C++中繼承的內存分佈

class Base 
{ 
public: 
 int m_base; 
}; 

class DerivedA: public Base 
{ 
public: 
 int m_derivedA; 
}; 

class DerivedB: public Base 
{ 
public: 
 int m_derivedB; 
}; 

class DerivedC: public DerivedA, public DerivedB 
{ 
public: 
 int m_derivedC; 
}; 

 類結構圖：

內存分佈圖：

 

DerivedC:

                    DerivedA::m_base

                    m_derivedA

                    DerivedB::m_base

                    m_derivedB

                    m_derivedC

 

====================================================

如果DerivedB 和 DerivedC 都是虛繼承 ， 即 virtual public Base
這時內存佈局：

    

DerivedC：

                    objDerivedA::vbptr

                    objDerivedA::m_derivedA

                    objDerivedB::vbptr

                    objDerivedB::m_derivedB

                    m_derivedC

                    m_base    只有一份

 

類似於這個：

=================================================================
Base, DerivedA, DerivedB 各增加一個虛函數

則內存佈局爲：

 

 

DerivedB：

                    DerivedB::vfptr

                    DerivedB::vbptr

                    DerivedB::m_derivedB

                    Base::vfptr

                    Base::m_base

 

DerivedC:

                    DerivedA::vfptr                  04

                    DerivedA::vbptr                 08

                    DevivedA::m_derivedA      0C

                    DevivedB::vfptr                  10

                    DerivedB::vbptr                  14
                    DerivedB::m_derivedB        18

                    m_derivedC                         1C

                    Base::vfptr                           20

                    m_base                                 24               

 

如果是這樣呢？

class A{}

class B1: classA{virtual fun()}

class  B2: virtual public classA{virtual fun()}

class C: public B1,public B2{}               這樣Virtual繼承起不到作用， C還是有兩個內嵌A對象，就是有兩個m_base

 

Class C:    

                DerivedA::vfptr          

                DerivedA::m_base

                DerivedA::m_derivedA  

                DerivedB::vfptr

                DerivedB::vbptr

                DerivedB::m_derivedB

                m_derivedC

                DerivedB::m_base

總結：

        先基類元素後繼承類元素

        有虛函數只是增加vfptr；繼承的類如果有增加虛函數，向vtable增加函數指針

        虛繼承增加vbptr，注意：虛基類元素排在最後（這個是和 先基類後繼承 不同之處）

               注意上面，凡是打上了vbptr的類，  DerivedB::m_base都被打到了最後。

        vfptr在vbptr之前

轉載於：http://www.cnblogs.com/DylanWind/archive/2009/01/12/1373919.html