Home » 2019 » October

Monthly Archives: October 2019

Templated implementation of the Visitor Pattern for C++11

14th October 2019 / Leave a comment

This is an update to the previous post about templating the Visitor pattern.
Templated implementation of the Visitor Pattern for C++03

I won’t repeat the bulk of the post as the examples will be identical.

The new code uses ‘variadic temples’ that were introduced with C++11, and as such, greatly simplifies the code required.

Visitable

  //*****************************************************************
  /// The visitable class for N types.
  //*****************************************************************
  template <typename T1, typename... Types>
  class Visitable : public Visitable<T1>, public Visitable<Types...>
  {
  public:

    using Visitable<T1>::Accept;
    using Visitable<Types...>::Accept;
  };

  //*****************************************************************
  /// The specialised visitable class for 1 type.
    //*****************************************************************
  template <typename T1>
  class Visitable<T1>
  {
  public:

    virtual void Accept(T1&) = 0;
  };
Visitiable

There, that’s it! It is able to handle any number of visitor types and doesn’t require N specialisations of the template.
The change to Visitor is just as radical.

Visitor

  //*****************************************************************
  /// The visitor class for N types.
  //*****************************************************************
  template <typename T1, typename... Types>
  class Visitor : public Visitor<T1>, public Visitor<Types...>
  {
  public:

    using Visitor<T1>::Visit;
    using Visitor<Types...>::Visit;
  };

  //*****************************************************************
  /// The specialised visitor class for 1 type.
  //*****************************************************************
  template <typename T1>
  class Visitor<T1>
  {
  public:

    virtual void Visit(T1&) = 0;
  };
Visitor