Polymorphism involving objects with value semantics (C++)

polymorphicValues.cpp

/*
(c) 2013 +++ Filip Stoklas, aka FipS, http://www.4FipS.com +++
THIS CODE IS FREE - LICENSED UNDER THE MIT LICENSE
ARTICLE URL: http://forums.4fips.com/viewtopic.php?f=3&t=1194
*/
 
#include <iostream>
#include <vector>
#include <memory>
 
struct Circle
{
    void draw(std::ostream &out) { out << "Circle::draw()\n"; }
};
 
struct Square
{
    void draw(std::ostream &out) { out << "Square::draw()\n"; }
};
 
struct List
{
    template <typename Shape_T>
    void push(Shape_T shape)
    {
        _shapes.emplace_back(new Slot<Shape_T>(std::move(shape)));
    }
 
    void draw(std::ostream &out)
    {
        for(auto &shape : _shapes)
        {
            shape->draw(out);
        }
    }
 
 private:
 
    struct If
    {
        virtual ~If() {}
        virtual void draw(std::ostream &out) = 0;
    };
 
    template <typename Shape_T>
    struct Slot : public If
    {
        virtual void draw(std::ostream &out) { shape.draw(out); }
 
        Slot(Shape_T shape) : shape(shape) {}
        Shape_T shape;
    };
 
    typedef std::unique_ptr<If> If_ptr;
    std::vector<If_ptr> _shapes;
};
 
int main()
{
    Square s;
    Circle c;
    List l;
 
    l.push(s);
    l.push(c);
    l.push(c);
    l.push(c);
 
    l.draw(std::cout);
 
    return 0;
}
 
// Compiled under Visual C++ 2012, output:
// Square::draw()
// Circle::draw()
// Circle::draw()
// Circle::draw()

//
//original found at
//http://code.4fips.com/browse/forums/2013/polymorphism_value_semantics_sample.cpp/
//