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一、基本概念
- 一个基类可以派生多个派生类，一个派生类也可以由多个基类派生而成
- 继承
- 单一继承
- 多重继承
- 继承方式（缺省默认：private）
- public
- private
- protected
| 公有继承 | 保护继承 | 私有继承 | |
| 公有成员 | public | protected | private |
| 保护成员 | protected | protected | private |
| 私有成员 | 不可见 | 不可见 | 不可见 |
- 基类的 private 成员不可以被继承
二、派生类的构造及析构
#include <iostream>
using namespace std;class a1
{
public:a1(){cout << "a1 Constructor called" << endl;}~a1(){cout << "a1 Destructor called" << endl;}
};class a2
{
public:a2(){cout << "a2 Constructor called" << endl;}~a2(){cout << "a2 Destructor called" << endl;}
};class Derived : public a1, public a2
{
public:Derived(){cout << "Derived Constructor called" << endl;}~Derived(){cout << "Derived Destructor called" << endl;}
};int main()
{Derived obj;return 0;
}输出结果：
a1 Constructor called
a2 Constructor called
Derived Constructor called
Derived Destructor called
a2 Destructor called
a1 Destructor called
在定义一个派生类对象时，构造函数的调用顺序：
基类 >>> 派生类对象成员（按定义顺序） >>> 派生类
析构函数调用顺序恰好相反
//将 Derived 修改如下
class Derived : public a2
{
private:a1 obj1;
public:Derived(){cout << "Derived Constructor called" << endl;}~Derived(){cout << "Derived Destructor called" << endl;}
};a2 Constructor called
a1 Constructor called
Derived Constructor called
Derived Destructor called
a1 Destructor called
a2 Destructor called
有参情况
-
派生类只需要负责直接基类构造函数的调用
-
如果基类构造函数不需要提供参数，则无需在初始化列表中给出
-
创建对象构造函数的调用顺序与声明顺序有关，而非在初始化列表中的顺序
-
其他初始化项包括对象成员，常成员和引用成员
示例
#include <iostream>
using namespace std;class Base
{
private:static int count;int x;
public:Base(int i){x=i;cout<<"Base constructor called"<<count++<<endl;}void display(){cout<<"x = "<<x<<endl;}
};class Derived : public Base
{
private:Base b;
public:Derived (int i): Base(i),b(i){cout<<"Derived constructor called"<<endl;}
};int Base::count=0;int main()
{Derived d(3);d.display();return 0;
}多重继承示例
#include <iostream>
using namespace std;class Grand
{
private:int a;
public:Grand(int n):a(n){cout << "Grand c,a=" << a << endl;}~Grand(){cout << "Grand d" << endl;}
};class Father:public Grand
{
private:int b;
public:Father(int n1,int n2):Grand(n1),b(n2){cout << "Father c,b=" << b << endl;}~Father(){cout << "Father d" << endl;}
};class Mother
{
private:int c;
public:Mother(int n):c(n){cout << "Mother c,c=" << c << endl;}~Mother(){cout << "Mother d" << endl;}
};class Child:public Father,public Mother
{
private:int d;
public:Child(int n1,int n2,int n3,int n4):Father(n4,n3),Mother(n2),d(n1){cout << "Child d=" << d << endl;}~Child(){cout << "Child d" << endl;}
};int main()
{Child c(1,2,3,4);return 0;
}Grand c,a=4
Father c,b=3
Mother c,c=2
Child d=1
Child d
Mother d
Father d
Grand d
三、同名冲突
基类与派生类的同名冲突
同名覆盖原则：新成员名称与基类某个成员同名时，若未加任何特殊标识，访问派生类中新定义的同名成员
需要访问基类：使用 “基类名::” 进行限定
- 通过派生类的指针或引用，访问的是派生类的同名成员（同名覆盖√）
