虚函数和虚拟基类用法小结

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申明：本文主要参考MSDN

虚继承：在继承定义中包含了virtual关键字的继承关系；

虚基类：在虚继承体系中的通过virtual继承而来的基类；

声明虚基类的一般形式为
class 派生类名: virtual 继承方式基类名

需要注意: 为了保证虚基类在派生类中只继承一次，应当在该基类的所有直接派生类中声明为虚基类。否则仍然会出现对基类的多次继承。

将类A声明为虚基类，方法如下:
class A//声明基类A
{…};
class B :virtual public A//声明类B是类A的公用派生类，A是B的虚基类
{…};
class C :virtual public A//声明类C是类A的公用派生类，A是C的虚基类
{…};

虚继承 Demo 参考来自：http://www.cnblogs.com/itech/archive/2009/03/01/1399996.html

#include <iostream>
using namespace std;class B
{
public:int i;virtual void vB(){ cout << "B::vB" << endl; }void fB(){ cout << "B::fB" << endl;}
};class D1 : virtual public B
{
public:int x;virtual void vD1(){ cout << "D1::vD1" << endl; }void fD1(){ cout << "D1::fD1" << endl;}
};class D2 : virtual public B
{
public:int y;void vB(){ cout << "D2::vB" << endl;}virtual void vD2(){ cout << "D2::vD2" << endl;}void fD2(){ cout << "D2::fD2" << endl;}
};class GD :  public D1, public D2
{
public:int a;void vB(){ cout << "GD::vB" << endl;}void vD1(){cout << "GD::vD1" << endl;}virtual void vGD(){cout << "GD::vGD" << endl;}void fGD(){cout << "GD::fGD" << endl;}
};

虚基类：

用于某类从多个类继承，这多个基类有共同基类时，这个最上层基类的成员会多次在最终派生类出现而产生二义性，为避免二义性，使得最终派生类中，最上层的基类成员只有一份，这时需要虚拟继承，该最上层类就是虚基类，需要注意的是，该类在第一层派生时就要虚拟继承才行，使用方法是在继承方式前加上一个 virtual就可以了

#include <iostream>
using namespace std;
class CBase
{
protected:int a;
public:CBase(int na){a = na;cout << "CBase constructor" << endl;}~CBase(){cout << "CBase deconstructor!" << endl;}
};
class CDerive1: virtual public CBase
{
public:CDerive1(int na) : CBase(na){cout << "CDerive1 constructor" << endl;}~CDerive1(){cout << "CDerive1 deconstructor" << endl;}int GetA(){return a;}
};
class CDerive2: virtual public CBase
{
public:CDerive2(int na): CBase(na){cout << "CDerive2 constructor" << endl;}~CDerive2(){cout << "CDerive2 deconstructor" << endl;}int GetA(){return a;}
};class CDerive12: public CDerive1,public CDerive2
{
public:CDerive12(int na1,int na2,int na3): CDerive1(na1), CDerive2(na2), CBase(na3){cout << "CDerive12 constructor" << endl;}~CDerive12(){cout << "CDerive12 deconstructor" << endl;}
};
void main()
{CDerive12 obj(100,200,300);cout << "frome CDerive1: a = " << obj.CDerive1::GetA() << endl;cout << "frome CDerive2: a = " << obj.CDerive2::GetA() << endl << endl;system("pause");
}

效果：