C++中继承的内存分布

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; 
}; 

 类结构图：

内存分布图：

 

DerivedC:

                    DerivedA::m_base

                    m_derivedA

                    DerivedB::m_base

                    m_derivedB

                    m_derivedC

 

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

如果DerivedB 和 DerivedC 都是虚继承 ， 即 virtual public Base
这时内存布局：

    

DerivedC：

                    objDerivedA::vbptr

                    objDerivedA::m_derivedA

                    objDerivedB::vbptr

                    objDerivedB::m_derivedB

                    m_derivedC

                    m_base    只有一份

 

类似于这个：

=================================================================
Base, DerivedA, DerivedB 各增加一个虚函数

则内存布局为：

 

 

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               

 

如果是这样呢？

class A{}

class B1: classA{virtual fun()}

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

class C: public B1,public B2{}               这样Virtual继承起不到作用， C还是有两个内嵌A对象，就是有两个m_base

 

Class C:    

                DerivedA::vfptr          

                DerivedA::m_base

                DerivedA::m_derivedA  

                DerivedB::vfptr

                DerivedB::vbptr

                DerivedB::m_derivedB

                m_derivedC

                DerivedB::m_base

总结：

        先基类元素后继承类元素

        有虚函数只是增加vfptr；继承的类如果有增加虚函数，向vtable增加函数指针

        虚继承增加vbptr，注意：虚基类元素排在最后（这个是和 先基类后继承 不同之处）

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

        vfptr在vbptr之前

转载于：http://www.cnblogs.com/DylanWind/archive/2009/01/12/1373919.html