深入C++对象模型&虚函数表

来源：互联网发布：java输入输出流考题编辑：程序博客网时间：2024/05/16 15:53

多态的实现机制：

C++中虚函数的主要作用就是用来实现多态，就是使用基类的指针或者引用调用重写的虚函数，当父类的指针或引用指向父类对象时调用的是父类虚函数，当指向子类对象时调用的是子类的虚函数。那么这又是怎么实现的呢？？？

这都是通过虚函数表实现的，虚函数表是通过一块连续内存来存储虚函数的地址。这张表解决了虚函数重写（地址进行覆盖）的问题。在有虚函数的对象实例中都有一张虚函数表，虚函数表就像一张地图，指明了实际调用的虚函数函数。

例：

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

虚函数表的最后一个元素是一个空指针。

既然我们知道了虚函数的地址，那么就可以通过过找到这块地址来调用这个虚函数。这也导致了多态的不安全性,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、单继承对象模型

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、多重继承的对象模型

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、菱形继承的对象模型

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、菱形的虚拟继承

<span style="font-size:14px;">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>

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