? ? ? ? ?在<<反C++>>文中提到的 VC6.0 上的編譯器支持一個d1reportAllClassLayout的開關, 能夠輸出全部對象的內存布局信息,?我自己經常使用vs2005來開發，所以這里我就以vs2005為例講怎么設置d1reportAllClassLayout。

? ? ? ? 右鍵項目屬性(Properties)—》配制屬性(Configuration Properties)—》C/C++--》命令行（Command Line）的框里輸入/d1reportAllClassLayout，就可以看類的對象布局。例如以下圖：（當然也能夠使用命令行：cl –d1reportSingleClassLayout[classname] test.cpp）

測試代碼：

class CBase { public:virtual void fun(void) {}private:int m_valuable; }; 輸出結果：

class CBase size(8):+---0 | {vfptr}4 | m_valuable+--- CBase::$vftable@:| &CBase_meta| 00 | &CBase::fun CBase::fun this adjustor: 0 CDerived：非virtual繼承CBase，持有一個virtual重寫方法和一個virtual新方法。一個成員變量 class CDerived: public CBase { public:void fun(void) { }virtual void vfun(void) { }public:int m_derived; }; 編譯后輸出結果： class CDerived size(12):+---| +--- (base class CBase)0 | | {vfptr}4 | | m_valuable| +---8 | m_derived+--- CDerived::$vftable@:| &CDerived_meta| 00 | &CDerived::fun1 | &CDerived::vfun CDerived::fun this adjustor: 0 CDerived::vfun this adjustor: 0 當中vftable@CBase的CBase::fun項在這里更新為CDerived::fun，同一時候添加了一項CDerived::vfun。 CDerived2：virtual繼承CBase，持有一個virtual重寫方法和一個virtual新方法，一個成員變量。因此結構為vftable@自身，vbtable@自身，member@自身，以及CBase結構。 總結： class CDerived2: virtual public CBase { public:void fun(void) { }virtual void vfun(void) { }public:int m_derived; }; 結果輸出： class CDerived2 size(20):+---0 | {vfptr}4 | {vbptr}8 | m_derived+---+--- (virtual base CBase) 12 | {vfptr} 16 | m_valuable+--- CDerived2::$vftable@CDerived2@:| &CDerived2_meta| 00 | &CDerived2::vfun CDerived2::$vbtable@:0 | -41 | 8 (CDerived2d(CDerived2+4)CBase) CDerived2::$vftable@CBase@:| -120 | &CDerived2::fun CDerived2::fun this adjustor: 12 CDerived2::vfun this adjustor: 0 vbi: class offset o.vbptr o.vbte fVtorDispCBase 12 4 4 0 當中vftable@自身僅僅有一項：CDerived2::vfun()，vbtable@自身僅僅有一項：它virtual繼承的父類CBase。而vftable@CBase原來的CBase::fun更新為CDerived2::fun。CDerived3：virtual繼承CBase，因此結構為vbtable@自身，member@自身。CBase結構 class CDerived3: virtual public CBase { public:void fun(void) { }public:int m_derived3; }; 結果輸出： class CDerived3 size(16):+---0 | {vbptr}4 | m_derived3+---+--- (virtual base CBase)8 | {vfptr} 12 | m_valuable+--- CDerived3::$vbtable@:0 | 01 | 8 (CDerived3d(CDerived3+0)CBase) CDerived3::$vftable@:| -80 | &CDerived3::fun CDerived3::fun this adjustor: 8 vbi: class offset o.vbptr o.vbte fVtorDispCBase 8 0 4 0 CGDerived：繼承CDerived2、CDerived3 class CGDerived: public CDerived2, public CDerived3 { public:void vfun() { }virtual void vgfun() { }public:int m_gd; }; 輸出： class CGDerived size(32):+---| +--- (base class CDerived2)0 | | {vfptr}4 | | {vbptr}8 | | m_derived| +---| +--- (base class CDerived3) 12 | | {vbptr} 16 | | m_derived3| +--- 20 | m_gd+---+--- (virtual base CBase) 24 | {vfptr} 28 | m_valuable+--- CGDerived::$vftable@CDerived2@:| &CGDerived_meta| 00 | &CGDerived::vfun1 | &CGDerived::vgfun CGDerived::$vbtable@CDerived2@:0 | -41 | 20 (CGDerivedd(CDerived2+4)CBase) CGDerived::$vbtable@CDerived3@:0 | 01 | 12 (CGDerivedd(CDerived3+0)CBase) CGDerived::$vftable@CBase@:| -240 | &thunk: this-=12; goto CDerived2::fun CGDerived::vfun this adjustor: 0 CGDerived::vgfun this adjustor: 0 vbi: class offset o.vbptr o.vbte fVtorDispCBase 24 4 4 0 因此首先是CDerived2的結構和CDerived3的結構，自己的新virtual方法vgfun則加入在最左父類CDerived2的虛函數表中。然后是自己的成員。

