Visitor Pattern

Default Role Model

Participants
Collaborations

Known Implementation Models

see also abstract model

Participants

declares a Visit operation for each class of ConcreteElement in the object structure. The operation's name and signature identifies the class that sends the Visit request to the visitor. That lets the visitor determine the concrete class of the element being visited. Then the visitor can access the element directly through its particular interface.

ConcreteVisitor

implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class of object in the structure. ConcreteVisitor provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.

defines an Accept operation that takes a visitor as an argument.

ConcreteElement

implements an Accept operation that takes a visitor as an argument.

ObjectStructure

can enumerate its elements.

may provide a high-level interface to allow the visitor to visit its elements.

may either be a composite (see Composite) or a collection such as a list or a set.

Collaborations

A client that uses the Visitor pattern must create a ConcreteVisitor object and then traverse the object structure, visiting each element with the visitor.
When an element is visited, it calls the Visitor operation that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its state, if necessary.

The following interaction diagram illustrates the collaborations between an object structure, a visitor, and two elements:

Diagramm is missing.