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class A { virtual void aa(){}; }; class B : public virtual A { char j[3]; //加入一个变量是为了看清楚class中的vfptr放在什么位置 public: virtual void bb(){}; }; class C : public virtual A { char i[3]; public: virtual void cc(){}; }; class C1 : public A { char i1[3]; public: virtual void cc1(){}; }; class D : public C { char d[3]; public: virtual void dd(){}; }; class D1 : public C1 { char d1[3]; public: virtual void dd1(){}; }; class E : public virtual C { char e[3]; public: virtual void ee(){}; }; class E1 : public virtual C1 { char e1[3]; public: virtual void ee1(){}; }; class F : public C, public virtual B { char f[3]; public: virtual void ff(){}; }; class F1 : public virtual C, public B { char f1[3]; public: virtual void ff1(){}; }; class G : public virtual E { char g[3]; public: virtual void gg(){}; }; class H : public virtual E, public virtual C { char h[3]; public: virtual void hh(){}; }; class H1 : public E, public C1 { char h1[3]; public: virtual void hh1(){}; };
在VS2013下，在项目->属性->C/C++->命令行中添加/d1reportAllClassLayout即可查看所有类的内存分布情况:
1> class A size(4): 1> +--- 1> 0 | {vfptr} 1> +--- 1> 1> A::$vftable@: 1> | &A_meta 1> | 0 1> 0 | &A::aa 1> 1> A::aa this adjustor: 0 1> 1> 1> class B size(16): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | j 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 12 | {vfptr} 1> +--- 1> 1> B::$vftable@B@: 1> | &B_meta 1> | 0 1> 0 | &B::bb 1> 1> B::$vbtable@: 1> 0 | -4 1> 1 | 8 (Bd(B+4)A) 1> 1> B::$vftable@A@: 1> | -12 1> 0 | &A::aa 1> 1> B::bb this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 12 4 4 0 1> 1> 1> class C size(16): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | i 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 12 | {vfptr} 1> +--- 1> 1> C::$vftable@C@: 1> | &C_meta 1> | 0 1> 0 | &C::cc 1> 1> C::$vbtable@: 1> 0 | -4 1> 1 | 8 (Cd(C+4)A) 1> 1> C::$vftable@A@: 1> | -12 1> 0 | &A::aa 1> 1> C::cc this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 12 4 4 0 1> 1> 1> class C1 size(8): 1> +--- 1> | +--- (base class A) 1> 0 | | {vfptr} 1> | +--- 1> 4 | i1 1> | <alignment member> (size=1) 1> +--- 1> 1> C1::$vftable@: 1> | &C1_meta 1> | 0 1> 0 | &A::aa 1> 1 | &C1::cc1 1> 1> C1::cc1 this adjustor: 0 1> 1> 1> class D size(20): 1> +--- 1> | +--- (base class C) 1> 0 | | {vfptr} 1> 4 | | {vbptr} 1> 8 | | i 1> | | <alignment member> (size=1) 1> | +--- 1> 12 | d 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 16 | {vfptr} 1> +--- 1> 1> D::$vftable@C@: 1> | &D_meta 1> | 0 1> 0 | &C::cc 1> 1 | &D::dd 1> 1> D::$vbtable@: 1> 0 | -4 1> 1 | 12 (Dd(C+4)A) 1> 1> D::$vftable@A@: 1> | -16 1> 0 | &A::aa 1> 1> D::dd this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 16 4 4 0 1> 1> 1> class D1 size(12): 1> +--- 1> | +--- (base class C1) 1> | | +--- (base class A) 1> 0 | | | {vfptr} 1> | | +--- 1> 4 | | i1 1> | | <alignment member> (size=1) 1> | +--- 1> 8 | d1 1> | <alignment member> (size=1) 1> +--- 1> 1> D1::$vftable@: 1> | &D1_meta 1> | 0 1> 0 | &A::aa 