最后。CDerived2和CDerived3的父類CBase結構也帶入當中。

它的fun默認指向CGDerived的最左父類CDerived2::fun。

CGG：繼承CGDerived class CGG: public CGDerived { public: int m_kc; }; 輸出： 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 添加一個變量，基本僅僅是把CGDerived的結構再套一層。最后加上自己的成員變量。

vbtable所指向的父類結構依舊在最后。 空類 class CBase2 { }; 輸出： class CBase2 size(1): +--- +--- CD2：virtual繼承則一定會創建vbtable，用vbptr指針指向，因此size為4 class CD2: virtual public CBase2 { }; 輸出： class CD2 size(4): +--- 0 | {vbptr} +--- +--- (virtual base CBase2) +--- CD2::$vbtable@: 0 | 0 1 | 4 (CD2d(CD2+0)CBase2) vbi: class offset o.vbptr o.vbte fVtorDisp CBase2 4 0 4 0 CE：非virtual繼承CD2和CDerived2。于是依照類的聲明順序，先帶入CDerived2的結構，再帶入CD2的結構（而不是按繼承順序） class CE: public CD2, public CDerived2 { }; 輸出： class CE size(24): +--- | +--- (base class CDerived2) 0 | | {vfptr} 4 | | {vbptr} 8 | | m_derived | +--- | +--- (base class CD2) 12 | | {vbptr} | +--- +--- +--- (virtual base CBase2) +--- +--- (virtual base CBase) 16 | {vfptr} 20 | m_valuable +--- CE::$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::$vftable@CBase@: | -16 0 | &thunk: this-=4; goto CDerived2::fun vbi: class offset o.vbptr o.vbte fVtorDisp CBase2 16 12 4 0 CBase 16 12 8 0 CF：virtual繼承CBase和CBase2，vbtable此時的項便有兩個。此時依照繼承的順序，而不是依照類的聲明順序 class CF: virtual public CBase2, virtual public CBase { }; 輸出： class CF size(12): +--- 0 | {vbptr} +--- +--- (virtual base CBase2) +--- +--- (virtual base CBase) 4 | {vfptr} 8 | m_valuable +--- CF::$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

總結：

?繼承方式：非virtual繼承：導入各個父類的結構（依照父類聲明的順序，從上到下），自身member在最后

? ? ? ? ?重寫virtual方法：更新該方法最早定義的類的vftable

? ? ? ??新的virtual方法：在最左父類的vftable添加

?繼承方式：有virtual繼承：在自身member后添加virtual父類的結構（依照子類繼承的順序從左到右）。同一時候在最前面添加vbtable（假設沒有的話），添加一項指向父類結構

? ? ? ? ?重寫virtual方法：更新該方法的最早定義的類的vftable

? ? ? ? ?新的virtual方法：在自身最前面添加vftable（假設沒有的話），在自己的vftable添加

附 全部源代碼：

#ifndef TEST_CBASE_H #define TEST_CBASE_Hclass 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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