- 基类指针/引用，访问基类同名成员
#include <iostream>
using namespace std;class Base
{
public:int a;Base(int x){a = x;}void Print(){cout << "Base::a = " << a << endl;}
};class Derived : public Base
{
public:int a; //欸这里也有个a耶Derived(int x, int y) : Base(x){a = y;Base::a *= 2;}void Print(){Base::Print();cout << "Derived::a = " << a << endl;}
};void Test1(Base& b)
{b.Print();
}void Test2(Derived& d)
{d.Print();
}int main()
{Derived d(200, 300);d.Print();d.a = 400;d.Base::a = 500;d.Base::Print();Base* pb;pb = &d;pb->Print();Test1(d);Derived *pd;pd = &d;pd->Print();Test2(d);return 0;
}多重继承中直接基类的同名冲突
通过域解析符解决
#include <iostream>
using namespace std;class Base1
{
protected:int a;Base1(int x){a = x;cout<<"Base1 a="<<a<<endl;}void Print(){cout << "Base::a = " << a << endl;}
};class Base2
{
protected:int a;
public:Base2(int x){a = x;cout<<"Base2 a="<<a<<endl;}
};class Derived:public Base1, public Base2
{
public:Derived(int x,int y):Base1(x),Base2(y){Base1::a *=2;Base2::a *=2;cout<<"Derived from Base1::a="<<Base1::a<<endl;cout<<"Derived from Base2::a="<<Base2::a<<endl;}
};int main()
{Derived d(10,20);return 0;
}共同祖先基类引发的同名冲突
- 域解析符
- 虚基类
虚基类
virtual 确保虚基类最多被调用一次
#include <iostream>
using namespace std;class Base
{
protected:int a;
public:Base (int x):a(x){cout<<"Base a="<<a<<endl;}~Base (){cout<<"Base destructor"<<endl;}
};class Base1 : public virtual Base
{
protected:int b;
public:Base1(int x,int y):Base(y),b(x){cout<<"Base1 from Base a="<<a<<endl;cout<<"Base1 b="<<b<<endl;}
};class Base2 : public virtual Base
{
protected:int c;
public:Base2(int x,int y):Base(y),c(x){cout<<"Base2 from Base a="<<a<<endl;cout<<"Base2 c="<<c<<endl;}
};class Derived : public Base1, public Base2
{
public:Derived(int x,int y):Base1(x,y),Base2(2*x,2*y),Base(3*x){cout<<"a="<<a<<endl;cout<<"Base::a="<<Base::a<<endl;cout<<"Base1::a="<<Base1::a<<endl;cout<<"Base2::a="<<Base2::a<<endl;cout<<"b="<<b<<endl;cout<<"c="<<c<<endl;}~Derived (){cout<<"Derived destructor"<<endl;}
};int main()
{Derived d(10,20);return 0;
}其中 Base 类只有一份复制
只有最后一层派生类对虚基类构造函数的调用发挥作用
创建一个对象，构造函数调用次序：
虚基类的构造函数
直接基类的构造函数
对象成员的构造函数
派生类自己的构造函数
四、赋值兼容规则
使公有派生类可以当作基类来使用
1. 派生类对象 -> 基类对象
2. 派生类对象地址 -> 基类指针
3. 派生类对象指针 -> 基类指针
3.派生类对象 -> 基类引用
#include <iostream>
using namespace std;class Base
{
private:int b;
public:Base(int x):b(x){}int getB(){return b;}
};class Derived : public Base
{
private:int d;
public:Derived(int x, int y):Base(x), d(y){}int getD(){return d;}
};int main()
{Base b(11);Derived d(22, 33);b = d;cout << "b.getB() = " << b.getB() << endl;Base *bp = &d;cout << "bp->getB() = " << bp->getB() << endl;Derived *dp = &d;Base *bp2 = dp;cout << "bp2->getB() = " << bp2->getB() << endl;Base &rb = d;cout << "rb.getB() = " << rb.getB() << endl;return 0;
}
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