Discussion Session 3: Generic Programming, Templates, Containers and Iterators

1
Discussion Session 3 Generic Programming,
Templates, Containers and Iterators

• CS 225 Data Structures Software Principles

2
Templates
3
Swapping integers

• void swap (int a, int b)
• int tmp a
• a b
• b tmp

4
Swapping characters

• void swap (char a, char b)
• char tmp a
• a b
• b tmp
• (repeat for types float, double, String, ?)

5
Generic swapping

• We could just copy/paste. Why not?
• Code reuse is a Good Thing
• Waste of our time this is easy to automate
• It would be better if we could just specify a
  generic type for swap ()

6
Templates

• Provide a way to re-use source code
• Two kinds of templates
• Class Templates
• Function Templates
• The compiler generates specific code from
  templates template instantiation
• Using templates just means learning new syntax

7
Function Template swap( )

• template lttypename Tgt
• void swap (T a, T b)
• T tmp a
• a b
• b tmp
• Use swap(foo1, foo2)

8
Class Template SimpleArray

• See Array wrapper class at /homesta/cs225/src/libr
  ary/03-cgeneric/_simpleGenArray
• Declaration syntax
• templatelttypename Etypegt
• class SimpleArray
• //
• Etype operator (int index)
• //
• Etype data // Pointer to array elements.

9
Class Template SimpleArray

• Definition syntax
• templatelttypename Etypegt
• SimpleArrayltEtypegtSimpleArray ()
• Instantiation syntax
• SimpleArrayltintgt intArray

10
SimpleArray vs. IntArray

• Syntax changes
• Replace int with Etype
• Extra template syntax
• SimpleArray is not a type SimpleArrayltgt is
• SimpleArrayltintgt
• SimpleArrayltStringgt
• An array of Etypes is stored with a pointer to
  Etype
• Memory is allocated using new Etype
• Check _runThisCode for examples of use

11
Containers and Iterators
12
Containers

• A container is an object that holds other objects
• Examples Arrays, Vectors, Lists
• The C Standard Library (STL) was designed so
  that containers have a standard interface
• In general, you can add objects to containers, or
  remove objects from containers
• Iterators provide a general way to access the
  stuff inside containers

13
Iterators

• Iterators provide a way to access elements inside
  a container
• Abstraction of the idea of a pointer to an
  element in a sequence
• Iterators act as an interface between containers
  and algorithms they hold them together by
  providing an abstract view of the data

14
Categories of Iterators

• Based on access types
• Input
• Output
• Forward
• Bidirectional access
• Random access
• In CS225, well use mainly Bidirectional access
• iterators

15
Iterator Categories and Operations
16
Common Functions for (almost) all STL Containers
Functions that return Iterators
begin( ) Points to first element end(
) Points to one-past-last
element rbegin( ) Points to first
element of reverse sequence rend( )
Points to one-past-last element of reverse
sequence
Element Access
front( ) First element back( ) Last element
Subscripting, unchecked access at(
) Subscripting, checked access
17
Using Iterators

• templatelttypename Tgt
• void print_all(T c)
• Titerator i
• cout ltlt ""
• for (i c.begin() i ! c.end() i)
• cout ltlt i
• cout ltlt ""

18
Example IntArrayIter
19
Jasons IntArrayIteriterator class

• /homesta/cs225/src/library/03-cgeneric/_intIter
• // nested inside class IntArrayIter
• class iterator
• public
• iterator()
• int operator()
• IntArrayIteriterator operator() //
  prefix
• IntArrayIteriterator operator--() //
  prefix
• bool operator(IntArrayIteriterator
  const origVal) const
• // note not as full-featured as an STL iterator
  implementation
• private
• int ptr
• friend class IntArrayIter // so
  IntArrayIter can set our private pointer
• Iterator implementation just a wrapper around a
  pointer

20
Jasons IntArrayIter class

• class IntArrayIter
• public
• class iterator / see previous slide /
• //
• // same interface as the IntArray class, plus
• IntArrayIteriterator begin() // returns
  itor to start of array
• IntArrayIteriterator end() // returns itor
  to one-past-the-end
• private
• int lowerBound // lower bound
• int upperBound // upper bound
• int data // pointer to the start of the
  array elements.
• int onePastLast // pointer to the memory
  one past end of array
• end() is particularly useful when looping tells
  us when were done
• Understand implementations of these functions
  the iterator class!

