【C++初学】C++核心编程技术详解（二）：类与继承

3. 函数提高
   3.1 函数默认参数
   函数的形参可以有默认值，调用时可以省略这些参数。
   示例代码：

int func(int a, int b = 10, int c = 10) {return a + b + c;
}int main() {cout << "ret = " << func(20, 20) << endl;cout << "ret = " << func(100) << endl;system("pause");return 0;
}

3.2 函数占位参数

占位参数在函数声明中占位，调用时必须提供值。
示例代码：

void func(int a, int) {cout << "this is func" << endl;
}int main() {func(10, 10); // 占位参数必须提供值system("pause");return 0;
}

3.3 函数重载

函数重载允许函数名相同，但参数类型、个数或顺序不同。
示例代码：

void func() {cout << "func 的调用！" << endl;
}void func(int a) {cout << "func (int a) 的调用！" << endl;
}void func(double a) {cout << "func (double a)的调用！" << endl;
}void func(int a, double b) {cout << "func (int a ,double b) 的调用！" << endl;
}int main() {
func();
func(10);
func(3.14);
func(10, 3.14);
func(3.14, 10);
system("pause");
return 0;
}

void func(int& a)
{cout << "func(int &a)调用" << endl;
}void func(const int& a)
{cout << "func(const int &a)调用" << endl;
}void func2(int a, int b = 10)
{cout << "func2(int a , int b = 10)调用" << endl;
}void func2(int a)
{cout << "func2(int a)调用" << endl;
}
int main()
{int a = 10;func(a);func(10);//func2(10);system("pause");return 0;
}

4 类和对象

面向对象的三大特性是封装、继承和多态。类是对象的抽象，对象是类的实例。

4.1 封装

封装是将属性和行为作为一个整体，表现生活中的事物，并加以权限控制。

4.1.1 封装的意义

示例代码：

// 圆类
const double PI = 3.14;
class Circle {
public:int m_r; // 半径double calculateZC() { // 计算周长return 2 * PI * m_r;}
};int main() {Circle c1;c1.m_r = 10;cout << "圆的周长为： " << c1.calculateZC() << endl;system("pause");return 0;
}

案例2：

#include <string>
class Student
{
public:void setName(string name){m_name = name;}void setID(int id){m_id = id;}void showStudent(){cout << "name: " << m_name << "id: " << m_id << endl;}
public:string m_name;int m_id;
};
int main()
{Student stu;stu.setName("德玛西亚");stu.setID(250);stu.showStudent();system("pause");return 0;
}

4.1.2 struct 和 class 的区别

struct 默认权限为公共。
class 默认权限为私有。
示例代码：

class c1   //默认私有
{int m_A;};
struct c2 //默认公共
{int m_A;};
int main()
{c1 c1;c1.m_A = 10;//errc2 c2;c2.m_A = 10;system("pause");return 0;
}

4.1.3 成员属性设置为私有

将成员属性设置为私有可以控制读写权限，并检测数据有效性。
示例代码：

class Person {
public:void setName(string name) {m_Name = name;}string getName() {return m_Name;}int getAge() {return m_Age;}void setAge(int age) {if (age < 0 || age > 150) {cout << "你个老妖精!" << endl;return;}m_Age = age;}void setLover(string lover) {m_Lover = lover;}
private:string m_Name; // 姓名int m_Age; // 年龄string m_Lover; // 情人
};int main() {Person p;p.setName("张三");cout << "姓名： " << p.getName() << endl;p.setAge(50);cout << "年龄： " << p.getAge() << endl;p.setLover("苍井");system("pause");return 0;
}

4.2 对象的初始化和清理

构造函数和析构函数是C++中用于对象初始化和清理的重要机制。

4.2.1 构造函数和析构函数

构造函数用于对象创建时的初始化，析构函数用于对象销毁前的清理。
示例代码：

class Person {
public:Person() {cout << "Person的构造函数调用" << endl;}~Person() {cout << "Person的析构函数调用" << endl;}
};void test01() {Person p;
}int main() {test01();system("pause");return 0;
}

