8 June 2005
A truly decoupled visitor
From the Design Patterns [Amazon] book to Modern C++ Design [Amazon], I've been frustrated that the technique I use to implement the visitor pattern is not offered or even refuted. I'm either on to something or, quite the opposite, off of something. Or maybe simply on something.
The visitor pattern describes how to act upon collections of objects in a generic manner. Collection elements contain a single function that accept a visitor class and pass themselves to that class when called. Visitor classes accept elements and act upon them depending on their type. A separate function (usually in the owner of the collection) performs the iteration.
// Have the document expand each element by 1 pica
Document::Expand()
{
ExpandVisitor v;
for each element
{
element.Accept(v);
}
}
If an element cannot accept the visitor (e.g. perhaps a watermark would not be expanded), the visitor skips it. This pattern requires alteration of the elements themselves, so even with Alexandrescu's elegant and generic template approach it's a little intrusive.
I still use the variation I came up with in my first implementation of the visitor pattern, and although programmers often fail to unlearn bad habits, I feel that it's a more useable solution. Because I generally am not able to edit either the elements or the owner of the collection, I create a traversal class that accepts visitor-derived classes. The traversal class performs the element iteration, and passes each element to the visitor class.
// Construct with a reference to the container.
Traversal::Traversal(Container & c)
Traversal::Process(Visitor & v)
{
for each element in c
{
v.Visit(element);
}
}
New actions are created (as with the standard implementation) by deriving from the base visitor class. The traversal class, once written, is static.
- Techniques after using Swift for a month posted by sstrader on 26 August 2015 at 11:41:51 PM
- Some thoughts on Ruby after finishing a week (minus one day) of training posted by sstrader on 15 December 2011 at 8:59:30 PM
- Links on the singleton pattern posted by sstrader on 9 December 2011 at 9:19:50 AM
- Phonebot posted by sstrader on 29 October 2011 at 6:37:05 PM
- The labeled break in Java posted by sstrader on 4 September 2011 at 11:27:56 AM