OO Design and STL

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OO Design (and STL)

• CSE 399
• Mar 19, 2008

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Reminders

• Homework 4 Due today
• Exam April 2nd
• You may bring 1 page (8.5 x 11 inches) of
  handwritten notes to the exam.

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Today

• OO Design
• How to design an OO program well
• Brief intro to design patterns
• Standard Template Library
• vectors, etc
• some examples of design concepts

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First one thing about types

• If I write
• x x
• What does it mean?
• x times x you say?
• Always?
• Are you sure?

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Maybe I said

• Maybe above that I wrote
• typedef int x
• .
• Now what does this mean
• x x
• Declaration of an int pointer called x.
• You cant locally tell what is a type in C.

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Non-silly consequence

• Silly example, but real consequences.
• In templates need to be explicit what is a type
  sometimes
• templateltclass Tgt void foo(int x)
• Tx(y)
• .
• Is Tx a type gt declare variable y
• Is Tx a function gt call it with y as argument

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The rule

• If you have a qualified (with ) name that
  depends on a template parameter
• The default is to assume its not a type
• If you want a type, use typename
• typename vectorltTgtiterator it
• If you dont use typename when you need it, you
  get confusing errors

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Quotation

• Computer science is the opposite of biology
• In biology one seeks to classify things by
  their differences. In computer science one seeks
  to find the similarities.
• -- Yannis Smaragdakis

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Goals of good design

• Maintainable
• Readable
• Understandable
• Flexible
• I just need you to change one thing
• Unfortunately
• School get it to work fast, ignore design

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(Hopefully) A quick review

• Object
• noun
• encapsulates data
• with methods that act on it
• Method
• verb
• allows you to ask an object to act on itself
• vectorltintgt myVector
• myVector.push_back(7)

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Other considerations

• Classes should be re-usable/flexible
• vectorltTgt hold any type
• STL lots of templates
• A class should represent one type of thing
• Dont use monolithic design
• (everything in one big class)

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Slightly humourus story

• In my undergrad OO design class, on the practice
  test What is wrong with these classes
• .
• ProcessMap

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Speaking of vector..

• vectorltTgt similar to java vector
• has overloaded myVect1
• has push_back() to add to end
• has pop_back() to remove from end
• Supports iterators
• include ltvectorgt
• google for other functionality

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Iterator

• Iterator container position
• vectorltintgtiterator it myVect.begin()
• while (it ! myVect.end())
• int x it
• cout ltlt Current thing is ltlt x ltlt endl
• it

iterator type is inside vectorltintgt
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Many STL classes Iterator

• listltTgtiterator
• mapltkey,datagtiterator
• setltTgtiterator
• All iterators look the same
• go to next
• current item
• Decouples element access from structure

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Why iterators?

• templateltclass itgt
• int sum (it cur, it last)
• int sum 0
• while (cur ! last)
• sum cur
• cur

• Can sum iterators from any containers (of ints)
• Lists
• Maps
• Vectors
• Your own container?
• .

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STL has various algorithms

• include ltalgorithmgt
• sort
• min_element
• set_union
• count
• etc
• Almost all act on iterators
• Allows to act on any type of container

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Design pattern

• General reusable software concept
• Just saw Iterator design pattern
• Canonical book Gang of Four
• Design Patterns
• Elements of Reusable Object-Oriented Software
• Erich Gamma, Richard Helm, Ralph Johnson, and
  John Vlissides

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Abstract Factory

• Suppose we are doing something with crypto
• Different crypto algorithms (RSA, AES, etc)
• For each algorithm
• Encrypter class (handles encryption/decryption)
• KeyGenerator class (to make keys)
• AESEncrypter/AESKeyGen
• RSAEncrypter/RSAKeyGen
• Once we select an algorithm, we want to make
  instances of the proper classes in various places

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Abstract factory

• class CryptoFactory
• virtual Encrypter makeEncrypter() 0
• virtual KeyGen makeKeyGen() 0
• class AESFactory public CryptoFactory
• virtual Encrypter makeEncrypter()
• return new AESEncrypter()

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Abstract Factory in use

• CryptoFactor cf createFactoryFromUserChoice()
  
• // which returns new AESFactory()
• // or new RSAFactory()
• KeyGen g cf-gtmakeKeyGen()
• Encrypter e cf-gtmakeEncrypter()
• //use g and e as appropriate

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Opposite of biology

• Remember the quote at the start?
• Abstract Factory is a great example
• Decide once what factory to make
• After just call methods on it to make things
• Call methods on objects to do things
• Dynamic dispatch handles all of the decision
  making

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Decorator Java IO

• FileInputStream fis new FileInputStream(f)
• DataInputStream dis new DataInputStream(fis)
• Sound familiar?
• Java IO uses the Decorator pattern
• DIS wraps FIS and decorates it with new
  functionality (in this case readInt() etc).