4.2.2 构造函数的分类及调用
构造函数可以分为有参构造和无参构造，还可以分为普通构造和拷贝构造。
示例代码：

class Person {
public:Person() {cout << "无参构造函数!" << endl;}Person(int a) {age = a;cout << "有参构造函数!" << endl;}Person(const Person& p) {age = p.age;cout << "拷贝构造函数!" << endl;}~Person() {cout << "析构函数!" << endl;}
public:int age;
};void test01() {Person p; // 调用无参构造函数
}void test02() {Person p1(10); // 调用有参构造函数Person p2 = Person(10); // 调用拷贝构造函数Person p3 = p1; // 调用拷贝构造函数
}int main() {test01();test02();system("pause");return 0;
}

4.2.3 拷贝构造函数调用时机

拷贝构造函数在以下三种情况下被调用：
使用一个已经创建完毕的对象来初始化一个新对象。
以值传递的方式给函数参数传值。
以值方式返回局部对象。
示例代码：

class Person
{
public:Person(){cout << "无参构造函数" << endl;}Person(int a){age = a;cout << "有参构造函数" << endl;}Person(const Person& p){cout << "拷贝构造函数" << endl;age = p.age;}~Person(){cout << "析构函数" << endl;}
public:int age;
};void test01()
{Person p;
}void test02()
{Person p1(10);Person p2 = Person(10);Person p3 = Person(p2);Person p4 = 10;Person p5 = p4;
}
int main()
{test01();test02();system("pause");return 0;
}

class Person
{
public:Person(){cout << "无参构造函数" << endl;mAge = 0;}Person(int age){cout << "有参构造函数" << endl;}Person(const Person& p){cout << "拷贝构造函数" << endl;mAge = p.mAge;}~Person(){cout << "析构函数" << endl;}
public:int mAge;
};void test01()
{Person man(100);Person newman(man);Person newman2 = man;
}void dowork(Person p1) {};
void test02()
{Person p;dowork(p);
}Person dowork2()
{Person p1;cout << (int*)&p1 << endl;return p1;
}void test03()
{Person p = dowork2();cout << (int*)&p << endl;
}
int main()
{//test01();//test02();test03();system("pause");return 0;
}

4.2.4 构造函数调用规则

如果用户定义了有参构造函数，编译器不会再提供默认无参构造函数，但会提供默认拷贝构造函数。
示例代码：

class Person {
public:Person() {cout << "无参构造函数!" << endl;}Person(int a) {age = a;cout << "有参构造函数!" << endl;}Person(const Person& p) {age = p.age;cout << "拷贝构造函数!" << endl;}~Person() {cout << "析构函数!" << endl;}
public:int age;
};void test01() {Person p1(18);Person p2(p1); // 调用拷贝构造函数cout << "p2的年龄为： " << p2.age << endl;
}void test02() {Person p1; // 如果用户没有提供默认构造函数，会报错Person p2(10); // 用户提供的有参构造函数
}int main() {test01();test02();system("pause");return 0;
}

4.2.5 深拷贝与浅拷贝

浅拷贝只是简单地赋值，而深拷贝会在堆区重新申请空间。
示例代码：

class Person {
public:Person() {cout << "无参构造函数!" << endl;}Person(int age, int height) {cout << "有参构造函数!" << endl;m_age = age;m_height = new int(height);}Person(const Person& p) {cout << "拷贝构造函数!" << endl;m_age = p.m_age;m_height = new int(*p.m_height); // 深拷贝}~Person() {cout << "析构函数!" << endl;if (m_height != NULL) {delete m_height;}}
public:int m_age;int* m_height;
};void test01() {Person p1(18, 180);Person p2(p1);cout << "p1的年龄： " << p1.m_age << " 身高： " << *p1.m_height << endl;cout << "p2的年龄： " << p2.m_age << " 身高： " << *p2.m_height << endl;
}int main() {test01();system("pause");return 0;

