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COSC3557: Object-Oriented Programming

Haibin Zhu, Ph. D.

Professor of CS, Nipissing University

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Lecture 9

Container classes & STL

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Contents General principles about Containers

Substitution/Downcast/Overriding/Generic Object-Based/Template based

C++ STL Containers Iterators Algorithms

Function Object: Everything is an object!

C++ Container classes List Map

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Containers in Dynamically Typed Languages

Collection classes are simple to write in dynamically typed languages, Since all variables are polymorphic. Containers simply hold values in variables (which can hold anything), and when they are removed from the container they can be assigned to any variable.

Dynamically typed languages have historically come with a rich set of container classes in their standard library.

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Tension between Strong Typing and Reuse

The situation is very different when we move to statically typed languages. There, the strong typing can get in the way of software reuse.

type Link = Record value : integer nextelement : ^ Link end; What happens when we need a linked list of

doubles? or of a user defined type?

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Can OO Techniques Solve This

Can we bring any of the OO techniques (subsitution, polymorphism) into the solution of this problem? We examine three techniques:

Using Substitution and Downcasting Using Substitution and Overriding Using Templates (or Generics)

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Using Substitution and Downcasting

In Java and other languages, (almost) all values can be stored in a variable of type Object.

Therefore, we write containers that store Object values. Problem. Requires a cast when a value is removed from the

container. Vector aVector = new Vector(); Cat Felice = new Cat(); aVector.addElement(Felice); ... // cast used to convert Object value to Cat Cat animal = (Cat) aVector.elementAt(0); Worse problem, typing errors are detected when a value is

removed from the collection, not when it is inserted.

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Generic Algorithms versus Encapsulation

Object-Oriented programming holds encapsulation as an ideal, bundling all actions in one place. It can make for ``fat'' classes.

The STL separates data structures and algorithms. The data structures are relatively small. The algorithms are many. They can be mixed and matched.

It allows for a much smaller class library.

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Using Substitution and Overriding

In certain situations we can eliminate the downcast, replacing it with overriding.

This only works, however, if you can predict ahead of time what operations you want to do on a collection.

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An Example, Java Window Events

A good example is the way that window events are handled in Java. Each window maintains a list of listeners, each listener must implement a fixed interface (WindowListener).

public class CloseQuit extends WindowAdapter { // execute when the user clicks in the close box public void windowClosing (WindowEvent e) { System.exit(0); // halt the program } } When an event occurs the window simply runs down the list of

listener, invoking the appropriate method in each. No downcasting required.

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A Third Alternative, Generics Generics, as we described in the last chapter, are a third approach. template <class T> class List { public: void addElement (T newValue); T firstElement (); private: Link * firstLink; private class Link { // nested class public: T value; Link * nextLink; Link (T v, Link * n) : value(v), nextLink(n) { } }; }; Allow for both strong typing and reuse.

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Collection Traversal

Here is another difficult problem. How to you allow access to elements of a collection without exposing the internal structure?

Consider the conventional solution: var aList : List; (* the list being manipulated *) p : Link; (* a pointer for the loop *) begin ... p := aList.firstLink; while (p <> nil) do begin writeln (p.value); p := p^.nextElement; end; Needed to expose the link type, and the field nextElement. Can we avoid this problem?

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Two Solutions There are two common solutions:

· Create an iterator; an object that facilitates access to elements and enumeration over the collection.

· The visitor. Bundle the action to be performed as an object, and pass it to the collection.

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Iterator

An iterator is an object that has two major responsibilities: Provide access to the current element Provide a way to move to the next element

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Alternative, the Visitor An alternative to iterators is the idea of a visitor.

Requires the ability to bundle the action to be performed into an object, and hand it to the collection.

This is most common technique used in Smalltalk aList do: [:x | ('element is' + x) print ].

The block is passed as argument to the list, which turns around and executes the block on each element.

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List A set of sequentially organized

elements. A specific type of graph, where each node

except the first has a single preceding node, and each node except the last has a single following node.

Contains 0-n nodes. Implementation: array and linked list.

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How to have lists with different types

By template With the same structure, we can have

integer, float, or character string as a value of a node.

But, can you have a list with values in different types by C++?

Yes! But with the same base class.

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Template-based List

head

infonext

tail

Same class

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Tnodetemplate <class T>class Tnode { friend class TList<T>;public: Tnode():next(0){ } Tnode( const T & val ); Tnode<T> * Next() const; friend ostream & operator <<(ostream & os, const Tnode<T> & N);private: T value; // data stored in node Tnode * next; // points to next node};

value next

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TList template <class T> class TList { public: TList ); ~TList ); int Advance ); // Return 0 if current is already at the end of the list; //otherwise, current will point

to the next node and return 1. void Append const T & nodeVal ); // Add a new node to the end //of the list. void Clear ); // Remove all nodes. T Get ) const; // Get the data at the current position. void GoLast ); // Set current to the last node in the list. void GoTop ); // Set current to the header node. int AtEnd ); // return true if at end. void InsertAfter const T & nodeVal ); // Insert new node after current one. int IsEmpty ) const; // Return 1 if the list is empty; otherwise,return 0. void Prepend const T & nodeVal ); // Insert a node at the beginning of the list. void Replace const T & newVal ); // Replace the data in the current node. private: Tnode<T> * head; // head node Tnode<T> * tail; // tail node Tnode<T> * current; // current node };//Tlist.cpp

headtailcurrent

value next

value null

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Object-Based List

In this section, we will introduce the method to design a List not based on template.

