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Concept
Visitor pattern
Summary
A visitor pattern is a software design pattern and separates the algorithm from the object structure. Because of this separation new operations can be added to existing object structures without modifying the structures. It is one way to follow the open/closed principle in object-oriented programming and software engineering.
In essence, the visitor allows adding new virtual functions to a family of classes, without modifying the classes. Instead, a visitor class is created that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal through double dispatch.
Programming languages with sum types and pattern matching obviate many of the benefits of the visitor pattern, as the visitor class is able to both easily branch on the type of the object and generate a compiler error if a new object type is defined which the visitor does not yet handle.
The Visitor
design pattern is one of the twenty-three well-known Gang of Four design patterns
that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is,
objects that are easier to implement, change, test, and reuse.
It should be possible to define a new operation for (some) classes of an object structure without changing the classes.
When new operations are needed frequently and the object structure consists of many unrelated classes,
it's inflexible to add new subclasses each time a new operation is required
because "[..] distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."
Define a separate (visitor) object that implements an operation to be performed on elements of an object structure.
Clients traverse the object structure and call a dispatching operation accept (visitor) on an element — that "dispatches" (delegates) the request to the "accepted visitor object". The visitor object then performs the operation on the element ("visits the element").
Official source
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