4.2.6 初始化列表

初始化列表用于在构造函数中初始化成员变量。
示例代码：

class Person {
public:Person(int a, int b, int c) : m_A(a), m_B(b), m_C(c) {cout << "构造函数调用" << endl;}void PrintPerson() {cout << "mA:" << m_A << endl;cout << "mB:" << m_B << endl;cout << "mC:" << m_C << endl;}
private:int m_A;int m_B;int m_C;
};int main() {Person p(1, 2, 3);p.PrintPerson();system("pause");return 0;
}

4.2.7 类对象作为类成员

类中的成员可以是另一个类的对象。
示例代码：

class Phone {
public:Phone(string name) {m_PhoneName = name;cout << "Phone构造" << endl;}~Phone() {cout << "Phone析构" << endl;}string m_PhoneName;
};class Person {
public:Person(string name, string pName) : m_Name(name), m_Phone(pName) {cout << "Person构造" << endl;}~Person() {cout << "Person析构" << endl;}void playGame() {cout << m_Name << " 使用" << m_Phone.m_PhoneName << " 牌手机! " << endl;}string m_Name;Phone m_Phone;
};void test01() {Person p("张三", "苹果X");p.playGame();
}int main() {test01();system("pause");return 0;
}

4.2.8 静态成员

静态成员变量和静态成员函数是类的所有对象共享的。
示例代码：

class Person {
public:static int m_A; // 静态成员变量static void func() {cout << "func调用" << endl;m_A = 100;}int m_B; // 非静态成员变量
};int Person::m_A = 10;void test01() {Person p1;p1.func();Person::func();cout << "p1.m_A = " << p1.m_A << endl;cout << "m_A = " << Person::m_A << endl;
}int main() {test01();system("pause");return 0;
}

class Person
{
public:static int m_A;
private:static int m_B;};
int Person::m_A = 10;
int Person::m_B = 10;void test01()
{Person p1;p1.m_A = 100;cout << "p1.m_A = " << p1.m_A << endl;Person p2;p2.m_A = 200;cout << "p1.m_A = " << p1.m_A << endl;cout << "p2.m_A = " << p2.m_A << endl;cout << "m_A = " << Person::m_A << endl;}
int main()
{test01();system("pause");return 0;
}

4.3 C++对象模型和this指针

成员变量和成员函数分开存储

this指针指向被调用的成员函数所属的对象。
示例代码：

class Person {
public:Person(int age) {this->age = age;}Person& PersonAddPerson(Person p) {this->age += p.age;return *this;}int age;
};void test01() {Person p1(10);cout << "p1.age = " << p1.age << endl;Person p2(10);p2.PersonAddPerson(p1).PersonAddPerson(p1).PersonAddPerson(p1);cout << "p2.age = " << p2.age << endl;
}int main() {test01();system("pause");return 0;
}

空指针访问成员数组

4.4 友元

友元可以访问类的私有成员。

4.4.1 全局函数做友元

示例代码：

class Building {friend void goodGay(Building *building);string m_BedRoom; // 卧室
public:Building() {m_BedRoom = "卧室";}string m_SittingRoom; // 客厅
};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;
}

4.4.2 类做友元

示例代码：

class Building {friend class goodGay;string m_BedRoom; // 卧室
public:Building() {m_BedRoom = "卧室";}string m_SittingRoom; // 客厅
};class goodGay {
public:void visit(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 gg;gg.visit(&b);
}int main() {test01();system("pause");return 0;
}

4.4.3 成员函数做友元

示例代码：

class Building {friend void goodGay::visit(Building *building);string m_BedRoom; // 卧室
public:Building() {m_BedRoom = "卧室";}string m_SittingRoom; // 客厅
};class goodGay {
public:void visit(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 gg;gg.visit(&b);
}int main() {test01();system("pause");return 0;
}

4.5 运算符重载

运算符重载可以为自定义数据类型重新定义运算符的行为。

4.5.1 加号运算符重载

示例代码：

class Person
{
public:Person() {};Person(int a, int b){this->m_A = a;this->m_B = b;}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;};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);Person p3 = p2 + p1;cout << "mA:" << p3.m_A << "mB:" << p3.m_B << endl;Person p4 = p3 + 10;cout << "mA:" << p4.m_A << "mB:" << p4.m_B << endl;}
int main()
{test();system("pause");return 0;
}