21
IntArrayIter summary

• Things to Note
• The iterator class is nested in the IntArrayIter
  class
• We implement the iterator with an int
• How we implement the member functions via simple
  pointer manipulation
• The implementations of begin( ) and end( )
• The IntArray class has onePastEnd as a member

22
The Vector class
23
STL Class vector

• STL container that stores objects in a sequence,
  with lower bound at 0
• Auto-expanding array (unlike the Array classes)
• Does this behind the scenes as necessary when you
  add elements with push_back() or insert()
• Runtime complexity amortized constant time for
  access
• Has 4 iterator types. Know how they are used.
• iterator, reverse_iterator, their const versions

24
STL Class vector
begin() / rend()
end() / rbegin()
2
5
4
6
1
start
finish
onePastEnd
vectorSize 5 vectorCapacity 8
PRIVATE

• vectorSize how many elements the vector holds
• vectorCapacity how many elements the vector
  could hold before expanding

25
The (modified) STL Vector class

• 4 iterators
• iterator, reverse_iterator, const versions
• Element access
• unchecked access
• at( ) checked access
• Rapid access, but expensive insert/delete (except
  for push_back( ) and pop_back( ) )

26
iterator class in vector

• class iterator // iterator class nested inside
  vector class
• public
• // Bunch of typedefs, just like vector.
  Useful when writing generic functions
• iterator() // initalized to point to NULL
• reference operator()
• pointer operator-gt()
• iterator operator() // prefix
• iterator operator--() // prefix
• iterator operator(int ignore) // postfix
• iterator operator--(int ignore) // postfix
• bool operator(iterator const origVal) const
• bool operator!(iterator const origVal) const
• private
• pointer ptr
• iterator(pointer assignPtr)
• friend class vectorltvalue_typegt // so vector
  can set private members
• friend class vectorltvalue_typegtconst_iterator
  // similarly

27
Function Objects
28
Function Objects (Functors)

• Class objects that overload the ( ) operator for
  implementing desired behavior
• also ordinary function is a function object
• and so is a function pointer
• See Jasons _genericFns/ code!

class Negate public int operator() (int n)
return -n
void Callback(int n, Negate neg) int val
neg(n)
29
A Functor Example

• templatelttypename Tgt class Sum
• private
• T res
• public
• Sum() res 0 // init
• void operator( ) (T x) res x
  // accumulate
• T result( ) const return res //
  return sum
• void getSum(vectorltdoublegt dVec)
• Sumltdoublegt s
• for(iterator iter dVec.begin() iter !
  dVec.end() iter)
• s(iter)
• cout ltlt "The sum is " ltlt s.result() ltlt endl

30
More Examples

• /homesta/cs225/src/library/03-cgeneric/_genericF
  ns
• main.cpp
• 29-47 one way to load data into a vector
• 50-60 iterator loop you saw in class (almost)
• 62-78 calls to print() with different formatting
  functors
• 80-95 print a subset dont have to use
  begin(), end()
• 101-126 more functor fun with printRelevant()
• 126 same stuff, but with the Array class

31
Generic Programming
32
Generic Programming

• Decide which algorithms you want parameterize
  them so that they work for a variety of suitable
  types and data structures
• Containers that are templated so they can hold
  elements of any type
• Generic algorithms which operate on containers
  using iterators and assume nothing about the type
  of object they contain.
• Examples copy( ), sort( ), search( ),
  partition( ),
• How do templates, functors, iterators, etc.
  support these ideas?

33
Summary

• Templates
• IntArray vs. SimpleArray
• Containers and Iterators
• Vector Class
• Function Objects
• Generic Programming
• From an EWS machine
• /homesta/cs225/src/library/03-cgeneric/