c++虚函数继承与虚继承（五）





虚函数继承和虚继承是完全不同的两个概念。

虚函数继承是解决多态性的，当用基类指针指向派生类对象的时候，基类指针调用虚函数的时候会自动调用派生类的虚函数，这就是多态性，也叫动态编联。

虚继承就是为了节约内存，他是多重继承中的特有的概念。适用于菱形继承形式。

#include<iostream>

using namespace std;

class Shape                   //抽象类,不能创建对象
{
protected:
	int m_a;
	int m_b;
	
public:
	virtual double GetS()=0;//纯虚函数
	Shape(int a,int b=0);
};

Shape::Shape(int a,int b)
{
	m_a = a;
	m_b = b;
}


class Square : public Shape
{

public:
	Square(int a,int b);
	double GetS();
};

Square::Square(int a,int b):Shape(a,b)
{
	
}

double Square:: GetS()
{
	return  m_a*m_b;
}

class Circle : public Shape
{

public:
	Circle(int a);
	double GetS();
	
};

Circle::Circle(int a) : Shape(a)
{
	
}

double Circle::GetS()
{
	return 3.14*m_a*m_a;
}

int main()
{
	Square s(2,3);
	cout<<"矩形的面积"<<s.GetS()<<endl;
	
	Circle c(2);
	cout<<"圆的面积"<<c.GetS()<<endl;
	
	Shape *p = new Square(3,3);
	cout<<"square: "<<p->GetS()<<endl;
	
	p = new Circle(4);
	cout<<"circle: "<<p->GetS()<<endl;
	
	
	
	return 0;
}

答案

矩形的面积6
圆的面积12.56
square: 9
circle: 50.24

GetS()为虚函数，可以使基类指针调用派生类函数，根据对象类型决定调用的函数

#include<iostream>
#include<cstring>

using namespace std;

class Animal
{
public:

	int m_age;
public:
	Animal(int a);
	~Animal();
	
};

Animal::Animal(int a)
{
	cout<<"animal constructor"<<endl;
	m_age = a;
}
Animal::~Animal()
{
	cout<<"animal dextructor"<<endl;
}

class Bird: virtual public Animal
{
public:
	int m_height;	
public:
	Bird(int h,int a);
	~Bird();
};
Bird::Bird(int h,int a):Animal(a)
{
	cout<<"bird constructor"<<endl;
	m_height = h;
}
Bird::~Bird()
{
	cout<<"bird destructor"<<endl;
}


class Fish: virtual public Animal//虚继承
{
public:
	int m_swim;	
public:
	Fish(int s,int a);
	~Fish();
};

Fish::Fish(int s,int a):Animal(a)
{
	cout<<"fish constructor"<<endl;
	m_swim = s;
}
Fish::~Fish()
{
	cout<<"fish destructor"<<endl;
}

class Waterbird : public Bird,public Fish
{
public:
	char *m_name;
	
public:
	Waterbird(int h,int s,char *n,int a);
	~Waterbird();
	void show();
};

Waterbird::Waterbird(int h,int s,char *n,int a):Bird(h,a),Fish(s,a),Animal(a)//
{
	cout<<"waterbird constructor"<<endl;
	m_name = new char[10];
	strcpy(m_name,n);
}

void Waterbird::show()
{
	//cout<<" 高度 "<<m_height<<" 速度 "<<m_swim<<" 年龄 "<<Fish::m_age<<" 名字 "<<m_name<<endl;
	cout<<" 高度 "<<m_height<<" 速度 "<<m_swim<<" 年龄 "<<m_age<<" 名字 "<<m_name<<endl;//虚继承解决二义性问题
}

Waterbird::~Waterbird()
{
	cout<<"waterbird destructor"<<endl;
	if(m_name!=NULL)
	{
		delete [] m_name;
	}
}
int main()
{
	Waterbird b(12,22,"bird",111);
	b.show();
	cout<<"&b   "<<&b<<endl;
	cout<<"&b.m_height  "<<&b.m_height<<endl;
	cout<<"&b.m_swim    "<<&b.m_swim<<endl;
	cout<<"&b.m_age     "<<&b.m_age<<endl;//地址顺序：虚基类指针 m_height 虚基类指针 m_swim  m_name  m_age
	
	Bird *pb = &b;
	Fish *pf = &b;
	Animal *pa = &b;
	Waterbird *pw = &b;
	
	//pb.m_height = 1;
	//pb.m_age = 1;
	
	cout<<"pb: "<<pb<<endl;
	cout<<"pf: "<<pf<<endl;
	cout<<"pa: "<<pa<<endl;
	cout<<"pw: "<<pw<<endl;
	
	cout<<"pb+1: "<<pb+1<<endl;
	cout<<"pf+1: "<<pf+1<<endl;
	cout<<"pa+1: "<<pa+1<<endl;
	cout<<"pw+1: "<<pw+1<<endl;
	
	return 0;
}

animal constructor
bird constructor
fish constructor
waterbird constructor
 高度 12 速度 22 年龄 111 名字 bird
&b   0xbfe88a48
&b.m_height  0xbfe88a4c
&b.m_swim    0xbfe88a54
&b.m_age     0xbfe88a5c
pb: 0xbfe88a48
pf: 0xbfe88a50
pa: 0xbfe88a5c
pw: 0xbfe88a48
pb+1: 0xbfe88a54
pf+1: 0xbfe88a5c
pa+1: 0xbfe88a60
pw+1: 0xbfe88a60
waterbird destructor
fish destructor
bird destructor
animal dextructor