深入C++對(duì)象模型&虛函數(shù)表

多態(tài)的實(shí)現(xiàn)機(jī)制：

       C++中虛函數(shù)的主要作用就是用來(lái)實(shí)現(xiàn)多態(tài)，就是使用基類(lèi)的指針或者引用調(diào)用重寫(xiě)的虛函數(shù)，當(dāng)父類(lèi)的指針或引用指向父類(lèi)對(duì)象時(shí)調(diào)用的是父類(lèi)虛函數(shù)，當(dāng)指向子類(lèi)對(duì)象時(shí)調(diào)用的是子類(lèi)的虛函數(shù)。那么這又是怎么實(shí)現(xiàn)的呢？？？

       這都是通過(guò)虛函數(shù)表實(shí)現(xiàn)的，虛函數(shù)表是通過(guò)一塊連續(xù)內(nèi)存來(lái)存儲(chǔ)虛函數(shù)的地址。這張表解決了虛函數(shù)重寫(xiě)（地址進(jìn)行覆蓋）的問(wèn)題 。在有虛函數(shù)的對(duì)象實(shí)例中都有一張?zhí)摵瘮?shù)表，虛函數(shù)表就像一張地圖，指明了實(shí)際調(diào)用的虛函數(shù)函數(shù)。

例：

class Base
{
public:
       Base()
              :_b(1){}
       virtual void fun1()
       { }
       virtual void fun2()
       { }
private:
       int _b;
};

虛函數(shù)表的最后一個(gè)元素是一個(gè)空指針。

       既然我們知道了虛函數(shù)的地址，那么就可以通過(guò)過(guò)找到這塊地址來(lái)調(diào)用這個(gè)虛函數(shù)。這也導(dǎo)致了多態(tài)的不安全性,效率降低。

typedef void (* pfun)();
void PrintfBase(pfun *_ppfun)
{
       int i = 0;
       for (i = 0; _ppfun[i] != NULL; i++)
       {
              _ppfun[i] ();
       }
}
void test()
{
       Base b;
       PrintfBase((pfun *)*((int *)(&b)));
}

1、單繼承對(duì)象模型

class Base
{
public:
       Base()
              :_b(1){}
       virtual void fun1()
       {
              cout << "Base::fun1()" <<endl;
       }
       virtual void fun2()
       {
              cout << "Base::fun2()" << endl;
       }
private:
       int _b;
};
class Deriver :public Base
{
public:
       Deriver()
              :_d(2){}
       virtual void fun1()
       {
              cout << "Deriver::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Deriver::fun3()" << endl;
       }
private:
       int _d;
};
typedef void (* pfun)();
void PrintfBase(pfun *_ppfun)
{
       int i = 0;
       for (i = 0; _ppfun[i] != NULL; i++)
       {
              _ppfun[i] ();
       }
}
void test()
{
       Base b;
       PrintfBase((pfun *)*((int *)(&b)));
}

2、多重繼承的對(duì)象模型

class Base1
{
public:
       Base1()
              :_b1(1){}
       virtual void fun1()
       {
              cout << "Base1::fun1()" <<endl;
       }
       virtual void fun2()
       {
              cout << "Base1::fun2()" << endl;
       }
private:
       int _b1;
};
class Base2
{
public:
       Base2()
              :_b2(2){}
       virtual void fun1()
       {
              cout << "Base2::fun1()" << endl;
       }
       virtual void fun2()
       {
              cout << "Base1::fun2()" << endl;
       }
private:
       int _b2;
};
class Deriver :public Base1,public Base2
{
public:
       Deriver()
              :_d3(3){}
       virtual void fun1()
       {
              cout << "Deriver::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Deriver::fun3()" << endl;
       }
private:
       int _d3;
};

3、菱形繼承的對(duì)象模型

class Base
{
public:
       Base()
              :_b1(1){}
       virtual void fun1()
       {
              cout << "Base1::fun1()" << endl;
       }
       virtual void fun2()
       {
              cout << "Base1::fun2()" << endl;
       }
private:
       int _b1;
};
class Base2:public Base
{
public:
       Base2()
              :_b2(1){}
       virtual void fun1()
       {
              cout << "Base2::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Base2::fun2()" << endl;
       }
private:
       int _b2;
};
class Base3:public Base
{
public:
       Base3()
              :_b3(1){}
       virtual void fun1()
       {
              cout << "Base3::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Base3::fun2()" << endl;
       }
private:
       int _b3;
};
class Deriver:public Base2,public Base3
{
public:
       Deriver()
              :_d3(3){}
       virtual void fun1()
       {
              cout << "Deriver::fun1()" << endl;
       }
       virtual void fun4()
       {
              cout << "Deriver::fun3()" << endl;
       }
private:
       int _d3;
};

4、菱形的虛擬繼承

<span >class Base
{
public:
       Base()
              :_b1(1){}
       virtual void fun1()
       {
              cout << "Base1::fun1()" << endl;
       }
       virtual void fun2()
       {
              cout << "Base1::fun2()" << endl;
       }
private:
       int _b1;
};
class Base2:virtual public Base
{
public:
       Base2()
              :_b2(2){}
       virtual void fun1()
       {
              cout << "Base2::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Base2::fun2()" << endl;
       }
private:
       int _b2;
};
class Base3:virtual public Base
{
public:
       Base3()
              :_b4(3){}
       virtual void fun1()
       {
              cout << "Base3::fun1()" << endl;
       }
       virtual void fun3()
       {
              cout << "Base3::fun2()" << endl;
       }
private:
       int _b3;
};
class Deriver:public Base2,public Base3
{
public:
       Deriver()
              :_d3(4){}
       virtual void fun1()
       {
              cout << "Deriver::fun1()" << endl;
       }
       virtual void fun4()
       {
              cout << "Deriver::fun4()" << endl;
       }
private:
       int _d4;
};</span>