4.5.2 左移运算符重载

示例代码：

class Person {friend ostream& operator<<(ostream& out, Person& p);
public:Person(int a, int b) {this->m_A = a;this->m_B = b;}
private:int m_A;int m_B;
};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;
}int main() {test();system("pause");return 0;
}

4.5.3 递增运算符重载

示例代码：

class MyInteger {friend ostream& operator<<(ostream& out, MyInteger myint);
public:MyInteger() {m_Num = 0;}MyInteger& operator++() {m_Num++;return *this;}MyInteger operator++(int) {MyInteger temp = *this;m_Num++;return temp;}
private:int m_Num;
};ostream& operator<<(ostream& out, MyInteger myint) {out << myint.m_Num;return out;
}void test01() {MyInteger myInt;cout << ++myInt << endl;cout << myInt << endl;
}void test02() {MyInteger myInt;cout << myInt++ << endl;cout << myInt << endl;
}int main() {test01();test02();system("pause");return 0;
}

4.5.4 赋值运算符重载

示例代码：

class Person {
public:Person(int age) {m_Age = new int(age);}Person& operator=(Person &p) {if (m_Age !=NULL) {
delete m_Age;
m_Age = NULL;
}
m_Age = new int(*p.m_Age);
return *this;
}
~Person() {
if (m_Age != NULL) {
delete m_Age;
m_Age = NULL;
}
}
int *m_Age;
};
void test01() {
Person p1(18);
Person p2(20);
Person p3(30);
p3 = p2 = p1;
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();
system("pause");
return 0;
}

4.5.5 关系运算符重载

示例代码：

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() {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;
}

4.5.6 函数调用运算符重载

示例代码：

class MyPrint {
public:void operator()(string text) {cout << text << endl;}
};class MyAdd {
public:int operator()(int v1, int v2) {return v1 + v2;}
};void test01() {MyPrint myFunc;myFunc("hello world");
}void test02() {MyAdd add;int ret = add(10, 10);cout << "ret = " << ret << endl;cout << "MyAdd()(100,100) = " << MyAdd()(100, 100) << endl;
}int main() {test01();test02();system("pause");return 0;
}

5. 继承

继承是面向对象的三大特性之一，允许子类继承父类的成员。

5.1 继承的基本语法

示例代码：

class BasePage {
public:void header() {cout << "首页、公开课、登录、注册...（公共头部）" << endl;}void footer() {cout << "帮助中心、交流合作、站内地图...(公共底部)" << endl;}void left() {cout << "Java,Python,C++...(公共分类列表)" << endl;}
};class Java : public BasePage {
public:void content() {cout << "JAVA学科视频" << endl;}
};class Python : public BasePage {
public:void content() {cout << "Python学科视频" << endl;}
};class CPP : public BasePage {
public:void content() {cout << "C++学科视频" << endl;}
};void test01() {cout << "Java下载视频页面如下： " << endl;Java ja;ja.header();ja.footer();ja.left();ja.content();cout << "--------------------" << endl;cout << "Python下载视频页面如下： " << endl;Python py;py.header();py.footer();py.left();py.content();cout << "--------------------" << endl;cout << "C++下载视频页面如下： " << endl;CPP cp;cp.header();cp.footer();cp.left();cp.content();
}int main() {test01();system("pause");return 0;
}

5.2 继承方式

继承方式有三种：公共继承、保护继承和私有继承。
示例代码：

class Base1 {
public:int m_A;
protected:int m_B;
private:int m_C;
};class Son1 : public Base1 {
public:void func() {m_A; // 可访问 public 权限m_B; // 可访问 protected 权限// m_C; // 不可访问}
};class Base2 {
public:int m_A;
protected:int m_B;
private:int m_C;
};class Son2 : protected Base2 {
public:void func() {m_A; // 可访问 protected 权限m_B; // 可访问 protected 权限// m_C; // 不可访问}
};class Base3 {
public:int m_A;
protected:int m_B;
private:int m_C;
};class Son3 : private Base3 {
public:void func() {m_A; // 可访问 private 权限m_B; // 可访问 private 权限// m_C; // 不可访问}
};class GrandSon3 : public Son3 {
public:void func() {// Son3 是私有继承，所以继承 Son3 的属性在 GrandSon3 中都无法访问到// m_A;// m_B;// m_C;}
};

