，代码实战验证

互为友元类

(1)2个类可以互为友元类，代码实战验证
(2)互为友元类要注意互相引用的细节规则

(1)友元类其实就是批量制造友元函数
(2)友元类中所有全部成员都成为了友元函数，相当于一次打了很多洞，极大破坏了面向对象
(3)除非确实有必要，否则建议按需定义友元函数，尽量维护面向对象，让代码更安全健壮

#include <iostream>
#include <string>class Country;  // Forward declaration of Country// External friend function declaration
void disp_info_external(const Country &ct);class InfoDisplayer {public:// Member function declarationvoid disp_info_member(const Country &ct) const;
};class InfoFriendClass {public:// Member function to display informationvoid display(const Country &ct) const;
};class Country {private:std::string privateName;protected:int protectedPopulation;public:std::string publicCapital;// ConstructorCountry(std::string name, int population, std::string capital): privateName(name),protectedPopulation(population),publicCapital(capital) {}// Friend function declarationfriend void disp_info_external(const Country &ct);// friend void InfoDisplayer::disp_info_member(const Country &ct);//error: no// declaration matches ‘void InfoDisplayer::disp_info_member(const Country&)’friend class InfoFriendClass;  // Friend class declaration
};// Definition of the external friend function
void disp_info_external(const Country &ct) {std::cout << "disp_info_external Name: " << ct.privateName << "\n";std::cout << "disp_info_external Population: " << ct.protectedPopulation<< "\n";std::cout << "disp_info_external Capital: " << ct.publicCapital << "\n";
}// Definition of the member friend function
void InfoDisplayer::disp_info_member(const Country &ct) const {// std::cout << "disp_info_member Name: " << ct.privateName << "\n";// error: ‘int Country::protectedPopulation’ is protected within this context//  std::cout << "disp_info_member Population: " << ct.protectedPopulation <<//  "\n"; std::cout << "disp_info_member Capital: " << ct.publicCapital <<//  "\n";
}// Definition of the friend class member function
void InfoFriendClass::display(const Country &ct) const {std::cout << "InfoFriendClass Name: " << ct.privateName << "\n";std::cout << "InfoFriendClass Population: " << ct.protectedPopulation << "\n";std::cout << "InfoFriendClass Capital: " << ct.publicCapital << "\n";
}int test070401() {// Create an object of CountryCountry myCountry("Wonderland", 5000000, "Magic City");// Display information using the external friend functiondisp_info_external(myCountry);// Create an object of InfoDisplayerInfoDisplayer displayer;// Display information using the member friend functiondisplayer.disp_info_member(myCountry);// Create an object of InfoFriendClassInfoFriendClass friendClass;// Display information using the friend class member functionfriendClass.display(myCountry);return 0;
}

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#include <iostream>
#include <string>class Vegetable;  // Forward declaration of Vegetableclass Fruit {private:std::string name;protected:std::string color;public:// ConstructorFruit(std::string n, std::string c) : name(n), color(c) {}// Friend class declarationfriend class Vegetable;// Definition of the friend functionvoid displayVegetable(const Vegetable &veg);
};class Vegetable {private:std::string name;protected:std::string color;public:// ConstructorVegetable(std::string n, std::string c) : name(n), color(c) {}// Friend class declarationfriend class Fruit;// Member function declarationvoid displayFruit(const Fruit &fruit) const {std::cout << "Vegetable accessing Fruit's name: " << fruit.name << "\n";std::cout << "Vegetable accessing Fruit's color: " << fruit.color << "\n";}
};void Fruit::displayVegetable(const Vegetable &veg) {std::cout << "Fruit accessing Vegetable's name: " << veg.name << "\n";std::cout << "Fruit accessing Vegetable's color: " << veg.color << "\n";
}int test070402() {// Create objects of Fruit and VegetableFruit apple("Apple", "Red");Vegetable spinach("Spinach", "Green");apple.displayVegetable(spinach);// Display information using the member function of Vegetablespinach.displayFruit(apple);return 0;
}

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为什么会有友元函数

使用友元函数的优缺点
(1)缺点：破坏了封装机制，尽量不使用友元函数，不得已才使用友元函数
(2)优点：在实现类之间数据共享时，减少系统开销，提高效率。

使用友元函数的两种情况
(1)运算符重载的某些场合需要使用友元
(2)两个类要共享数据的时候
两个类如何共享数据
(1)类内的数据，其实就是类的成员变量
(2)2个类共享数据方法1：将共享数据访问权限设置为public。
(3)2个类共享数据方法2：通过第三个专门封装数据的类，和2个类中带参数的成员函数来传参共享
(4)2个类共享数据方法3：通过友元函数打洞

友元函数和类的成员函数的区别
(1)成员函数有this指针，而友元函数没有this指针。为什么？因为友元只是朋友，并不是类内“自家人”
(2)友元函数是不能被继承的，就像父亲的朋友未必是儿子的朋友。
(3)友元关系不具有传递性。类B是类A的友元，类C是B的友元，类C不一定是类A的友元，要看类中是否有相应的声明

共有友元函数
(1)1个函数同时成为2个类的友元函数
(2)共有友元函数可以是外部函数，也可以是某个（第3个）类的成员函数
(3)共有友元函数内可同时访问2个类的受保护成员，间接将2个完全无关的类的数据打通了