Positive polymorphism

Question

I read Concept Based Inheritance in C ++. I have an attached sample code for everyone. I basically ask if this concept is correct? I am new to this, so I just put off what is on my mind. Any comments / criticisms are welcome.

#include "stdafx.h" #include <memory> #include <vector> #include <algorithm> #include <iostream> using namespace std; struct Point{ int x; int y; }; class graphics_surface{ class drawable_concept{ public: virtual void draw(Point const& coordinate) {}; virtual ~drawable_concept() {}; }; template<class T> class drawable_model : public drawable_concept{ public: drawable_model(T& item) : item_(item){} void draw(Point const& coordinate){ item_.draw(coordinate); } ~drawable_model(){} private: T item_; }; public: template<class T> void push_back(T& drawable){ v_.push_back(shared_ptr<drawable_concept>( new drawable_model<T>(drawable))); } void draw(Point const& coordinate) { for_each(v_.begin(), v_.end(), [&](shared_ptr<drawable_concept>& concept){ concept->draw(coordinate); }); } private: vector<shared_ptr<drawable_concept>> v_; }; struct triangle{ void draw(Point const& p){ cout << "Triangle: " << px << "," << py << endl; } }; struct square{ void draw(Point const& p){ cout << "Sqaure: " << px << "," << py << endl; } }; int _tmain(int argc, _TCHAR* argv[]) { Point p; px = 1; py = 2; graphics_surface surface; surface.push_back(triangle()); surface.draw(p); return 0; } 

Thanks in advance.

Blair

Answer 1

A few points:

• I see no good reason to put drawable_concept or drawable_model inside graphics_surface - you simply prohibit reusing something that is potentially useful in other types of containers ...

• you have problems with const

  • draw should probably be const (and should not follow semicolon definitions ;-)

  • drawable_model(T& item) should take item with const reference

  • push_back(T& drawable) shoudl take drawable using const reference

• you should use make_shared for exception safety

• the functionality of "factory" may be better divided into a separate function, rather than enclosed inside push_back

Answer 2

Your approach here is more about erasing styles than about concept-based programming. This is an extension of the idea used by boost :: any. Concepts are a set of constraints on the type required by a class or function template. STL has concepts such as ForwardIterator and InputIterator. These are limitations that are expected to be true for parameters passed, for example, to some std algorithms.

Answer 3

The triangle and square classes must inherit from graphics_surface::drawable_concept . Read about virtual method tables .

See also wikipage in C ++ Concept .