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