COTS Challenges for Embedded Systems

1
E61 CS 342S Object-Oriented Software Development
The STL and the Iterator Pattern
Christopher Gill Department of Computer Science
and Engineering Washington University, St.
Louis cdgill_at_cse.wustl.edu
Thanks to Jim Wang, Peng Wang
2
What is an Iterator?

• GoF book suggests an interface
• First (), Next(), IsDone(), CurrentItem()
• However, intent they give is more essential
• Provide a way to access the elements of an
  aggregate object sequentially without exposing
  its underlying representation
• Having different representations
• means aggregate objects may have different
  semantics for the interface above (some trivial)
• Well consider interface methods above with
• istream, ostream, array, linked list, bi-linked
  list

3
Syntax and Semantics

• Syntax governs how interface is written
• First () Next() IsDone() CurrentItem()
• p A0 p p A gt 3 p
• Semantics governs what implementation does
• Move to start, move forward one position, test
  completion, return item at current position
• Compilers can only check syntax (C, Java)
• The programmer must address semantics
• Including memory allocation, safe aliasing, etc.
• Is it bad to implement Next () as --p ?

4
A Few Key Ideas to Consider

• Difference Type
• Type for distance between two iterators i1 and
  i2
• E.g., ptrdiff_t
• Reference, Value Types
• For T p, value type is T, p normally returns T
  
• For const T p, value type is const T, p gives
  const T
• Iterator Category
• What concept(s) it models

how far? units?
how far? units?
5
Review Iterator Concept Hierarchy
destructive read at head of stream
write to stream
read/write once
Input Iterator
Output Iterator
Linked-list style access
Forward Iterator
value persists after read/write, values have
locations, can express distance between two
iterators
Bi-linked-list style access
Bidirectional Iterator
Array/buffer style access
Random Access Iterator
6
Iterator Concepts Expressions, Models

• Input Iterator (read from a stream/container)
• i (destructive), i t, i-gtm, i, i , i
• istream_iterator, most other STL iterators
• Output Iterator (write to a stream/container)
• X y(x), X yx, y x, x t, x, x , x t
• front_insert_iterator, back_insert_iterator,
  insert_iterator, ostream_iterator
• How are First(), Next(), IsDone(), CurrentItem()
  implemented for each of these?

7
Iterator Concepts Expressions, Models

• Forward Iterator (moves in one direction)
• Can pass same forward iterator in multiple args
• Dereference does not move the iterator
• X x, X(), i, i , i (non-destructive) in
  addition to the expressions for Input Iterator
  and Output Iterator
• slistltintgtiterator, slistltdoublegtconst_iterato
  r, hash_setltstringgtiterator
• Bidirectional Iterator (moves both ways)
• Forward Iterator expressions,--i, i--
• listltintgtiterator, setltstringgtiterator
• How are First(), Next(), IsDone(), CurrentItem()
  implemented for each of these?
• Can we do more than with Input/Output Iterator?

8
Iterator Concepts Expressions, Models

• Random Access Iterator (constant time index)
• Concept fully expresses C pointers
• Bidirectional Iterator expressions and in, in,
  ni, i-n, i-n, n-i, i-j, iltj, in, int
• Question we can express the distance between two
  Forward Iterators, so why dont those iterators
  have i-j, iltj ?
• vectorltintgtiterator, dequeltintgtiterator, char
  
• How are First(), Next(), IsDone(), CurrentItem()
  implemented for each of these?
• Can we do more than with Forward/Bidirectional ?

9
Conclusions

• What really matters with a pattern is
• Its intent (the problem it solves)
• Its context (when and how it applies)
• The consequences of different solutions
• Comparing Iterator pattern and STL iterators
• Gives a deeper understanding of both
• Offers clues about implementation using STL
• A design and implementation approach
• Design your solution according to given problem
• Keep track of the patterns used in your design
• Map design patterns into libraries/frameworks
• Write code only when you have to