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;
}
