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Iterator Intent – Provide a means to Iterate through a series of nodes in specific way(s) Motivation – To traverse lists of information without having to know anything too specific about the objects – To implement multiple algorithms to traverse those nodes.
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Behavioural Patterns
GoF pg. 221 -222Iterator GoF pg. 257 – 271Memento GoF pg. 283-291
By: Dan Sibbernsen
Overview
• Behavioural – Concerned with algorithms and responsibilities
among objects.– Also concerned with methods of communication
between them– Allow you to put your “interconnected” hat on
while taking off your “control” hat.
Iterator
• Intent– Provide a means to Iterate through a series of
nodes in specific way(s)• Motivation– To traverse lists of information without having to
know anything too specific about the objects– To implement multiple algorithms to traverse
those nodes.
Class Diagram
Iterator:Implementation
• Who Controls the Iteration?– External Iterator• Client controls iteration• Client must explicitly request next() on each node
– Internal Iterator• Iterator controls iteration• Client hands an operation to the Iterator, Iterator tells
every child in the aggregate to perform it.
Implementation Benefits
• External Iterator– More flexible than Internal. Can compare 2
collections easily• Internal Iterator– Easier to use, as they define the iteration logic for
you. Makes portability easier.
Who defines the traversal Algorithm?
• Algorithm can be held by a class outside of the Iterator. It could be held by the aggregate can hold it.– Called “the cursor.”– Allows the algorithm to access private members
of the aggregate (and not break encapsulation)• Algorithm is held by the iterator– Allows algorithms to be swapped out easily.
How Robust is the Iterator?
• If an iterator is robust it– Will allow for removal of nodes in the middle of
traversal.– It does this through registering the iterator with
the aggregate. If any nodes are removed, it is notified.
Additional Iterator Operations
• All come with first, next, isdone, and currentItem
• Previous– Goes to the previous node that was iterated– Can be helpful in certain situations
• SkipTo– Skips to a certain node.– Goes to a node matching specific criteria
Using Polymorphic Iterators in c++
• Drawbacks– Allocated dynamically by a factory method.– Client is responsible for deleting them. Can be
error prone due to multiple levels of calls, and also because of exceptions• Proxy can be used to remedy this.
• Only use them when Polymorphism is required.
Iterators may have privileged access
• Makes coupling tighter between Iterator and Aggregate
• Can be problematic if there are multiple iterators you want to apply– Solution: make private iterator-specific functions
“protected” so that only Iterator subclasses can access them
Iterators for composites
• Use an Internal iterator to keep track of the levels-deep it has traversed. This happens recursively so everything is stored on the stack.
Null Iterators
• Makes traversal of Composite classes easier.• Always returns IsDone() as true.
Related Patterns
• Composite• Factory Method• Memento
Consequences (good)
• “Supports variations in the traversal of an aggregate.”
• “Iterators simplify the Aggregate interface.”• “More than one traversal can be pending on
an aggregate.”
Memento
• Intent– “Without violating encapsulation, capture and
externalize an object’s internal state so that the object can be restored to this state later.”
• Motivation– When we want to store off an object’s internal
state without adding any complication to the object’s interface.
– Perhaps for an undo mechanism
Class Diagram
Undo Mechanism Complications
• Constraint-Solver problem– Uses multiple levels deep of objects to draw
objects.– What if we want to support undo but can’t
localize the changes to just one class?– Propose a Memento that gathers information
from objects contained in a certain class
Applicability
• Use this– when you want to save state on a hierarchy’s
elements.– When the hierarchy’s interface would be broken if
implementation details were exposed.
Participants
• Memento– stores the state of the Originator
• Originator– Creates the memento– “Uses the memento to restore its internal state”
• CareTaker– Keeps track of the Memento– Never uses the Memento’s Interface to the
Originator
Collaboration
• Pg. 286 (can’t find image)• Caretaker requests a memento from an
Originator.• Caretaker passes back memento.• Originator uses it to restore state.
Consequences (good)
• “Preserves Encapsulation Boundaries”• “It simplifies Originator”• You can name your class
“mementos_TheFreshMaker”!!!
Consequences (bad)
• Might be expensive• Difficulty defining interfaces to keep
Originator encapsulated• Hidden costs in caring for mementos– Caretaker could have to keep track of a lot of
information for the memento• You can name your class
“mementos_TheFreshMaker” =(
Implementation
• Language Support– C++ lets you call Originator a friend of Memento,
thus giving it access to all private members of Memento that it needs
Storing Incremental Changes
• If storing state happens incrementally, then we can just record the changes of what’s happened in a new memento object.
• This helps with memory difficulties.
Related Patterns
• Command• Iterator