The Visitor design pattern provides a solution:
Define a separate Visitor object that implements operations to be performed on the elements of an object structure.
Clients traverse the object structure and call accept(visitor) on each element to delegate the request to the "accepted visitor object".
The visitor object then performs the request ("visits" the element).
Describing the Visitor design in more detail is the theme of the following sections. See Applicability section for all problems Visitor can solve.

• The key idea in this pattern is to define a double-dispatch operation accept(visitor) for each Element class (see also Collaboration and Implementation).
  "This is the key to the Visitor pattern. The operation that gets executed depends on both the type of Visitor and the type of Element it visits." [GoF, p339]
• Define separate Visitor objects:
  • For all supported Element classes, define a common Visitor interface by defining a "visit" operation for each Element class (Visitor | visitElementA(e),visitElementB(e),…).
    "We'll use the term visitor to refer generally to classes of objects that "visit" other objects during a traversal and do something appropriate." [GoF, p74]
  • Define classes (Visitor1,…) that implement the Visitor interface.
• This enables compile-time flexibility (via class inheritance).
  "You create a new operation by adding a new subclass to the visitor class hierarchy." [GoF, p333]