仅供互相学习，请勿喷，有观点欢迎指出~
class A
{
virtual void aa(){};
};
class B : public virtual A
{
char j[]; //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[];
public:
virtual void cc(){};
};
class C1 : public A
{
char i1[];
public:
virtual void cc1(){};
};
class D : public C
{
char d[];
public:
virtual void dd(){};
};
class D1 : public C1
{
char d1[];
public:
virtual void dd1(){};
};
class E : public virtual C
{
char e[];
public:
virtual void ee(){};
};
class E1 : public virtual C1
{
char e1[];
public:
virtual void ee1(){};
};
class F : public C, public virtual B
{
char f[];
public:
virtual void ff(){};
};
class F1 : public virtual C, public B
{
char f1[];
public:
virtual void ff1(){};
};
class G : public virtual E
{
char g[];
public:
virtual void gg(){};
};
class H : public virtual E, public virtual C
{
char h[];
public:
virtual void hh(){};
};
class H1 : public E, public C1
{
char h1[];
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:
> class A size():
> +---
> | {vfptr}
> +---
>
> A::$vftable@:
> | &A_meta
> |
> | &A::aa
>
> A::aa this adjustor:
>
>
> class B size():
> +---
> | {vfptr}
> | {vbptr}
> | j
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
>
> B::$vftable@B@:
> | &B_meta
> |
> | &B::bb
>
> B::$vbtable@:
> | -
> | (Bd(B+)A)
>
> B::$vftable@A@:
> | -
> | &A::aa
>
> B::bb this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
>
>
> class C size():
> +---
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
>
> C::$vftable@C@:
> | &C_meta
> |
> | &C::cc
>
> C::$vbtable@:
> | -
> | (Cd(C+)A)
>
> C::$vftable@A@:
> | -
> | &A::aa
>
> C::cc this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
>
>
> class C1 size():
> +---
> | +--- (base class A)
> | | {vfptr}
> | +---
> | i1
> | <alignment member> (size=)
> +---
>
> C1::$vftable@:
> | &C1_meta
> |
> | &A::aa
> | &C1::cc1
>
> C1::cc1 this adjustor:
>
>
> class D size():
> +---
> | +--- (base class C)
> | | {vfptr}
> | | {vbptr}
> | | i
> | | <alignment member> (size=)
> | +---
> | d
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
>
> D::$vftable@C@:
> | &D_meta
> |
> | &C::cc
> | &D::dd
>
> D::$vbtable@:
> | -
> | (Dd(C+)A)
>
> D::$vftable@A@:
> | -
> | &A::aa
>
> D::dd this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
>
>
> class D1 size():
> +---
> | +--- (base class C1)
> | | +--- (base class A)
> | | | {vfptr}
> | | +---
> | | i1
> | | <alignment member> (size=)
> | +---
> | d1
> | <alignment member> (size=)
> +---
>
> D1::$vftable@:
> | &D1_meta
> |
> | &A::aa
> | &C1::cc1
> | &D1::dd1
>
> D1::dd1 this adjustor:
>
>
> class E size():
> +---
> | {vfptr}
> | {vbptr}
> | e
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base C)
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
>
> E::$vftable@:
> | &E_meta
> |
> | &E::ee
>
> E::$vbtable@E@:
> | -
> | (Ed(E+)A)
> | (Ed(E+)C)
>
> E::$vftable@A@:
> | -
> | &A::aa
>
> E::$vftable@C@:
> | -
> | &C::cc
>
> E::$vbtable@C@:
> | -
> | - (Ed(C+)A)
>
> E::ee this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> C
>
>
> class E1 size():
> +---
> | {vfptr}
> | {vbptr}
> | e1
> | <alignment member> (size=)
> +---
> +--- (virtual base C1)
> | +--- (base class A)
> | | {vfptr}
> | +---
> | i1
> | <alignment member> (size=)
> +---
>
> E1::$vftable@E1@:
> | &E1_meta
> |
> | &E1::ee1
>
> E1::$vbtable@:
> | -
> | (E1d(E1+)C1)
>
> E1::$vftable@C1@:
> | -
> | &A::aa
> | &C1::cc1
>
> E1::ee1 this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> C1
>
>
> class F size():
> +---
> | +--- (base class C)
> | | {vfptr}
> | | {vbptr}
> | | i
> | | <alignment member> (size=)
> | +---
> | f
