Flexible and efficient variation of the Visitor design pattern in C++ that accommodates arbitrary argument and return types.
This implementation is based on Anand Shankar Krishnamoorthi's:
Cooperative Visitor: A Template Technique for Visitor Creation
Cooperative Visitor is a template-based technique that allows: <br>
- Flexibility of acyclic visitor with performance close to cyclic visitor
- Easier to use, maintain and modify
- Better semantic names rather than "visit"
- Easy creation of const/non-const visitors
- Flexible return and argument types
- Dynamic configuration of visitation behavior
- Implementing a default handler
- Developing template code upon visitor
- Fallback to nearest ancestor
- Custom signature for 'visit' methods
This implementation uses C++11 and replaced the "Loki" TypeList by C++11 variadic templates. <br>
Only one virtual method is needed for the visitation (dispatching is performed via a custom virtual table (more information in the link above)).
Some efforts have been made to simplify the Visitor utilisation via some macro helpers.
Example:
#include <cassert>
#include <Visitor.hpp>
#include <Visitable.hpp>
class Shape : public Visitable<Shape>
{
public:
META_BaseVisitable(Shape)
};
class Circle : public Shape
{
public:
META_Visitable(Circle, Shape)
};
class Polygon : public Shape
{
public:
META_Visitable(Polygon, Shape)
};
class PolyPolygon : public Polygon
{
public:
META_Visitable(PolyPolygon, Polygon)
};
class PolyPolyPolygon : public PolyPolygon
{
public:
META_Visitable(PolyPolyPolygon, PolyPolygon)
};
class ShapeVisitor : public Visitor<Shape, bool, float, std::string const &>
{
public:
META_Visitor(ShapeVisitor, draw)
ShapeVisitor()
{
META_Visitables(Shape, Circle, Polygon);
}
protected:
bool draw(Shape & shape, float, std::string const &)
{
return false;
}
bool draw(Circle & circle, float, std::string const &)
{
return true;
}
bool draw(Polygon & polygon, float, std::string const &)
{
return true;
}
};
int main(int argc, char const ** argv)
{
Shape shape;
Circle circle;
Polygon polygon;
PolyPolygon pp;
PolyPolyPolygon ppp;
ShapeVisitor sv;
bool b;
b = sv(shape, 1.0f, "shape"); assert(!b);
b = sv(circle, 2.0f, "circle"); assert(b);
b = sv(polygon, 3.0f, "polygon"); assert(b);
b = sv(pp, 4.0f, "pp"); assert(b); // Fallback to ShapeVisitor::draw(Polygon &, ...)
b = sv(ppp, 5.0f, "ppp"); assert(b); // Fallback to ShapeVisitor::draw(Polygon &, ...)
return 0;
}
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