C++虚函数表分析

在<<反C++>>文中提到的 VC6.0 上的编译器支持一个d1reportAllClassLayout的开关, 能够输出全部对象的内存布局信息, 我自己经常使用vs2005来开发，所以这里我就以vs2005为例讲怎么设置d1reportAllClassLayout。

右键项目属性(Properties)—》配制属性(Configuration Properties)—》C/C++--》命令行（Command Line）的框里输入/d1reportAllClassLayout，就可以看类的对象布局。例如以下图：（当然也能够使用命令行：cl –d1reportSingleClassLayout[classname] test.cpp）

測试代码：

class CBase

{

public:

        virtual void fun(void) {}

private:

        int m_valuable;

};

输出结果：

class CBase size(8):

 +---

 0 | {vfptr}

 4 | m_valuable

 +---

CBase::$vftable@:

 | &CBase_meta

 |  0

 0 | &CBase::fun

CBase::fun this adjustor: 0

CDerived：非virtual继承CBase，持有一个virtual重写方法和一个virtual新方法。一个成员变量

class CDerived: public CBase

{

public:

	void fun(void) { }

	virtual void vfun(void) { }

public:

	int m_derived;

};

编译后输出结果：

class CDerived size(12):

 +---

 | +--- (base class CBase)

 0 | | {vfptr}

 4 | | m_valuable

 | +---

 8 | m_derived

 +---

CDerived::$vftable@:

 | &CDerived_meta

 |  0

 0 | &CDerived::fun

 1 | &CDerived::vfun

CDerived::fun this adjustor: 0

CDerived::vfun this adjustor: 0

当中vftable@CBase的CBase::fun项在这里更新为CDerived::fun，同一时候添加了一项CDerived::vfun。

CDerived2：virtual继承CBase，持有一个virtual重写方法和一个virtual新方法，一个成员变量。因此结构为vftable@自身，vbtable@自身，member@自身，以及CBase结构。

总结：

class CDerived2: virtual public CBase

{

public:

	void fun(void) { }

	virtual void vfun(void) { }

public:

	int m_derived;

};

结果输出：

class CDerived2 size(20):

 +---

 0 | {vfptr}

 4 | {vbptr}

 8 | m_derived

 +---

 +--- (virtual base CBase)

12 | {vfptr}

16 | m_valuable

 +---

CDerived2::$vftable@CDerived2@:

 | &CDerived2_meta

 |  0

 0 | &CDerived2::vfun

CDerived2::$vbtable@:

 0 | -4

 1 | 8 (CDerived2d(CDerived2+4)CBase)

CDerived2::$vftable@CBase@:

 | -12

 0 | &CDerived2::fun

CDerived2::fun this adjustor: 12

CDerived2::vfun this adjustor: 0

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

           CBase      12       4       4 0

当中vftable@自身仅仅有一项：CDerived2::vfun()，vbtable@自身仅仅有一项：它virtual继承的父类CBase。而vftable@CBase原来的CBase::fun更新为CDerived2::fun。

CDerived3：virtual继承CBase，因此结构为vbtable@自身，member@自身。CBase结构

class CDerived3: virtual public CBase

{

public:

	void fun(void) { }

public:

	int m_derived3;

};

结果输出：

class CDerived3 size(16):

 +---

 0 | {vbptr}

 4 | m_derived3

 +---

 +--- (virtual base CBase)

 8 | {vfptr}

12 | m_valuable

 +---

CDerived3::$vbtable@:

 0 | 0

 1 | 8 (CDerived3d(CDerived3+0)CBase)

CDerived3::$vftable@:

 | -8

 0 | &CDerived3::fun

CDerived3::fun this adjustor: 8

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

           CBase       8       0       4 0

CGDerived：继承CDerived2、CDerived3

class CGDerived: public CDerived2, public CDerived3

{

public:

	void vfun() { }

	virtual void vgfun() { }

public:

	int m_gd;

};

输出：

class CGDerived size(32):

 +---

 | +--- (base class CDerived2)

 0 | | {vfptr}

 4 | | {vbptr}

 8 | | m_derived

 | +---

 | +--- (base class CDerived3)

12 | | {vbptr}

16 | | m_derived3

 | +---

20 | m_gd

 +---

 +--- (virtual base CBase)

24 | {vfptr}

28 | m_valuable

 +---

CGDerived::$vftable@CDerived2@:

 | &CGDerived_meta

 |  0

 0 | &CGDerived::vfun

 1 | &CGDerived::vgfun

CGDerived::$vbtable@CDerived2@:

 0 | -4

 1 | 20 (CGDerivedd(CDerived2+4)CBase)

CGDerived::$vbtable@CDerived3@:

 0 | 0

 1 | 12 (CGDerivedd(CDerived3+0)CBase)

CGDerived::$vftable@CBase@:

 | -24

 0 | &thunk: this-=12; goto CDerived2::fun

CGDerived::vfun this adjustor: 0

CGDerived::vgfun this adjustor: 0

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

           CBase      24       4       4 0

因此首先是CDerived2的结构和CDerived3的结构，自己的新virtual方法vgfun则加入在最左父类CDerived2的虚函数表中。然后是自己的成员。

最后。CDerived2和CDerived3的父类CBase结构也带入当中。

它的fun默认指向CGDerived的最左父类CDerived2::fun。

CGG：继承CGDerived

class CGG: public CGDerived

{

public:

	int m_kc;

};

