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

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