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

Your BaseVisitor does nothing for you, other than allowing arbitrary visitees to delete the visitor. Instead, you want to have a base class for the visitor which provides all of the different accept functions that could be called on it, and for the Visitable to accept this visitor.

To do this, you could use a type list to define the types the visitor can accept, have a base visitee class which takes the type list, and add the type list as a parameter to your visitee implementation.

sketch of example:

// assuming a typelist has typedefs first and second and a 
// type 'empty' representing end of type list

template
class Visitor : public Visitor {
public:
    // visitor has a visit function for each type in Types
    virtual void visit(typename Types::first& visitable) = 0;
};

template<> class Visitor { };

template
class Visitable{
    public:
    // base accepts a visitor which can visit any type in Types
    virtual void accept(Visitor& visitor) = 0;
};

template
class VisitableImpl : public Visitable {
public:
    // impl calls specific visit function 
    virtual void accept(Visitor& visitor) override {
        visitor.visit(static_cast(*this));
    }
};