musteresel / climbing-acyclic-visitor

An acyclic visitor that climbs up a class hierarchy to find a matching method to call.

climbing-acyclic-visitor

An acyclic visitor that climbs up a class hierarchy to find a matching method to call.

Usage

The code is written such that you only have to include the header file found in the include directory. The relevant classes are in the cav namespace, so for demonstration purposes it's probably the best to just use that entire namespace:

#include "climbing-acyclic-visitor.hpp"
using namespace cav;

Making classes visitable

For a class to be visitable, it must declare a "parent visitable type" parent_visitable (to which it must be castable, too) and inherit from the is_visitable template class, providing itself as template parameter.

So to declare the root of a hierarchy as being visitable, the following is sufficient:

class root : /* possibly other superclasses */ public is_visitable<root>
{
  public:
    using parent_visitable = base_visitable;
    // ...
};

In this case root implicitely inherits from its "parent visitable type" through the is_visitable template class, so there is no need to directly derive from base_visitable. The class root now has an method accept through which it can be fed to a visitor class.

To declare a sub class as being visitable, there is the additional "burden" of explicitely declaring the accept method to use the correct accept method from the is_visitable parent (and not the one from the "real" parent class):

class sub : public root, public is_visitable<sub>
{
  public:
    using parent_visitable = root;
    void accept(base_visitor & visitor)
    {
      is_visitable<sub>::accept(visitor);
    }
};

Now, a visitor may define a special behaviour for the sub class. The advantage compared to a regular acyclic visitor is, that the visitor doesn't NEED to define a special behaviour. In case no special behaviour is defined, an object of type sub is handled the same way as an object of type root.

Further subclassing works analogous.

Of course, one can also define a "regular" sub class of root:

class regular_sub : public root
{
  // ... anything ...
};

regular_sub now inherits the accept method from its parent, but a visitor may not define special behaviour on regular_sub. Further subclassing regular_sub is a bit special: One must not declare regular_sub as parent_visitable (because it's not a "real" visitable) but instead use the next parent that really is a "visitable":

class sub_regular_sub : public regular_sub, public is_visitable<sub_regular_sub>
{
  public:
    using parent_visitable = root; // Important to choose the right class here!
    void accept(base_visitor & visitor)
    {
      is_visitable<sub_regular_sub>::accept(visitor);
    }
};

One additional note here: One can also declare a sub class as being visitable, but "skipping" some parent visitable classes or start a complete new visitable hierarchy:

class subsub : public sub, public is_visitable<subsub>
{
  public:
    using parent_visitable = root; // Skip sub, i.e. don't let a visitor threat this as a sub.
    void accept(base_visitor & visitor)
    {
      is_visitable<subsub>::accept(visitor);
    }
};
class subsub_new_hierarchy : public sub, public is_visitable<subsub_new_hierarchy>
{
  public:
    using parent_visitable = base_visitable; // Threat this as the root of a visitable hierarchy.
    void accept(base_visitor & visitor)
    {
      // But must resolve ambiguities!
      is_visitable<subsub_new_hierarchy>::accept(visitor);
    }
};

The other side: Defining a visitor

To define a visitor, one must derive from the template class can_visit, giving the class the visitor is able to visit as template parameter. One must then provide an implementation of the visit method with one parameter (whose type is the same as the template parameter):

class some_visitor : public can_visit<root>,
                     public can_visit<sub_regular_sub>,
                     public can_visit<sub>
{
  public:
    void unknown_visitable(base_visitable & bv)
    {
      // handle it somehow
      std::cout << "unknown" << std::endl;
    }
    void visit(root & r)
    {
      // do something ...
      std::cout << "Is root" << std::endl;
    }
    void visit(sub & s)
    {
      // do something ...
      std::cout << "Is sub" << std::endl;
    }
    void visit(sub_regular_sub & srs)
    {
      // do something different ...
      std::cout << "Is sub_regular_sub" << std::endl;
    }
};

Putting it together

The let a visitor visit a visitable, the visitable must accept the visitor:

some_visitor my_visitor;

root r;
sub s;
subsub subs;
regular_sub rs;
sub_regular_sub subrs;
subsub_new_hierarchy subs_nh;


r.accept(my_visitor);        // Is root
s.accept(my_visitor);        // Is sub
subs.accept(my_visitor);     // Is root
rs.accept(my_visitor);       // Is root
subrs.accept(my_visitor);    // Is sub_regular_sub
subs_nh.accept(my_visitor);  // unknown