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
#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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