C++虚基类(virtual base class)

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