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深入C++对象模型&虚函数表
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>
本文出自 “11132019” 博客，转载请与作者联系！
深入C++对象模型&虚函数表
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