How can I implement the visitor patter with return type in C++

Question

I want to implement the visitor pattern for one of my class without having to depend on the types that will implement the interface to visit them.

My solution was this:

class model::VisitableNode {
public:
   template<class T>
   virtual T accept(NodeVisitor<T>);
}

But C++ says that it doesn't support template + virtual methods

Node in my application will have only one implementation but if I don't use a template return type my model class will depend on the toolkit that I'm using to create the graphic for my app.

Answer 1

If a visitor needs to return a value, it is normal to store the returned value in the visitor itself. Thus:

NodeVisitor<double> dv;
node->accept(dv);
double result = dv.result();

If you don't like the boilerplate, you can wrap it in a non-virtual member:

class model::VisitableNode {
public:
   template<class T>
   /* non-virtual */ T accept(NodeVisitor<T>& v) {
      do_accept(v);
      return v.result;
   }
   virtual void do_accept(NodeVisitorBase& v) = 0;
}

Answer 2

Why not template the class itself?

template<class T>
class model::VisitableNode<T> {
public:
  
   virtual T accept(NodeVisitor<T>);
}