输出：

class CGG size(36):

 +---

 | +--- (base class CGDerived)

 | | +--- (base class CDerived2)

 0 | | | {vfptr}

 4 | | | {vbptr}

 8 | | | m_derived

 | | +---

 | | +--- (base class CDerived3)

12 | | | {vbptr}

16 | | | m_derived3

 | | +---

20 | | m_gd

 | +---

24 | m_kc

 +---

 +--- (virtual base CBase)

28 | {vfptr}

32 | m_valuable

 +---

CGG::$vftable@CDerived2@:

 | &CGG_meta

 |  0

 0 | &CGDerived::vfun

 1 | &CGDerived::vgfun

CGG::$vbtable@CDerived2@:

 0 | -4

 1 | 24 (CGGd(CDerived2+4)CBase)

CGG::$vbtable@CDerived3@:

 0 | 0

 1 | 16 (CGGd(CDerived3+0)CBase)

CGG::$vftable@CBase@:

 | -28

 0 | &thunk: this-=16; goto CDerived2::fun

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

           CBase      28       4       4 0

添加一个变量，基本仅仅是把CGDerived的结构再套一层。最后加上自己的成员变量。

vbtable所指向的父类结构依旧在最后。

空类

class CBase2

{

};

输出：

class CBase2 size(1):

 +---

 +---

CD2：virtual继承则一定会创建vbtable，用vbptr指针指向，因此size为4

class CD2: virtual public CBase2

{

};

输出：

class CD2 size(4):

 +---

 0 | {vbptr}

 +---

 +--- (virtual base CBase2)

 +---

CD2::$vbtable@:

 0 | 0

 1 | 4 (CD2d(CD2+0)CBase2)

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

          CBase2       4       0       4 0

CE：非virtual继承CD2和CDerived2。于是依照类的声明顺序，先带入CDerived2的结构，再带入CD2的结构（而不是按继承顺序）

class CE: public CD2, public CDerived2

{

};

输出：

class CE size(24):

 +---

 | +--- (base class CDerived2)

 0 | | {vfptr}

 4 | | {vbptr}

 8 | | m_derived

 | +---

 | +--- (base class CD2)

12 | | {vbptr}

 | +---

 +---

 +--- (virtual base CBase2)

 +---

 +--- (virtual base CBase)

16 | {vfptr}

20 | m_valuable

 +---

CE::$vftable@CDerived2@:

 | &CE_meta

 |  0

 0 | &CDerived2::vfun

CE::$vbtable@CD2@:

 0 | 0

 1 | 4 (CEd(CD2+0)CBase2)

 2 | 4 (CEd(CE+12)CBase)

CE::$vbtable@CDerived2@:

 0 | -4

 1 | 12 (CEd(CDerived2+4)CBase)

CE::$vftable@CBase@:

 | -16

 0 | &thunk: this-=4; goto CDerived2::fun

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

          CBase2      16      12       4 0

           CBase      16      12       8 0

CF：virtual继承CBase和CBase2，vbtable此时的项便有两个。此时依照继承的顺序，而不是依照类的声明顺序

class CF: virtual public CBase2, virtual public CBase

{

};

输出：

class CF size(12):

 +---

 0 | {vbptr}

 +---

 +--- (virtual base CBase2)

 +---

 +--- (virtual base CBase)

 4 | {vfptr}

 8 | m_valuable

 +---

CF::$vbtable@:

 0 | 0

 1 | 4 (CFd(CF+0)CBase2)

 2 | 4 (CFd(CF+0)CBase)

CF::$vftable@:

 | -4

 0 | &CBase::fun

vbi:    class  offset o.vbptr  o.vbte fVtorDisp

          CBase2       4       0       4 0

           CBase       4       0       8 0

总结：

继承方式：非virtual继承：导入各个父类的结构（依照父类声明的顺序，从上到下），自身member在最后

重写virtual方法：更新该方法最早定义的类的vftable

新的virtual方法：在最左父类的vftable添加

继承方式：有virtual继承：在自身member后添加virtual父类的结构（依照子类继承的顺序从左到右）。同一时候在最前面添加vbtable（假设没有的话），添加一项指向父类结构

重写virtual方法：更新该方法的最早定义的类的vftable

新的virtual方法：在自身最前面添加vftable（假设没有的话），在自己的vftable添加

附全部源代码：

#ifndef TEST_CBASE_H

#define TEST_CBASE_H

class CBase

{

public:

	CBase();

	explicit CBase(int valuabel);

	~CBase();

	virtual void fun(void) {}

private:

	int m_valuable;

};

class CDerived: public CBase

{

public:

	void fun(void) { }

	virtual void vfun(void) { }

public:

	int m_derived;

};

class CDerived2: virtual public CBase

{

public:

	void fun(void) { }

	virtual void vfun(void) { }

public:

	int m_derived;

};

class CDerived3: virtual public CBase

{

public:

	void fun(void) { }

public:

	int m_derived3;

};

class CGDerived: public CDerived2, public CDerived3

{

public:

	void vfun() { }

	virtual void vgfun() { }

public:

	int m_gd;

};

class CGG: public CGDerived

{

public:

	int m_kc;

};

class CBase2

{

};

class CD2: virtual public CBase2

{

};

class CE: public CD2, public CDerived2

{

};

class CF: virtual public CBase2, virtual public CBase

{

};

#endif//TEST_CBASE_H

巴特西

C++虚函数表分析

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