Polymorphism involving objects with value semantics (C++)
Posted: Tue Dec 17, 2013 10:03 pm
A few months ago, I really enjoyed two very interesting C++ talks (1) (2) given by a very clever guy named Sean Parent. I was immediately enchanted by his approach to polymorphism involving objects with value semantics, which makes virtual dispatch and dynamic memory allocation just an implementation detail rather than a primary means of writing client code. I had been interested in this topic for some time but never encountered such a good source of inspiration, so I've decided to give it a try and come up with this minimal experiment to help me study this approach:
VIEW THE CODE BELOW IN FULL-SCREEN (polymorphism_value_semantics_sample.cpp)
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VIEW THE CODE BELOW IN FULL-SCREEN (polymorphism_value_semantics_sample.cpp)
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/*
(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(If_ptr(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()