程式師世界 >> 編程語言 >> C語言 >> C++ >> C++入門知識 >> 探究C++對象模型

探究C++對象模型

編輯：C++入門知識

先引用兩句名言作為開場白：

"C++老手分兩類：一種人把語言用得爛熟，OO觀念也有；另一種人不但如此，還對於台面下的機制，如編譯器合成的default constructor、object的內存布局等有莫大的興趣。"

"了解C++對象模型，絕對有助於你在語言本身以及面向對象觀念兩方面的層次提升。"

首先以Point類為例

再看下對象模型

這個模型好像跟我們以往的認知有些不一致呀，vtable首四個字節怎麼是type_info的指針？type_info是RTTI的內容，是編譯器用來進行類型識別的，它到底存不存在呢？

用代碼測試一下

[cpp]
// 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 <<"虛函數表入口指針地址"<< (int*)(&b) << endl;
    cout <<"虛函數表的地址"<< (int*)*(int*)(&b) << endl;

    cout <<"Base — 第一個數據成員"<< *((int*)(&b)+1) << endl;
    cout <<"Derive — 第一個數據成員"<< *((int*)(&d)+1) << endl;
    cout <<"Derive — 第二個數據成員"<< *((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();
    //以上代碼說明虛函數表位於對象內存的最開始的位置
    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;
}

// 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 <<"虛函數表入口指針地址"<< (int*)(&b) << endl;
    cout <<"虛函數表的地址"<< (int*)*(int*)(&b) << endl;

cout <<"Base — 第一個數據成員"<< *((int*)(&b)+1) << endl;
cout <<"Derive — 第一個數據成員"<< *((int*)(&d)+1) << endl;
cout <<"Derive — 第二個數據成員"<< *((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();
//以上代碼說明虛函數表位於對象內存的最開始的位置
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;
}

沒有看到 type_info的影子，這也正是我們以往心目中Vtable的內存位置

這是神馬情況？上文的圖在胡扯？

再看下面代碼，

測試代碼（vc8.0執行正常，vc6會報錯，可能對象模型不一致）

[cpp]
#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;
}

#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;
}