C++：继承性

本文主要是介绍C++：继承性，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

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

1. 派生类只需要负责直接基类构造函数的调用

2. 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

3. 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

4. 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

1. 域解析符
2. 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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