5.3 继承中的对象模型

父类中的私有成员也会被子类继承，但无法直接访问。
示例代码：

class Base {
public:int m_A;
protected:int m_B;
private:int m_C;
};class Son : public Base {
public:int m_D;
};void test01() {cout << "sizeof Son = " << sizeof(Son) << endl;
}int main() {test01();system("pause");return 0;
}

5.4 继承中构造和析构顺序
创建子类对象时，先调用父类的构造函数，再调用子类的构造函数。析构顺序相反。
示例代码：

class Base {
public:Base() {cout << "Base构造函数!" << endl;}~Base() {cout << "Base析构函数!" << endl;}
};class Son : public Base {
public:Son() {cout << "Son构造函数!" << endl;}~Son() {cout << "Son析构函数!" << endl;}
};void test01() {Son s;
}int main() {test01();system("pause");return 0;
}

5.5 继承同名成员处理方式
子类与父类出现同名成员时，可以通过作用域区分。
示例代码：

class Base {
public:void func() {cout << "Base - func()调用" << endl;}void func(int a) {cout << "Base - func(int a)调用" << endl;}int m_A;
};class Son : public Base {
public:void func() {cout << "Son - func()调用" << endl;}int m_A;
};void test01() {Son s;cout << "Son下的m_A = " << s.m_A << endl;cout << "Base下的m_A = " << s.Base::m_A << endl;s.func();s.Base::func();s.Base::func(10);
}int main() {test01();system("pause");return 0;
}

5.6 继承同名静态成员处理方式

静态成员同名处理方式与非静态成员相同。
示例代码：

class Base {
public:static void func() {cout << "Base - static void func()" << endl;}static void func(int a) {cout << "Base - static void func(int a)" << endl;}static int m_A;
};int Base::m_A = 100;class Son : public Base {
public:
static void func() {
cout << "Son - static void func()" << endl;
}
static int m_A;
};
int Son::m_A = 200;
void test01() {
cout << "通过对象访问： " << endl;
Son s;
cout << "Son 下 m_A = " << s.m_A << endl;
cout << "Base 下 m_A = " << s.Base::m_A << endl;
cout << "通过类名访问： " << endl;
cout << "Son 下 m_A = " << Son::m_A << endl;
cout << "Base 下 m_A = " << Son::Base::m_A << endl;
}
void test02() {
cout << "通过对象访问： " << endl;
Son s;
s.func();
s.Base::func();
cout << "通过类名访问： " << endl;
Son::func();
Son::Base::func();
Son::Base::func(100);
}
int main() {
test01();
test02();
system("pause");
return 0;
}

5.7 多继承语法

C++允许一个类继承多个类。

示例代码：

class Base1 {
public:Base1() {m_A = 100;}int m_A;
};class Base2 {
public:Base2() {m_A = 200;}int m_A;
};class Son : public Base2, public Base1 {
public:Son() {m_C = 300;m_D = 400;}int m_C;int m_D;
};void test01() {Son s;cout << "sizeof Son = " << sizeof(s) << endl;cout << s.Base1::m_A << endl;cout << s.Base2::m_A << endl;
}int main() {test01();system("pause");return 0;
}

5.8 菱形继承

菱形继承是指两个派生类继承同一个基类，而另一个类又同时继承这两个派生类。
示例代码：

class Animal {
public:int m_Age;
};class Sheep : virtual public Animal {};class Tuo : virtual public Animal {};class SheepTuo : public Sheep, public Tuo {};void test01() {SheepTuo st;st.Sheep::m_Age = 100;st.Tuo::m_Age = 200;cout << "st.Sheep::m_Age = " << st.Sheep::m_Age << endl;cout << "st.Tuo::m_Age = " << st.Tuo::m_Age << endl;cout << "st.m_Age = " << st.m_Age << endl;
}int main() {test01();system("pause");return 0;
}