C++父类子类中虚函数的使用

构造函数不能是虚函数，因为在调用构造函数创建对象时，构造函数必须是确定的，所以构造函数不能是虚函数。

析构函数可以是虚函数。

1.父类Father.h：

#pragma once
class Father
{
public:
	Father(void);
	virtual ~Father(void);

	virtual int getCount();

public:
	int count;
};

Father.cpp

#include "StdAfx.h"
#include "Father.h"
#include <iostream>

using namespace std;

Father::Father(void)
{
	count = 1;
	cout<<"Father is called. count = "<<count<<endl;
}


Father::~Father(void)
{
	cout<<"~Father is called."<<endl;
}


int Father::getCount()
{
	cout<<"Father::getCount() count = "<<count<<endl;
	return count;
}

2.子类Child.h：

#pragma once
#include "father.h"
class Child :
	public Father
{
public:
	Child(void);
	virtual ~Child(void);

	virtual int getCount();
	int getAge();

public:
	int age;
};

Child.cpp

#include "StdAfx.h"
#include "Child.h"
#include <iostream>

using namespace std;

Child::Child(void)
{
	count = 2;
	age = 20;
	cout<<"Child is called. count = "<<count<<", age = "<<age<<endl;
}


Child::~Child(void)
{
	cout<<"~Child is called."<<endl;
}

int Child::getCount()
{
	cout<<"Child::getCount() count = "<<count<<endl;
	return count;
}

int Child::getAge()
{
	cout<<"Child::getAge() age = "<<age<<endl;
	return age;
}

3.测试类Test.cpp

#include "stdafx.h"
#include  <cstdlib>
#include <iostream>
#include "Child.h"

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
	Father *father1 = new Father();
	cout<<"father1 count = "<<father1->getCount()<<endl;
	delete father1;
	cout<<"************** father1 end *****************"<<endl<<endl;

	Father *father2 = new Child();
	cout<<"father2 count = "<<father2->getCount()<<endl; // father2 don't have getAge() method
	delete father2;
	cout<<"************** father2 end *****************"<<endl<<endl;

	Child *child = new Child();
	cout<<"child count = "<<child->getCount()<<endl;
	cout<<"child age = "<<child->getAge()<<endl;
	delete child;
	cout<<"************** child end *****************"<<endl<<endl;

	getchar();

	return 0;
}

4.输出结果：

Father is called. count = 1
Father::getCount() count = 1
father1 count = 1
~Father is called.
************** father1 end *****************

Father is called. count = 1
Child is called. count = 2, age = 20
Child::getCount() count = 2
father2 count = 2
~Child is called.
~Father is called.
************** father2 end *****************

Father is called. count = 1
Child is called. count = 2, age = 20
Child::getCount() count = 2
child count = 2
Child::getAge() age = 20
child age = 20
~Child is called.
~Father is called.
************** child end *****************