多重继承与虚继承对象布局

#include <iostream>

using namespace std;

class Base1

public:

    Base1(){ cout << "Base1::Base1()" << endl;}

    virtual ~Base1(){ cout << "Base1::~Base1()" << endl;}

    virtual void speakClearly(){ cout << "Base1::speakClearly()" << endl;}

    virtual Base1 *clone() const { cout << "Base1::clone()" << endl;}

protected:

    float data_Base1;

};

class Base2

public:

    Base2(){ cout << "Base2::Base2()" << endl;}

    virtual ~Base2(){ cout << "Base2::~Base2()" << endl;}

    virtual void mumble(){ cout << "Base2::mumble()" << endl;}

    virtual Base2 *clone() const{ cout << "Base2::clone()" << endl;}

protected:

    float data_Base2;

};

class Derived : public Base1, public Base2

public:

    Derived(){ cout << "Derived::Derived()" << endl;}

    virtual ~Derived(){ cout << "Derived::~Derived()" << endl;}

    virtual Derived *clone() const { cout << "Derived::clone()" << endl;}

protected:

    float data_Derived;

};

int main()

    Base1 *pBase1=new Derived;

    pBase1->clone();

    pBase1->speakClearly();

    //pBase1->mumble();//complie-time error

    delete pBase1;

    cout << "-----------------------------------" << endl;

    Base2 *pBase2=new Derived;

    pBase2->clone();

    pBase2->mumble();

    delete pBase2;

    system("PAUSE");

    return 0;

按照《Inside C++ Objecet Model》, 其内存布局应该为：

这里有3个问题：

1.第一个虚表中存有Base2的虚函数指针(图中红色椭圆框)，但是通过pBase1的指针多态却访问不到

2.我们之所有称第一个vtable为主表，它属于整个Derived类的虚表，所以它的slot含有所有的虚函数指针；但是它又被第一个基类所共享

3.后面的各个vtable成为次表，其中slot的虚函数指针只和其对应基类相关的。并且访问这些虚函数需要this指针调整

《Inside C++ Object Model》P123使用virtual table offset strategy实现的，下面我们来探讨一下：

1. 简单的虚继承

class Point2d

public:

    Point2d(float x=0.0, float=1.0);

    virtual ~Point2d();

    virtual void mumble();

    virtual float z();

    //...

protected:

    float _x, _y;

};

classs Point3d : public virtual Point2d

public:

    Point3d(float x=0.0, float y=0.0, float z=0.0);

    ~Point3d();

    virtual float z();

protected:

    float _z;

};

2. 多重继承情况下的虚继承：只有虚继承才会生成新虚指针，如果Vertex3D也是虚继承Vertex和Point3D，则Vertex3D也会产生一个vptr。

class Point2d

public:

    virual 2d_func(){}

protected:

    float _x, _y;

};

class Vertex : public virtual Point2d

public:

    virtual ver_func(){}

protected:

    Vertex *next;

};

class Point3d : public virtual Point2d

public:

    virual 3d_func(){}

protected:

    float _z;

};

class Vertex3d : public Vertex, public Point3d

public:

    virtual ver3d_func(){}

protected:

    float mumble;

原文链接: https://www.cnblogs.com/tracylee/archive/2012/12/13/2815846.html

欢迎关注

微信关注下方公众号，第一时间获取干货硬货；公众号内回复【pdf】免费获取数百本计算机经典书籍

原创文章受到原创版权保护。转载请注明出处：https://www.ccppcoding.com/archives/72381

非原创文章文中已经注明原地址，如有侵权，联系删除

关注公众号【高性能架构探索】，第一时间获取最新文章

转载文章受原作者版权保护。转载请注明原作者出处！

多重继承与虚继承 对象布局