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C++虚函数表分析

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         在<<逆向C++>>一文中提到 VC6.0 以上的编译器支持一个d1reportAllClassLayout的开关, 可以输出所有对象的内存布局信息, 我自己常用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,即可看类的对象布局,如下图:(当然也可以使用命令行:cl –d1reportSingleClassLayout[classname] test.cpp)

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测试代码:

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
其中vftable@CBase的CBase::fun项在这里更新为CDerived::fun,同时增加了一项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
其中vftable@自身只有一项:CDerived2::vfun(),vbtable@自身只有一项: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则添加在最左父类CDerived2的虚函数表中。然后是自己的成员。最后,CDerived2和CDerived3的父类CBase结构也带入其中。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
增加一个变量,基本只是把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然在最后。

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,于是按照类的声明顺序,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,此时按照继承的顺序,而不是按照类的声明顺序
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(按照父类声明的顺序,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增加

 继承方式:有virtual继承:在自身member后增加virtual父类的结构(按照子类继承的顺序从左到右),同时在最前面增加vbtable(如果没有的话),增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:在自身最前面增加vftable(如果没有的话),在自己的vftable增加


附 所有源码:

#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


C++虚函数表分析

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原文地址:http://blog.csdn.net/wenrenhua08/article/details/40043811

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