1> 1 | &C1::cc1 1> 2 | &D1::dd1 1> 1> D1::dd1 this adjustor: 0 1> 1> 1> class E size(28): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | e 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 12 | {vfptr} 1> +--- 1> +--- (virtual base C) 1> 16 | {vfptr} 1> 20 | {vbptr} 1> 24 | i 1> | <alignment member> (size=1) 1> +--- 1> 1> E::$vftable@: 1> | &E_meta 1> | 0 1> 0 | &E::ee 1> 1> E::$vbtable@E@: 1> 0 | -4 1> 1 | 8 (Ed(E+4)A) 1> 2 | 12 (Ed(E+4)C) 1> 1> E::$vftable@A@: 1> | -12 1> 0 | &A::aa 1> 1> E::$vftable@C@: 1> | -16 1> 0 | &C::cc 1> 1> E::$vbtable@C@: 1> 0 | -4 1> 1 | -8 (Ed(C+4)A) 1> 1> E::ee this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 12 4 4 0 1> C 16 4 8 0 1> 1> 1> class E1 size(20): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | e1 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base C1) 1> | +--- (base class A) 1> 12 | | {vfptr} 1> | +--- 1> 16 | i1 1> | <alignment member> (size=1) 1> +--- 1> 1> E1::$vftable@E1@: 1> | &E1_meta 1> | 0 1> 0 | &E1::ee1 1> 1> E1::$vbtable@: 1> 0 | -4 1> 1 | 8 (E1d(E1+4)C1) 1> 1> E1::$vftable@C1@: 1> | -12 1> 0 | &A::aa 1> 1 | &C1::cc1 1> 1> E1::ee1 this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> C1 12 4 4 0 1> 1> 1> class F size(32): 1> +--- 1> | +--- (base class C) 1> 0 | | {vfptr} 1> 4 | | {vbptr} 1> 8 | | i 1> | | <alignment member> (size=1) 1> | +--- 1> 12 | f 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 16 | {vfptr} 1> +--- 1> +--- (virtual base B) 1> 20 | {vfptr} 1> 24 | {vbptr} 1> 28 | j 1> | <alignment member> (size=1) 1> +--- 1> 1> F::$vftable@C@: 1> | &F_meta 1> | 0 1> 0 | &C::cc 1> 1 | &F::ff 1> 1> F::$vbtable@C@: 1> 0 | -4 1> 1 | 12 (Fd(C+4)A) 1> 2 | 16 (Fd(F+4)B) 1> 1> F::$vftable@A@: 1> | -16 1> 0 | &A::aa 1> 1> F::$vftable@B@: 1> | -20 1> 0 | &B::bb 1> 1> F::$vbtable@B@: 1> 0 | -4 1> 1 | -8 (Fd(B+4)A) 1> 1> F::ff this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 16 4 4 0 1> B 20 4 8 0 1> 1> 1> class F1 size(32): 1> +--- 1> | +--- (base class B) 1> 0 | | {vfptr} 1> 4 | | {vbptr} 1> 8 | | j 1> | | <alignment member> (size=1) 1> | +--- 1> 12 | f1 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 16 | {vfptr} 1> +--- 1> +--- (virtual base C) 1> 20 | {vfptr} 1> 24 | {vbptr} 1> 28 | i 1> | <alignment member> (size=1) 1> +--- 1> 1> F1::$vftable@B@: 1> | &F1_meta 1> | 0 1> 0 | &B::bb 1> 1 | &F1::ff1 1> 1> F1::$vbtable@B@: 1> 0 | -4 1> 1 | 12 (F1d(B+4)A) 1> 2 | 16 (F1d(F1+4)C) 1> 1> F1::$vftable@A@: 1> | -16 1> 0 | &A::aa 1> 1> F1::$vftable@C@: 1> | -20 1> 0 | &C::cc 1> 1> F1::$vbtable@C@: 1> 0 | -4 1> 1 | -8 (F1d(C+4)A) 1> 1> F1::ff1 this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 16 4 4 0 1> C 20 4 8 0 1> 1> 1> class G size(40): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | g 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 12 | {vfptr} 1> +--- 1> +--- (virtual base C) 1> 16 | {vfptr} 1> 20 | {vbptr} 1> 24 | i 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base E) 1> 28 | {vfptr} 1> 32 | {vbptr} 1> 36 | e 