65. Visitor

• Problem to solve: consider mixed hierarchy of nodes representing a messy^(*) three-dimensional scene in a graphics application.

  • ^(*)The nodes represent many unrelated classes, with no common parent class.

• How could we uniformly examine the nodes?

• We might try an Iterator pattern, which separates the algorithm from a collection of objects.

• However,

  • Iterators traverse objects only in homogeneous containers.

  • Iterators don't work with elements of unrelated data types.

  • To invoke operations via an iterator, the objects must derive from a common parent class.

  • Other iterator and container type-specific restrictions may exist.