C++虚基类(virtual base class)
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参考http://www.learncpp.com/cpp-tutorial/118-virtual-base-classes/
#include<iostream>using namespace std;class Base{public:Base(){cout<<"Base constructor"<<endl;}};class A:public Base{public:A(){cout<<"A constructor"<<endl;}};class B: public Base{public:B(){cout<<"B constructor"<<endl;}};class D:public A,B{public:D(){cout<<"D constructor"<<endl;}};int main(){D d;}
程序输出
Base constructor
A constructor
Base constructor
B constructor
D constructor
A的基类是Base,B的基类也是Base,而类D多继承自A,B,可以看到基类Base的构造函数被调用了两次
有没有方法可以使基类只调用一次呢,这就需要引入虚基类(virtual base class)的概念
虚基类只需在派生类的继承列表前加入virtual关键字即可。
如
Class A:virtua public Base{
}
虚基类只会创建虚基类的一个对象。 (只调用一次基类的构造函数)。虚基类的构造函数由继承层次最深的类调用(if a class inherits one or more classes that have virtual parents, the most derived class is responsible for constructing the virtual base class.)
如果要识别某类的父类为虚基类,该派生类的继承列表前必须添加virtual关键字。否则将不识别该类的父类为虚基类
比如下例中若类A或类B中有一个未加virtual关键字,仍将调用基类构造函数两次
#include<iostream>using namespace std;class Base{public:Base(){cout<<"Base constructor"<<endl;}};class A:virtual public Base{public:A(){cout<<"A constructor"<<endl;}};class B: virtual public Base{public:B(){cout<<"B constructor"<<endl;}};class D:public A,B{public:D(){cout<<"D constructor"<<endl;}};int main(){D d;}
以上程序输出
Base constructor
A constructor
B constructor
D constructor
基类构造函数只调用一次,基类构造函数由类D调用。
#include<iostream>using namespace std;class Base{public:Base(){cout<<"Base constructor"<<endl;}};class A:virtual public Base{public:A(){cout<<"A constructor"<<endl;}};class B: virtual public Base{public:B(){cout<<"B constructor"<<endl;}};class E:public Base{public:E(){cout<<"E constructor"<<endl;}};class D:public A,B,E{public:D(){cout<<"D constructor"<<endl;}};int main(){D d;}
以上程序输出
Base constructor
A constructor
B constructor
Base constructor
E constructor
D constructor
基类Base构造函数调用了两次,因为派生类E继承列表前未添加virtual关键字,不表明该类E继承自虚基类,仍会调用基类构造函数
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