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