探究C++对象模型
先引用两句名言作为开场白：
"C++老手分两类：一种人把语言用得烂熟，OO观念也有；另一种人不但如此，还对于台面下的机制，如编译器合成的default constructor、object的内存布局等有莫大的兴趣。"
"了解C++对象模型，绝对有助于你在语言本身以及面向对象观念两方面的层次提升。"
首先以Point类为例
再看下对象模型
这个模型好像跟我们以往的认知有些不一致呀，vtable首四个字节怎么是type_info的指针？type_info是RTTI的内容，是编译器用来进行类型识别的，它到底存不存在呢？
用代码测试一下
// VirtualTable.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
using namespace std;
class Base {
private:
int x;
public:
virtual void f() { cout<<"Base::f"<<endl; };
virtual void g() { cout<<"Base::g"<<endl; };
virtual void h() { cout<<"Base::h"<<endl; };
Base(){x = 2;};
};
class Derive : public Base{
private:
int y;
public:
virtual void f1() { cout<<"Derive::f1"<<endl; };
virtual void g1() { cout<<"Derive::g1"<<endl; };
virtual void h1() { cout<<"Derive::h1"<<endl; };
Derive(){y = 3;};
};
int main(int argc, char* argv[])
{
typedef void(*Fun)(void);
Base b;
Derive d;
Fun pFun = NULL;
cout <<"虚函数表入口指针地址"<< (int*)(&b) << endl;
cout <<"虚函数表的地址"<< (int*)*(int*)(&b) << endl;
cout <<"Base — 第一个数据成员"<< *((int*)(&b)+1) << endl;
cout <<"Derive — 第一个数据成员"<< *((int*)(&d)+1) << endl;
cout <<"Derive — 第二个数据成员"<< *((int*)(&d)+2) << endl;
// Invoke the first virtual function
pFun = (Fun)*((int*)*(int*)(&b));
pFun = (Fun)*((int*)*(int*)(&b)+0); // Base::f()
pFun();
pFun = (Fun)*((int*)*(int*)(&b)+1); // Base::g()
pFun();
pFun = (Fun)*((int*)*(int*)(&b)+2); // Base::h()
pFun();
//以上代码说明 虚函数表位于对象内存的最开始的位置
pFun = (Fun)*((int*)*(int*)(&d)+0); // Base::f()
pFun();
pFun = (Fun)*((int*)*(int*)(&d)+1); // Base::g()
pFun();
pFun = (Fun)*((int*)*(int*)(&d)+2); // Base::h()
pFun();
pFun = (Fun)*((int*)*(int*)(&d)+3); // Derive::f1()
pFun();
pFun = (Fun)*((int*)*(int*)(&d)+4); // Derive::g1()
pFun();
pFun = (Fun)*((int*)*(int*)(&d)+5); // Derive::h1()
pFun();
system("pause");
return 0;
}
没有看到 type_info的影子，这也正是我们以往心目中Vtable的内存位置
这是神马情况？上文的图在胡扯？
再看下面代码，
测试代码（vc8.0执行正常，vc6会报错，可能对象模型不一致）
#include "iostream"
#include "string"
using namespace std;
class Aclass
{
public:
int a;
virtual void setA(int tmp)
{
a=tmp;
cout<<a<<endl;
}
};
class Bclass:public Aclass
{
public:
virtual void setA(int tmp)
{
a=tmp+10;
cout<<a<<endl;
}
public:
void print()
{
cout<<a<<endl;
}
};
class Cclass:public Bclass
{
};
typedef unsigned long DWORD;
struct TypeDescriptor
{
DWORD ptrToVTable;
DWORD spare;
char name[8];
};
struct PMD
{
int mdisp; //member displacement
int pdisp; //vbtable displacement
int vdisp; //displacement inside vbtable
};
struct RTTIBaseClassDescriptor
{
struct TypeDescriptor* pTypeDescriptor; //type descriptor of the class
DWORD numContainedBases; //number of nested classes following in the Base Class Array
struct PMD where; //pointer-to-member displacement info
DWORD attributes; //flags, usually 0
};
struct RTTIClassHierarchyDescriptor
{
DWORD signature; //always zero?
DWORD attributes; //bit 0 set = multiple inheritance, bit 1 set = virtual inheritance
DWORD numBaseClasses; //number of classes in pBaseClassArray
struct RTTIBaseClassArray* pBaseClassArray;
};
struct RTTICompleteObjectLocator
{
DWORD signature; //always zero ?
DWORD offset; //offset of this vtable in the complete class
DWORD cdOffset; //constructor displacement offset
struct TypeDescriptor* pTypeDescriptor; //TypeDescriptor of the complete class
struct RTTIClassHierarchyDescriptor* pClassDescriptor; //describes inheritance hierarchy
};
int main( )
{
Aclass* ptra=new Bclass;
int ** ptrvf=(int**)(ptra);
RTTICompleteObjectLocator str = *((RTTICompleteObjectLocator*)(*((int*)ptrvf[0]-1)));
//abstract class name from RTTI
string classname(str.pTypeDescriptor->name);
classname=classname.substr(4,classname.find("@@")-4);
cout<<classname<<endl;
system("pause");
return 0;
}
哈哈，原来在我们通常所认为VTable的前面的四个字节，你怎么看？
感谢 点击打开链接 的文章<<对象与对象的类型信息----获取对象的RTTI信息>>，引用了你的代码。