C ++ avoids code duplication for constant and non-constant visits

Question

I have a class that should call the visitor method for each member variable. Something like that:

class A{ int a, b, c; public: void accept(Visitor &visitor){ visitor.visit(a); visitor.visit(b); visitor.visit(c); } }; 

How can I get the void accept() const method with the same code without duplicating the code?

The obvious duplication solution is to add a method:

 void accept(Visitor &visitor) const { visitor.visit(a); visitor.visit(b); visitor.visit(c); } 

This method has exactly the meaning that I want, but I would like to avoid code duplication. The reason to have both methods is the ability to read variables by the visitor of the “read” and to have the accept method beautifully const . Then the non-constant accept can be used to "record / update" visitors.

Answer 1

You can create a helper function of a static class template that will output a constant based on the type of the this pointer that you provide to it. Like this:

 class A{ int a, b, c; public: void accept(Visitor &visitor){ acceptImpl(*this, visitor); } void accept(Visitor &visitor) const{ acceptImpl(*this, visitor); } private: template<typename t_A> static void acceptImpl(t_A& aObj, Visitor &visitor) { visitor.visit(aObj.a); visitor.visit(aObj.b); visitor.visit(aObj.c); } }; 

Answer 2

Template Helper:

 class A{ int a, b, c; private: template <typename T> static void do_visiting(T &self, Visitor &visitor) { visitor.visit(self.a); visitor.visit(self.b); visitor.visit(self.c); } public: void accept(Visitor &visitor) { do_visiting(*this, visitor); // calls do_visiting<A> } void accept(Visitor &visitor) const { do_visiting(*this, visitor); // calls do_visiting<const A> } };