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base B)
> | {vfptr}
> | {vbptr}
> | j
> | <alignment member> (size=)
> +---
>
> F::$vftable@C@:
> | &F_meta
> |
> | &C::cc
> | &F::ff
>
> F::$vbtable@C@:
> | -
> | (Fd(C+)A)
> | (Fd(F+)B)
>
> F::$vftable@A@:
> | -
> | &A::aa
>
> F::$vftable@B@:
> | -
> | &B::bb
>
> F::$vbtable@B@:
> | -
> | - (Fd(B+)A)
>
> F::ff this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> B
>
>
> class F1 size():
> +---
> | +--- (base class B)
> | | {vfptr}
> | | {vbptr}
> | | j
> | | <alignment member> (size=)
> | +---
> | f1
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base C)
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
>
> F1::$vftable@B@:
> | &F1_meta
> |
> | &B::bb
> | &F1::ff1
>
> F1::$vbtable@B@:
> | -
> | (F1d(B+)A)
> | (F1d(F1+)C)
>
> F1::$vftable@A@:
> | -
> | &A::aa
>
> F1::$vftable@C@:
> | -
> | &C::cc
>
> F1::$vbtable@C@:
> | -
> | - (F1d(C+)A)
>
> F1::ff1 this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> C
>
>
> class G size():
> +---
> | {vfptr}
> | {vbptr}
> | g
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base C)
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
> +--- (virtual base E)
> | {vfptr}
> | {vbptr}
> | e
> | <alignment member> (size=)
> +---
>
> G::$vftable@G@:
> | &G_meta
> |
> | &G::gg
>
> G::$vbtable@:
> | -
> | (Gd(G+)A)
> | (Gd(G+)C)
> | (Gd(G+)E)
>
> G::$vftable@A@:
> | -
> | &A::aa
>
> G::$vftable@C@:
> | -
> | &C::cc
>
> G::$vbtable@C@:
> | -
> | - (Gd(C+)A)
>
> G::$vftable@E@:
> | -
> | &E::ee
>
> G::$vbtable@E@:
> | -
> | - (Gd(E+)A)
> | - (Gd(E+)C)
>
> G::gg this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> C
> E
>
>
> class H size():
> +---
> | {vfptr}
> | {vbptr}
> | h
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base C)
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
> +--- (virtual base E)
> | {vfptr}
> | {vbptr}
> | e
> | <alignment member> (size=)
> +---
>
> H::$vftable@H@:
> | &H_meta
> |
> | &H::hh
>
> H::$vbtable@:
> | -
> | (Hd(H+)A)
> | (Hd(H+)C)
> | (Hd(H+)E)
>
> H::$vftable@A@:
> | -
> | &A::aa
>
> H::$vftable@C@:
> | -
> | &C::cc
>
> H::$vbtable@C@:
> | -
> | - (Hd(C+)A)
>
> H::$vftable@E@:
> | -
> | &E::ee
>
> H::$vbtable@E@:
> | -
> | - (Hd(E+)A)
> | - (Hd(E+)C)
>
> H::hh this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> C
> E
>
>
> class H1 size():
> +---
> | +--- (base class E)
> | | {vfptr}
> | | {vbptr}
> | | e
> | | <alignment member> (size=)
> | +---
> | +--- (base class C1)
> | | +--- (base class A)
> | | | {vfptr}
> | | +---
> | | i1
> | | <alignment member> (size=)
> | +---
> | h1
> | <alignment member> (size=)
> +---
> +--- (virtual base A)
> | {vfptr}
> +---
> +--- (virtual base C)
> | {vfptr}
> | {vbptr}
> | i
> | <alignment member> (size=)
> +---
>
> H1::$vftable@E@:
> | &H1_meta
> |
> | &E::ee
> | &H1::hh1
>
> H1::$vftable@C1@:
> | -
> | &A::aa
> | &C1::cc1
>
> H1::$vbtable@E@:
> | -
> | (H1d(E+)A)
> | (H1d(E+)C)
>
> H1::$vftable@A@:
> | -
> | &A::aa
>
> H1::$vftable@C@:
> | -
> | &C::cc
>
> H1::$vbtable@C@:
> | -
> | - (H1d(C+)A)
>
> H1::hh1 this adjustor:
>
> vbi: class offset o.vbptr o.vbte fVtorDisp
> A
> C
总结出单继承内存分布大致如下：
- 普通继承情况下，先父类元素后子类元素，若父类元素本身也是从某个爷爷类继承而来：父类是虚继承而来，则先父后子再爷爷（其实这个分布是满足规则2的）,即爷爷放在最后；父类是普通继承而来，先爷爷后父再子，即爷爷放在前面。
- 虚继承情况下，先子类元素后父类元素，如果父类元素本身也是从某个爷爷类继承（不论是虚继承还是普通继承）而来，则父类由类的深到浅依次分布（先爷爷后父，爷爷在子和父之间）。
多继承的内存分布情况如下：
- 虚继承和普通继承同时存在的情况下，先进行普通继承，再进行虚继承。
- 继承都是虚继承或者普通继承的时候，按照从左到右声明顺序进行继承，注意，都是虚继承的话，如果继承的相同的类，这个类只会在最先出现的地方出现一次而已；如果是普通继承，相同的类会多次出现。
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[]; //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[];
public:
virtual cc(){};
};
