ITEC 320

Lecture 19Higher level usage of pointers

Pointers

Review

• Linked list– Pointer usage– Package design

• HW 1–Write a doubly linked list that has insert

before / after and delete methods– Code will be posted by tomorrow– Due next Monday at 10PM

Pointers

Problems

• What are the major problems with pointers?

Pointers

Reference counting

• Each object has to be uniquely tracked in a system

• The number of pointers to that object is also counted

• Whenever a pointer is disposed, the reference count is decreased

• What happens when it reaches 0?• What problem does this resolve?

Pointers

Smart pointers

• Contain regular pointers• Also contain number of other

pointers that are pointing to the object

• Or do they…–What if an object contains the number of

pointers to itself?– Benefits / downsides

• Designs for this

Pointers

Factories

• In your words, what are they?

Pointers

Approaches

• Function that returns an object• Function that takes an object and

returns a pointer• Function that takes an id and returns

a pointer to it• Hash map for memory addresses and

# of accesses• Others?

Pointers

Advantages/Disadvantages

• Why use factories?• What do they mean for your code?• Benefits• Downsides

Pointers

Flyweight

• What does the word bring to mind?• One object / pointer reused lots and

lots of times• Similar to NULL except it is a type /

pointer• What reasons do you think you would

use a flyweight object?• Benefits / Downsides

Pointers

Disk buffer

• How many GB does a DVD hold?• 3 DVD game• How does that game load everything

into RAM? Or does it load everything into RAM?

• Memory Mapping to HD idea

Pointers

How it works

• Several blocks of data reserved in memory

• Each block maps to a unique block on a HD

• Data is requested, it is loaded from HD – If it is in list, use it, move to MRU

position– If it isn’t, load into least recently used

block and move it to the MRU position– Linked list of blocks?

Pointers

What it enables

Pointers

Design patterns

• Not language specific features• Encompasses a particular idea• May or may not be heavily supported

in a language– Java supports smart pointers by default– C++ requires a separate class

• Not a silver bullet

Pointers

Command pattern

• Signals when an action needs to be performed

CommandCentral

Contains pointers to objectsContains list of commandsTold to execute X, it actuallydoes it

Pointers

Summary

• Memory usage at a higher level