Hierarchical Visitor Pattern Examples In Real Life

Yes, that is indeed one attribute of the visitor pattern, although I’ve often seen it being used when late extension was a very rare use-case. Again – as mentioned in the conclusion – the visitor pattern is OK if you have many visitors (e.g. Film semi sub indo. By “late” extension) with a simple data structure.

@KonradRudolph thanks for that. Noting though, its not addressed explicitly in Patterns or the wikipedia article for instance. I don't disagree with you, but you could argue there are benefits to using a case stmt too so its strange its not generally contrasted: 1. You don't need an accept() method on objects of your collection. The ~visitor can handle objects of unknown type. Thus the case stmt seems a better fit for operating on object structures with a changeable collection of types involved. Patterns does concede that the Visitor pattern is not well suited to such a scenario (p333).

– Feb 17 '14 at 3:05 •. @SamPinkus konrad's spot on - that's why virtual like features are so useful in modern programming languages - they are the basic building block of extensible programs - in my opinion the c way (nested switch or pattern match, etc depending on your language of choice) is far cleaner in code that doesn't need to be extensible and I have been pleasantly surprised to see this style in complicated software like prover 9. More importantly any language that wants to provide extensibility should probably accommodate better dispatch patterns than recursive single dispatch (i.e. – Feb 17 '14 at 10:31. Everyone here is correct, but I think it fails to address the 'when'.

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First, from Design Patterns: Visitor lets you define a new operation without changing the classes of the elements on which it operates. Now, let's think of a simple class hierarchy. I have classes 1, 2, 3 and 4 and methods A, B, C and D. Lay them out like in a spreadsheet: the classes are lines and the methods are columns. Now, Object Oriented design presumes you are more likely to grow new classes than new methods, so adding more lines, so to speak, is easier. You just add a new class, specify what's different in that class, and inherits the rest.

Sometimes, though, the classes are relatively static, but you need to add more methods frequently -- adding columns. The standard way in an OO design would be to add such methods to all classes, which can be costly. The Visitor pattern makes this easy. By the way, this is the problem that Scala's pattern matches intends to solve. The Visitor design pattern works really well for 'recursive' structures like directory trees, XML structures, or document outlines.

A Visitor object visits each node in the recursive structure: each directory, each XML tag, whatever. The Visitor object doesn't loop through the structure. Instead Visitor methods are applied to each node of the structure. Here's a typical recursive node structure. Could be a directory or an XML tag. [If your a Java person, imagine of a lot of extra methods to build and maintain the children list.] class TreeNode( object ): def __init__( self, name, *children ): self.name= name self.children= children def visit( self, someVisitor ): someVisitor.arrivedAt( self ) someVisitor.down() for c in self.children: c.visit( someVisitor ) someVisitor.up() The visit method applies a Visitor object to each node in the structure. In this case, it's a top-down visitor.

You can change the structure of the visit method to do bottom-up or some other ordering. Here's a superclass for visitors.