1> | <alignment member> (size=1) 1> +--- 1> 1> G::$vftable@G@: 1> | &G_meta 1> | 0 1> 0 | &G::gg 1> 1> G::$vbtable@: 1> 0 | -4 1> 1 | 8 (Gd(G+4)A) 1> 2 | 12 (Gd(G+4)C) 1> 3 | 24 (Gd(G+4)E) 1> 1> G::$vftable@A@: 1> | -12 1> 0 | &A::aa 1> 1> G::$vftable@C@: 1> | -16 1> 0 | &C::cc 1> 1> G::$vbtable@C@: 1> 0 | -4 1> 1 | -8 (Gd(C+4)A) 1> 1> G::$vftable@E@: 1> | -28 1> 0 | &E::ee 1> 1> G::$vbtable@E@: 1> 0 | -4 1> 1 | -20 (Gd(E+4)A) 1> 2 | -16 (Gd(E+4)C) 1> 1> G::gg this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 12 4 4 0 1> C 16 4 8 0 1> E 28 4 12 0 1> 1> 1> class H size(40): 1> +--- 1> 0 | {vfptr} 1> 4 | {vbptr} 1> 8 | h 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 12 | {vfptr} 1> +--- 1> +--- (virtual base C) 1> 16 | {vfptr} 1> 20 | {vbptr} 1> 24 | i 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base E) 1> 28 | {vfptr} 1> 32 | {vbptr} 1> 36 | e 1> | <alignment member> (size=1) 1> +--- 1> 1> H::$vftable@H@: 1> | &H_meta 1> | 0 1> 0 | &H::hh 1> 1> H::$vbtable@: 1> 0 | -4 1> 1 | 8 (Hd(H+4)A) 1> 2 | 12 (Hd(H+4)C) 1> 3 | 24 (Hd(H+4)E) 1> 1> H::$vftable@A@: 1> | -12 1> 0 | &A::aa 1> 1> H::$vftable@C@: 1> | -16 1> 0 | &C::cc 1> 1> H::$vbtable@C@: 1> 0 | -4 1> 1 | -8 (Hd(C+4)A) 1> 1> H::$vftable@E@: 1> | -28 1> 0 | &E::ee 1> 1> H::$vbtable@E@: 1> 0 | -4 1> 1 | -20 (Hd(E+4)A) 1> 2 | -16 (Hd(E+4)C) 1> 1> H::hh this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 12 4 4 0 1> C 16 4 8 0 1> E 28 4 12 0 1> 1> 1> class H1 size(40): 1> +--- 1> | +--- (base class E) 1> 0 | | {vfptr} 1> 4 | | {vbptr} 1> 8 | | e 1> | | <alignment member> (size=1) 1> | +--- 1> | +--- (base class C1) 1> | | +--- (base class A) 1> 12 | | | {vfptr} 1> | | +--- 1> 16 | | i1 1> | | <alignment member> (size=1) 1> | +--- 1> 20 | h1 1> | <alignment member> (size=1) 1> +--- 1> +--- (virtual base A) 1> 24 | {vfptr} 1> +--- 1> +--- (virtual base C) 1> 28 | {vfptr} 1> 32 | {vbptr} 1> 36 | i 1> | <alignment member> (size=1) 1> +--- 1> 1> H1::$vftable@E@: 1> | &H1_meta 1> | 0 1> 0 | &E::ee 1> 1 | &H1::hh1 1> 1> H1::$vftable@C1@: 1> | -12 1> 0 | &A::aa 1> 1 | &C1::cc1 1> 1> H1::$vbtable@E@: 1> 0 | -4 1> 1 | 20 (H1d(E+4)A) 1> 2 | 24 (H1d(E+4)C) 1> 1> H1::$vftable@A@: 1> | -24 1> 0 | &A::aa 1> 1> H1::$vftable@C@: 1> | -28 1> 0 | &C::cc 1> 1> H1::$vbtable@C@: 1> 0 | -4 1> 1 | -8 (H1d(C+4)A) 1> 1> H1::hh1 this adjustor: 0 1> 1> vbi: class offset o.vbptr o.vbte fVtorDisp 1> A 24 4 4 0 1> C 28 4 8 0
总结出单继承内存分布大致如下：
多继承的内存分布情况如下：
总之，先满足基本的父子类分布情况，如果父类还有更深层次的基类，这些基类再依据普通继承和虚继承的情况进行内存分布（即红字标注部分）。另外，具体内存分布情况会不会还与编译环境有关就不得而知了，至少sizeof类的大小是会与编译环境有关的（http://www.cnblogs.com/yanqi0124/p/3829964.html文章最后对比了gcc和VC下sizeof的不同，因为对虚表指针的处理方式不同）
根据上述解释，就能解释程序员面试宝典中的一题：
class A { virtual aa(){}; }; class B : public virtual A { char j[3]; //加入一个变量是为了看清楚class中的vfptr放在什么位置 public: virtual bb(){}; }; class C : public virtual B { char i[3]; public: virtual cc(){}; };
C++ 继承之虚继承与普通继承的内存分布,布布扣,bubuko.com
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原文地址：http://www.cnblogs.com/yanqi0124/p/3830433.html
