some super-generic visitor API

Visitor.hpp

// This file is part of the "klex" project, http://github.com/christianparpart/klex>
//       (c) 2018 Christian Parpart <christian@parpart.family>
//
// Licensed under the MIT License (the "License"); you may not use this
// file except in compliance with the License. You may obtain a copy of
// the License at: http://opensource.org/licenses/MIT

#pragma once

#include <type_traits>

namespace klex::util {

namespace detail {
        template <const bool B, typename T = void> struct add_const_if {};
        template <typename T> struct add_const_if<true, T> {
                using type = typename std::add_const<T>::type;
        };

        template <const bool Mutable, typename T>
        class Visit {
          public:
                using Type = typename add_const_if<!Mutable, T>::type;

                virtual ~Visit() = default;

                virtual void visit(Type&) {}
                virtual void endVisit(Type&) {}
        };
}

template <const bool Mutable, typename... T>
class Visitor : public detail::Visit<Mutable, T>... {
  public:
        virtual ~Visitor() = default;

        using detail::Visit<Mutable, T>::visit...;
        using detail::Visit<Mutable, T>::endVisit...;
};

// Example:
// using ASTVisitor = util::Visitor<false, OptionalExpr, ClosureExpr, NonTerminalExpr, Terminal, Grammar>;
// using ASTRewriter = util::Visitor<true, OptionalExpr, ClosureExpr, NonTerminalExpr, Terminal, Grammar>;

}  // namespace klex::util