虚函数和虚继承

1、虚函数

#include <iostream>
using namespace std;

/* 定义基类CBase */
class CBase 
{
public:
    virtual void print() 
    {
        cout << "CBase print" << endl;
    }
};

/* 定义派生类CDerived */
class CDerived : public CBase 
{
public:
    /* 此处的override关键字显示声明子类覆盖基类print函数,
       基类函数名发生改变时，会有编译错误提醒检查 */
    void print() override  
    {
        cout << "CDerived print" << endl;
    }
};

int main(int argc, char* argv[]) 
{
    CBase* basePtr = new CDerived();
    basePtr->print();    
    delete basePtr;
    return 0;
}

xuanmiao@linux:~/Test/C++$ ./test38
CDerived print

 

2、虚继承

#include <iostream>
 
using namespace std;
 
class CAnimal
{
public:
    CAnimal()
    {
        cout << "CAnimal is constructed!"<< endl;
    }
    ~CAnimal()
    {
        cout << "CAnimal is deconstructed!"<< endl;
    }
    void move()
    {
        cout << " animal can move " << endl;
    }
};


class CBird : virtual public CAnimal //此处CBird虚继承CAnimal
{
public:
	void fly()
	{
		cout << "bird can fly" << endl;
	}
	void breath()
	{
		cout << "bird can breath" << endl;
	}
};
 
class CFish : virtual public CAnimal //此处CFish虚继承CAnimal
{
public:
	void swim()
	{
		cout << "fish can swim" << endl;
	}
	void breath()
	{
		cout << "fish can breath" << endl;
	}
};
 
class CWaterBird: public CBird, public CFish
{
public:
	void action()
	{
		cout << "waterbird can fly and swim"<< endl;
	}
};
 
 
int main(int argc, char *argv[])
{
	cout << endl <<"调用父类各自的成员函数：" << endl;	
	CWaterBird waterbird;
	waterbird.fly();
	waterbird.swim();
	waterbird.action();
 
	cout << endl <<"调用父类重复的成员函数：" << endl;
	waterbird.CBird::breath();  // 调用bird类的Breatha成员函数
	waterbird.CFish::breath();  // 调用fish类的Breatha成员函数
	cout << endl;
 
	return 0;	
}

xuanmiao@linux:~/Test/C++$ ./test39

调用父类各自的成员函数：
CAnimal is constructed!
bird can fly
fish can swim
waterbird can fly and swim

调用父类重复的成员函数：
bird can breath
fish can breath

CAnimal is deconstructed!

此处如果去掉 class CBird : virtual public CAnimal 和 class CFish : virtual public CAnimal 中的virtual关键字，那么CAnimal类中的构造函数和析构函数会被执行两遍

xuanmiao@linux:~/Test/C++$ ./test39

调用父类各自的成员函数：
CAnimal is constructed!
CAnimal is constructed!
bird can fly
fish can swim
waterbird can fly and swim

调用父类重复的成员函数：
bird can breath
fish can breath

CAnimal is deconstructed!
CAnimal is deconstructed!

 

3、虚函数表

（1）打印虚函数表

#include <iostream>
#include <stdio.h>
using namespace std;

class Base 
{
public:
	virtual void a() { cout << "Base a()" << endl; }
	virtual void b() { cout << "Base b()" << endl; }
	virtual void c() { cout << "Base c()" << endl; }
};

class Derive : public Base 
{
public:
	virtual void b() { cout << "Derive b()" << endl; }
};

int main(int argc, char* argv[])
{
	cout << "-----------Base------------" << endl;
	Base* q = new Base;
	long* tmp1 = (long*)q; 
	long* vptr1 = (long*)(*tmp1);
	for (int i = 0; i < 3; i++) 
    {
		printf("vptr[%d] : %p\n", i, (void*)vptr1[i]);
	}

    cout << endl << "---------Derive------------" << endl;
	Derive* p = new Derive;
	long* tmp = (long*)p;
	long* vptr = (long*)(*tmp);
	for (int i = 0; i < 3; i++) 
    {
		printf("vptr[%d] : %p\n", i, (void*)vptr[i]);
	}
	return 0;
}

xuanmiao@linux:~/Test/C++$ ./test43
-----------Base------------
vptr[0] : 0x561d7d7bf3b0
vptr[1] : 0x561d7d7bf3ec
vptr[2] : 0x561d7d7bf428

---------Derive------------
vptr[0] : 0x561d7d7bf3b0
vptr[1] : 0x561d7d7bf464
vptr[2] : 0x561d7d7bf428

 

（2）通过虚函数表中的指针调用函数

#include <iostream>
#include <stdio.h>
using namespace std;

class Base 
{
public:
	virtual void a() { cout << "Base a()" << endl; }
	virtual void b() { cout << "Base b()" << endl; }
	virtual void c() { cout << "Base c()" << endl; }
};

class Derive : public Base 
{
public:
	virtual void b() { cout << "Derive b()" << endl; }
};

int main(int argc, char* argv[])
{
	typedef void (*Func)();
	cout << "-----------Base------------" << endl;
	Base* q = new Base;
	long* tmp1 = (long*)q;
	long* vptr1 = (long*)(*tmp1);
	for (int i = 0; i < 3; i++) 
    {
		printf("vptr[%d] : %p\n", i, (void*)vptr1[i]);
	}
	Func a = (Func)vptr1[0];
	Func b = (Func)vptr1[1];
	Func c = (Func)vptr1[2];
	a();
	b();
	c();

	Derive* p = new Derive;
	long* tmp = (long*)p;
	long* vptr = (long*)(*tmp);
	cout << "---------Derive------------" << endl;
	for (int i = 0; i < 3; i++) 
    {
		printf("vptr[%d] : %p\n", i, (void*)vptr[i]);
	}
	Func d = (Func)vptr[0];
	Func e = (Func)vptr[1];
	Func f = (Func)vptr[2];
	d();
	e();
	f();

	
	return 0;
}

xuanmiao@linux:~/Test/C++$ ./test44
-----------Base------------
vptr[0] : 0x56451b1e740c
vptr[1] : 0x56451b1e7448
vptr[2] : 0x56451b1e7484
Base a()
Base b()
Base c()
---------Derive------------
vptr[0] : 0x56451b1e740c
vptr[1] : 0x56451b1e74c0
vptr[2] : 0x56451b1e7484
Base a()
Derive b()
Base c()

参考博客：https://cloud.tencent.com/developer/article/1599283