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Tons of other design patterns

• Visitor easy to add new operations to a
  structure
• Memento restore to previous state
• Object pool Recycle rather than free/allocate
• Many many more
• Good to learn about if you plan to design large
  OO systems

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Back to vectors..

• Suppose you want to remove from a vector
• myVect.remove(x) right?
• No
• remove(myVect.begin(), myVect.end(), x)

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Why?

• STL algorithms have remove() which works on
  iterators
• None of the STL containers have a remove
• Use the remove algorithm instead
• Good or bad design choice?
• What do you think?

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Other STL things map

• mapltstring,intgt m
• mabc 24
• mdef 42
• cout ltlt abc is ltlt mabc ltlt endl
• cout ltlt xyz is ltlt mxyz ltlt endl

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Other STL things map

• mapltstring,intgt m
• mabc 24
• mdef 42
• cout ltlt abc is ltlt mabc ltlt endl
• cout ltlt xyz is ltlt mxyz ltlt endl
• Output
• abc is 24
• xyz is 0

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map returns a

• Since the returns a reference
• It cant return NULL
• So if not found it returns a default object
• T() for objects
• 0 for ints
• etc
• My opinion terrible choice
• May seem convenient, but probably leads to bugs

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A better choice exceptions

• Recall from Java exceptions indicate erroneous
  conditions
• For example requested key not found in data
  structure
• Exceptions are thrown and caught
• In C any type can be thrown/caught

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I.e. could throw an int

• try
• throw 27
• catch (int x)
• cout ltlt Just threw int ltlt x ltlt endl

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More useful example

• class myexn public exception
• virtual const char what() const throw()
• return Some info about the exception
• try
• throw myexn() // create and throw myexn
• catch (exception e) ..

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More useful example

• class myexn public exception
• virtual const char what() const throw()
• return Some info about the exception
• try
• throw myexn() // create and throw myexn
• catch (exception e) ..

Like throws in Java specifies what types this
method can throw
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More useful example

• class myexn public exception
• virtual const char what() const throw()
• return Some info about the exception
• try
• throw myexn() // create and throw myexn
• catch (exception e) ..

Catching by reference allows us to catch subtypes
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What about resources?

• void foo(int x)
• MyObject y(x) //construct on stack
• .
• if(error_condition) throw something()
• What happens to y when something is thrown?

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Stack unwind destruct

• When an exception is thrown, C behaves like
  each function returns in order (until a handler
  is found)
• Called unwinding the stack
• When each stack frame is destroyed, objects in it
  are destructed
• y will have the destructor for MyObject run to
  clean it up

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What about pointers?

• void foo(int x)
• MyObject y new MyObject(x)
• .
• if (error_condition) throw something()
• Subtle, but important difference
• y is now a MyObject pointer on stack
• object is on heap

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What about pointers?

• void foo(int x)
• MyObject y new MyObject(x)
• myGlobalList.add(y)
• if (error_condition) throw something()
• Maybe we dont even want to delete y
• Could have added object to something
• But what if we didnt?

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auto_ptrltTgt

• C has an auto_ptrltTgt template
• Construct pass newed pointer
• Destruct will delete pointer
• Copy pointer out will give up ownership
• Internal pointer NULL, wont delet
• Can retrieve a copy with get()

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auto_ptr example

• auto_ptrltFoogt x (new Foo())
• auto_ptrltFoogt y
• y x // x looses ownership (ptr is NULL)
• y-gtmethod() //call method on the Foo
• //delete the Foo when y goes out of scope

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Usefulness

• auto_ptrs save simplify resource management
• If auto ptr owns resource when exception thrown
• Auto ptr gets destructed gt delete memory
• Use T release() to get raw pointer and disown
• Use T get() to get raw pointer, not disown (be
  careful)

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Next week Review

• Next week review for the exam
• Questions?
• Concerns?
• Confusions?
• Send me email Ill review whatever you all want