生活中你的家有客厅(Public),有你的卧室(Private)
客厅所有来的客人都可以进去,但是你的卧室是私有的,也就是说只有你能进去
但是呢,你也可以允许你的好闺蜜好基友进去。
在程序里,有些私有属性 也想让类外特殊的一些函数或者类进行访问,就需要用到友元的技术
友元的目的就是让一个函数或者类 访问另一个类中私有成员
友元的关键字为 friend
友元的三种实现
全局函数做友元
类做友元
成员函数做友元
全局函数不能,通过指针不能获取私有变量,只能获取公有变量!
但是如果全局函数在类中进行友元声明!
那么就可以进行私有变量的访问!
//告诉编译器 goodGay类是Building类的好朋友,可以访问到Building类中私有内容 friend class goodGay;
class Building { //告诉编译器 goodGay全局函数 是 Building类的好朋友,可以访问类中的私有内容 friend void goodGay(Building * building); public: Building() { this->m_SittingRoom = "客厅"; this->m_BedRoom = "卧室"; } public: string m_SittingRoom; //客厅 private: string m_BedRoom; //卧室 }; void goodGay(Building * building) { cout << "好基友正在访问: " << building->m_SittingRoom << endl; cout << "好基友正在访问: " << building->m_BedRoom << endl; } void test01() { Building b; goodGay(&b); } int main(){ test01(); system("pause"); return 0; }
class Building; class goodGay { public: goodGay(); void visit(); private: Building *building; }; class Building { //告诉编译器 goodGay类是Building类的好朋友,可以访问到Building类中私有内容 friend class goodGay; public: Building(); public: string m_SittingRoom; //客厅 private: string m_BedRoom;//卧室 }; Building::Building() { this->m_SittingRoom = "客厅"; this->m_BedRoom = "卧室"; } goodGay::goodGay() { building = new Building; } void goodGay::visit() { cout << "好基友正在访问" << building->m_SittingRoom << endl; cout << "好基友正在访问" << building->m_BedRoom << endl; } void test01() { goodGay gg; gg.visit(); } int main(){ test01(); system("pause"); return 0; }
class Building; class goodGay { public: goodGay(); void visit(); //只让visit函数作为Building的好朋友,可以发访问Building中私有内容 void visit2(); private: Building *building; }; class Building { //告诉编译器 goodGay类中的visit成员函数 是Building好朋友,可以访问私有内容 friend void goodGay::visit(); public: Building(); public: string m_SittingRoom; //客厅 private: string m_BedRoom;//卧室 }; Building::Building() { this->m_SittingRoom = "客厅"; this->m_BedRoom = "卧室"; } goodGay::goodGay() { building = new Building; } void goodGay::visit() { cout << "好基友正在访问" << building->m_SittingRoom << endl; cout << "好基友正在访问" << building->m_BedRoom << endl; } void goodGay::visit2() { cout << "好基友正在访问" << building->m_SittingRoom << endl; //cout << "好基友正在访问" << building->m_BedRoom << endl; } void test01() { goodGay gg; gg.visit(); } int main(){ test01(); system("pause"); return 0; }
运算符重载概念:对已有的运算符重新进行定义,赋予其另一种功能,以适应不同的数据类型
作用:实现两个自定义数据类型相加的运算
class Person { public: Person() {}; Person(int a, int b) { this->m_A = a; this->m_B = b; } //成员函数实现 + 号运算符重载 Person operator+(const Person& p) { Person temp; temp.m_A = this->m_A + p.m_A; temp.m_B = this->m_B + p.m_B; return temp; } public: int m_A; int m_B; }; //全局函数实现 + 号运算符重载 //Person operator+(const Person& p1, const Person& p2) { // Person temp(0, 0); // temp.m_A = p1.m_A + p2.m_A; // temp.m_B = p1.m_B + p2.m_B; // return temp; //} //运算符重载 可以发生函数重载 Person operator+(const Person& p2, int val) { Person temp; temp.m_A = p2.m_A + val; temp.m_B = p2.m_B + val; return temp; } void test() { Person p1(10, 10); Person p2(20, 20); //成员函数方式 Person p3 = p2 + p1; //相当于 p2.operaor+(p1) cout << "mA:" << p3.m_A << " mB:" << p3.m_B << endl; Person p4 = p3 + 10; //相当于 operator+(p3,10) cout << "mA:" << p4.m_A << " mB:" << p4.m_B << endl; } int main() { test(); system("pause"); return 0; }
总结1:对于内置的数据类型的表达式的的运算符是不可能改变的
总结2:不要滥用运算符重载
4.5.2 左移运算符重载
作用:可以输出自定义数据类型