友元
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
1 全局函数做友元
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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; } |
2 类做友元
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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; } |
3 成员函数做友元
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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
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
1 加号运算符重载
作用:实现两个自定义数据类型相加的运算
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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
2 左移运算符重载
作用:可以输出自定义数据类型
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class Person { friend ostream& operator<<(ostream& out, Person& p); public : Person( int a, int b) { this ->m_A = a; this ->m_B = b; } //成员函数 实现不了 p << cout 不是我们想要的效果 //void operator<<(Person& p){ //} private : int m_A; int m_B; }; //全局函数实现左移重载 //ostream对象只能有一个 ostream& operator<<(ostream& out, Person& p) { out << "a:" << p.m_A << " b:" << p.m_B; return out; } void test() { Person p1(10, 20); cout << p1 << "hello world" << endl; //链式编程 } int main() { test(); system ( "pause" ); return 0; } |
总结:重载左移运算符配合友元可以实现输出自定义数据类型
3 递增运算符重载
作用: 通过重载递增运算符,实现自己的整型数据
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class MyInteger { friend ostream& operator<<(ostream& out, MyInteger myint); public : MyInteger() { m_Num = 0; } //前置++ MyInteger& operator++() { //先++ m_Num++; //再返回 return * this ; } //后置++ MyInteger operator++( int ) { //先返回 MyInteger temp = * this ; //记录当前本身的值,然后让本身的值加1,但是返回的是以前的值,达到先返回后++; m_Num++; return temp; } private : int m_Num; }; ostream& operator<<(ostream& out, MyInteger myint) { out << myint.m_Num; return out; } //前置++ 先++ 再返回 void test01() { MyInteger myInt; cout << ++myInt << endl; cout << myInt << endl; } //后置++ 先返回 再++ void test02() { MyInteger myInt; cout << myInt++ << endl; cout << myInt << endl; } int main() { test01(); //test02(); system ( "pause" ); return 0; } |
总结: 前置递增返回引用,后置递增返回值
4 赋值运算符重载
c++编译器至少给一个类添加4个函数
- 默认构造函数(无参,函数体为空)
- 默认析构函数(无参,函数体为空)
- 默认拷贝构造函数,对属性进行值拷贝
- 赋值运算符 operator=, 对属性进行值拷贝
如果类中有属性指向堆区,做赋值操作时也会出现深浅拷贝问题
示例:
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class Person { public : Person( int age) { //将年龄数据开辟到堆区 m_Age = new int (age); } //重载赋值运算符 Person& operator=(Person &p) { if (m_Age != NULL) { delete m_Age; m_Age = NULL; } //编译器提供的代码是浅拷贝 //m_Age = p.m_Age; //提供深拷贝 解决浅拷贝的问题 m_Age = new int (*p.m_Age); //返回自身 return * this ; } ~Person() { if (m_Age != NULL) { delete m_Age; m_Age = NULL; } } //年龄的指针 int *m_Age; }; void test01() { Person p1(18); Person p2(20); Person p3(30); p3 = p2 = p1; //赋值操作 cout << "p1的年龄为:" << *p1.m_Age << endl; cout << "p2的年龄为:" << *p2.m_Age << endl; cout << "p3的年龄为:" << *p3.m_Age << endl; } int main() { test01(); //int a = 10; //int b = 20; //int c = 30; //c = b = a; //cout << "a = " << a << endl; //cout << "b = " << b << endl; //cout << "c = " << c << endl; system ( "pause" ); return 0; } |
5 关系运算符重载
**作用:**重载关系运算符,可以让两个自定义类型对象进行对比操作
示例:
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class Person { public : Person(string name, int age) { this ->m_Name = name; this ->m_Age = age; }; bool operator==(Person & p) { if ( this ->m_Name == p.m_Name && this ->m_Age == p.m_Age) { return true ; } else { return false ; } } bool operator!=(Person & p) { if ( this ->m_Name == p.m_Name && this ->m_Age == p.m_Age) { return false ; } else { return true ; } } string m_Name; int m_Age; }; void test01() { //int a = 0; //int b = 0; Person a( "孙悟空" , 18); Person b( "孙悟空" , 18); if (a == b) { cout << "a和b相等" << endl; } else { cout << "a和b不相等" << endl; } if (a != b) { cout << "a和b不相等" << endl; } else { cout << "a和b相等" << endl; } } int main() { test01(); system ( "pause" ); return 0; } |
6 函数调用运算符重载
- 函数调用运算符 () 也可以重载
- 由于重载后使用的方式非常像函数的调用,因此称为仿函数
- 仿函数没有固定写法,非常灵活
示例:
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class MyPrint { public : void operator()(string text) { cout << text << endl; } }; void test01() { //重载的()操作符 也称为仿函数 MyPrint myFunc; myFunc( "hello world" ); } class MyAdd { public : int operator()( int v1, int v2) { return v1 + v2; } }; void test02() { MyAdd add; int ret = add(10, 10); cout << "ret = " << ret << endl; //匿名对象调用 cout << "MyAdd()(100,100) = " << MyAdd()(100, 100) << endl; } int main() { test01(); test02(); system ( "pause" ); return 0; } |
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原文链接:https://blog.csdn.net/yang2330648064/article/details/124353287