virtual"构造函数 & 协变返回类

#include <iostream>
//Virtual constructor.
class Shape{
public:
    virtual ~Shape(void)
    { }

    virtual Shape* clone(void)const = 0;
    virtual Shape* create(void)const = 0;
};

class Circle : public Shape{
public:
    Circle* clone(void)const    //Covariant Return Types.See below.
    {
        std::cout<<"clone a circle.\n";
        return new Circle(*this);
    }
    Circle* create(void)const   //Covariant Return Types.
    {
        std::cout<<"create a circle.\n";
        return new Circle();
    }
};

class Rect : public Shape{
public:
    Rect* clone(void)const      //Covariant Return Types.
    {
        std::cout<<"clone a rectangle.\n";
        return new Rect(*this);
    }

    Rect* create(void)const     //Covariant Return Types.
    {
        std::cout<<"create a rectangle.\n";
        return new Rect();
    }
};
void user_code(const Shape& _Shape)
{
    Shape *s2 = _Shape.clone();
    Shape *s3 = _Shape.create();

    delete s2;
    delete s3;
}

int main(void)
{
    Circle c;
    user_code(c);

    Rect r;
    user_code(r);
}
//http://stackoverflow.com/questions/3593601/covariant-return-type
//The return type of an overriding function shall be either identical to the return
//type of the overridden function or covariant with the classes of the functions. 
//If a function D::f overrides a function B::f, the return types of the functions 
//are covariant if they satisfy the following criteria:
//
//— both are pointers to classes or references to classes)
//
//— the class in the return type of B::f is the same class as the class in the
//return type of D::f, or is an unambiguous and accessible direct or indirect 
//base class of the class in the return type of D::f
// 
//— both pointers or references have the same cv-qualification and the class type
//in the return type of D::f has the same cv-qualification as or less cv-qualification 
//than the class type in the return type of B::f.