多态是C++的重要特性，通过基类指针来访问派生类的函数。
虚函数就是为了实现这功能而定义的函数，虚函数可以在定义时实现也可以不实现，定义了虚函数的类可以实例化。
纯虚函数更多的是表示接口的含义，纯虚函数定义时不能实现，需要在派生的子类中实现且必须实现，含有纯虚函数的类不能实例化。
虚析构函数与虚函要成对出现，以便在通过基类指针释放派生对象时，调用派生类的析构函数。
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class VirtualFun
{
public:
VirtualFun(){std::cout<<"create VirtualFun"<<std::endl;};
virtual ~VirtualFun(){std::cout<<"delete VirtualFun"<<std::endl;};
virtual void Hello() {std::cout<<"hello VirtualFun"<<std::endl;};
};
class PureVirtualFun
{
public:
PureVirtualFun(){std::cout<<"create PureVirtualFun"<<std::endl;};
virtual ~PureVirtualFun(){std::cout<<"delete PureVirtualFun"<<std::endl;};
virtual void Hello() = 0;
};
class APureVirtualFun : public PureVirtualFun
{
public:
APureVirtualFun(){std::cout<<"create APureVirtualFun"<<std::endl;};
~APureVirtualFun(){std::cout<<"delete APureVirtualFun"<<std::endl;};
void Hello() {std::cout<<"hello APureVirtualFun"<<std::endl;};
};
int main()
{
VirtualFun * a = new VirtualFun();
a->Hello();
PureVirtualFun* b = new APureVirtualFun();
b->Hello();
delete a;
delete b;
}
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