Implementing the visitor pattern using C++ Templates

Question

I\'ve been trying to reduce the amount of boilerplate in my code, by using C++ Templates to implement the visitor pattern. So far I\'ve come up with this:

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Answer 1

This can be done in C++11 using variadic templates. Continuing from Pete's answer:

// Visitor template declaration
template
class Visitor;

// specialization for single type    
template
class Visitor {
public:
    virtual void visit(T & visitable) = 0;
};

// specialization for multiple types
template
class Visitor : public Visitor {
public:
    // promote the function(s) from the base class
    using Visitor::visit;

    virtual void visit(T & visitable) = 0;
};

template
class Visitable {
public:
    virtual void accept(Visitor& visitor) = 0;
};

template
class VisitableImpl : public Visitable {
public:
    virtual void accept(Visitor& visitor) {
        visitor.visit(static_cast(*this));
    }
};

Subclasses of Visitable:

class Mesh : public Object, public VisitableImpl {};
class Text : public Object, public VisitableImpl {};

A Visitor subclass:

class Renderer : public Visitor {};

It's not clear what the value_type of your Scene container is but you need to obtain a reference or pointer to Visitable on which to call accept:

for(Scene::iterator it = scene.begin(); it != scene.end(); ++it) {
    Visitable& object = static_cast&>(*it);
    if(pre_visit(object)) {
        object.accept(*this);
        post_visit(object);
    }
}