对象内存布局 (16) 转帖
原帖 [url=http://blog.csdn.net/pathuang68/article/details/4105922]http://blog.csdn.net/pathuang68/article/details/4105922[/url][color=#333333][font=Arial][size=12px]下面讨论虚基类和虚函数同时存在的时候，对对象内存布局的影响。[/size][/font][/color]
[color=#333333][font=Arial][size=12px]假定各个类之间的关系如下图：[/size][/font][/color]
[color=#333333][font=Arial][size=12px][img=459,330]http://p.blog.csdn.net/images/p_blog_csdn_net/pathuang68/EntryImages/20090424/1601.GIF[/img] [/size][/font][/color]
[color=#333333][font=Arial][size=12px]Base中声明了一个虚函数vfBase()和一个整形成员变量；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]Derived1 override了Base中声明的虚函数vfBase()，声明了一个虚函数vfDerived1()，另有一个整形成员变量derived1_member；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]Derived2 override了Base中声明的虚函数vfBase()，声明了一个虚函数vfDerived2()，另有一个整形成员变量derived2_member；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]ChildDerived分别override了Base、Derived1和Derived2中声明的虚函数vfBase()、vfDerived1() 和vfDerived2()，另外有一个整形成员变量childderived_member；[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]代码如下：[/size][/font][/color]
[color=#333333][font=Arial][size=12px]#include <iostream>[/size][/font][/color]
[color=#333333][font=Arial][size=12px]using namespace std;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]typedef void (*VFun)(void);[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]template<typename T>[/size][/font][/color]
[color=#333333][font=Arial][size=12px]VFun virtualFunctionPointer(T* b, int i)[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px] return (VFun)(*((int*)(*(int*)b) + i));[/size][/font][/color]
[color=#333333][font=Arial][size=12px]}[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]template<typename T>[/size][/font][/color]
[color=#333333][font=Arial][size=12px]int virtualBaseTableOffset(T* b, int i)[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px] return (int)*((int*)*(int*)b + i);[/size][/font][/color]
[color=#333333][font=Arial][size=12px]}[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]class Base[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px]public:[/size][/font][/color]
[color=#333333][font=Arial][size=12px] int base_member;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline virtual void vfBase()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in Base::vfBase()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px]};[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]class Derived1 : public virtual Base[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px]public:[/size][/font][/color]
[color=#333333][font=Arial][size=12px] int derived1_member;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline void vfBase()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in Derived1::vfBase()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline virtual void vfDerived1()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in Derived1::vfDerived1()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px]};[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]class Derived2 : public virtual Base[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px]public:[/size][/font][/color]
[color=#333333][font=Arial][size=12px] int derived2_member;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline void vfBase()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in Derived2::vfBase()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline virtual void vfDerived2()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in Derived2::vfDerived2()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px]};[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]class ChildDerived : public Derived1, public Derived2[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px]public:[/size][/font][/color]
[color=#333333][font=Arial][size=12px] int childderived_member;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline void vfBase()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in ChildDerived::vfBase()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline void vfDerived1()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in ChildDerived::vfDerived1()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px] inline void vfDerived2()[/size][/font][/color]
[color=#333333][font=Arial][size=12px] {[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "This is in ChildDerived::vfDerived2()" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] }[/size][/font][/color]
[color=#333333][font=Arial][size=12px]};[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]int main(void)[/size][/font][/color]
[color=#333333][font=Arial][size=12px]{[/size][/font][/color]
[color=#333333][font=Arial][size=12px] ChildDerived cd;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] VFun pVF;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] int* tmp;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << "sizeof(Base) = /t/t" << sizeof(Base) << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "sizeof(Derived1) = /t" << sizeof(Derived1) << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "sizeof(Derived2) = /t" << sizeof(Derived2) << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "sizeof(ChildDerived) = /t" << sizeof(ChildDerived) << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]cout << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "address of ChildDerived object:" << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "address = " << (int*)&cd << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px]cout << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "1st virtual function table: " << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF = virtualFunctionPointer(&cd, 0);[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF();[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << "1st virtual base table: " << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] tmp = (int*)((int*)&cd + 1);[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "address = " << tmp << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 0) << "/t<- not sure yet, but it doesn't matter here." << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 1) << "/t<- offset from Derived1 subobject's vbptr to Base subobject." << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 2) << "/t<- means the end of this virtual base table." << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] tmp = ((int*)&cd) + 3;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "2nd virtual function table: " << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF = virtualFunctionPointer(tmp, 0);[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF();[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << "2nd virtual base table: " << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] tmp = (int*)((int*)&cd + 4);[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "address = " << tmp << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 0) << "/t<- not sure yet, but it doesn't matter here." << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 1) << "/t<- offset from Derived2 subobject's vbptr to Base subobject." << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << virtualBaseTableOffset(tmp, 2) << "/t<- means the end of this virtual base table." << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] tmp = ((int*)&cd) + 7;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "3rd virtual function table: " << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF = virtualFunctionPointer(tmp, 0);[/size][/font][/color]
[color=#333333][font=Arial][size=12px] pVF();[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << endl;[/size][/font][/color]
[color=#333333][font=Arial]
[/font][/color]
[color=#333333][font=Arial][size=12px] cout << "Derived1 subobject address = /t" << (Derived1*)&cd << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "Derived2 subobject address = /t" << (Derived2*)&cd << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] cout << "Base subobject address = /t" << (Base*)&cd << endl;[/size][/font][/color]
[color=#333333][font=Arial][size=12px] return 0;[/size][/font][/color]
[color=#333333][font=Arial][size=12px]}[/size][/font][/color]
[color=#333333][font=Arial][size=12px]运行结果如下：[/size][/font][/color]
[color=#333333][font=Arial][img=636,591]http://p.blog.csdn.net/images/p_blog_csdn_net/pathuang68/EntryImages/20090424/1602.GIF[/img] [/font][/color]
[color=#333333][font=Arial][size=12px]ChildDerived、Derived1和Derived2对象memory layout分别图解如下：[/size][/font][/color]
[color=#333333][font=Arial][size=12px][img=490,260]http://p.blog.csdn.net/images/p_blog_csdn_net/pathuang68/EntryImages/20090424/1603.GIF[/img] [/size][/font][/color]
[color=#333333][font=Arial][size=12px]两个虚基类偏移量图解如下(虚函数表和虚基类表略)：[/size][/font][/color]
[color=#333333][font=Arial][img=302,260]http://p.blog.csdn.net/images/p_blog_csdn_net/pathuang68/EntryImages/20090424/1604.GIF[/img] [/font][/color]
[color=#333333][font=Arial][size=12px]结论：[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其一，只要涉及到虚基类，一切问题就变得复杂起来；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其二，如果同时存在vfptr和vbptr，vfptr居前，vbptr居后；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其三，普通基类居前，虚基类总是尽可能地排列在layout的最后；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其四，两个同一层次的虚基类subobject，先声明者居前，后声明者居后，这点和普通基类是一样的；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其五，两个不同层次的虚基类subobject，层次高者居前，层次低者居后；[/size][/font][/color]
[color=#333333][font=Arial][size=12px]其六，Stan Lippman建议，不要在一个virtual base class中声明nonstatic data member，理由是这样做会是问题变得非常复杂。[/size] [/font][/color] 此贴不错，比 inside the c++ object model 还好！
fsdafasfd
页:
[1]