Also, we implement a linked list that means, every node in the list has links to the previous one and the next one.

data next

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Template-based List

head

infonext

tail

Different classes

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The structure

ONode

Student Faculty Adminstrator. . . . .

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ONodeclass ONode { friend class OList;public: ONode(); virtual ~ ONode() { } ONode * Next() const; // Return pointer to next node. virtual int operator ==( const ONode & N ) const = 0; friend ostream & operator << (ostream & os, const ONode & N ); friend istream & operator >> (istream & inp, ONode & N );private: virtual void printOn( ostream & os ) const = 0; virtual void readFrom( istream & is ) = 0; ONode * next; // pointer to next node};

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OListclass OList {public: OList(); ~OList(); int Advance(); // Return 0 if current is already at the end of the list; //otherwise, current

will point to the next node and return 1. void Append( const ONode & nodeVal ); // Add a new node to the end of the list. void Clear(); // Remove all nodes. ONode Get() const; // Get the data at the current position. void GoLast(); // Set current to the last node in the list. void GoTop(); // Set current to the header node.void InsertAfter( const ONode & nodeVal ); // Insert new node after current one. int IsEmpty() const; // Return 1 if the list is empty; otherwise,return 0. void Prepend( const ONode & nodeVal ); // Insert a node at the beginning of the list. void Replace( const ONode & newVal ); // Replace the data in the current node. friend ostream & operator <<(ostream &, const OList &);private: ONode * head; // dummy head node ONode * tail; // dummy tail node ONode * current; // current position};

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What is the STL? The STL is a rich collection of standard

data structures, recently added to C++ Will elevate data structures to the level

of, say, the I/O stream package (i.e., something taken for granted)

In many ways, it is an beyond object-oriented design, yet it is powerful and it works.

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A very brief social history

The C++ Standard Library == STL + other stuff e.g., auto_ptr

The STL is primarily the design of one person, Alexander Stepanov, many have worked on it subsequently

Stepanov was a colleague of Bjarne Stroustrup at AT&T Research in the 1980s. Later he moved to HP and then SGI, taking the STL “ownership” with him.

Stroustrup liked the STL and lobbied for its inclusion in the 1998 ANSI standardization of C++.

The requirements on an implementation of the STL (as imposed by the ANSI standard) are tough; any correct and (compliantly) efficient implementation is very complex.

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Design Philosophy A collection of useful, efficient, type-safe, generic

containers A container has (almost) no understanding of the element

type, Exception: can-be-sorted via some operator< Each container should define its own iterators.

A collection of useful, efficient, generic algorithms that operate on iterators “Generic” => algorithm knows nothing about the structure it’s

operating on, apart from the fact that it can be traversed by an iterator, and knows almost) nothing about the elements in the structure

Define container methods only when the generic algorithms are unsuitable or much less efficient

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C++ STL The Standard Template Library, or STL, is a

C++ library of container classes, algorithms, and iterators.

Containers are classes whose purpose is to contain other objects.

It provides many of the basic algorithms and data structures normally used in programs.

Algorithms Containers Iterators

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Containers and algorithms

Sequences continuous blocks of objects Indexed by integers vector, list, deque,

Associative containers Might be indexed by any class of objects set, multiset, map, multimap

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Vectors

List Deque

0

1

2

3

4

5

0

1

2

3

4

0

1

2

3

4

5

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Set keys values

Map

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Container

Sequence Associative Container

Vector List deque Set Multiset Map Multimap

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C++ STL Algorithms(1) Non-Mutating Sequence Operations. Algorithms like

count and search which do not modify the iterator or its associated container.

Mutating Sequence Operations. Algorithms like copy, reverse, or swap which may modify a container or its iterator.

Searching and Sorting. Sorting, searching, and merging algorithms, such as stable_sort, binary_search, and merge.

Set Operations. Mathematical set operations, such as set_union and set_intersection. These algorithms only work on sorted containers.

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C++ STL Algorithms(2) Heap Operations. Heaps are a very useful and

efficient data structure that is often used to implement priority queues. The STL provides facilities for making heaps and using them.

Numeric Operations. The STL provides a few numerical routines to show how the STL might be used to provide a template-based numeric library. The STL provides algorithms for: accumulate, inner_product, partial_sum, and adjacent_difference.

Miscellaneous Operations. This final category is for algorithms, like min and next_permutation that don't quite fit in the above categories.

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An algorithm example vector<int> arr(3); arr[0] = 5; arr[1] =10; arr[2] = arr[0] + arr[1]; reverse(arr.begin(), arr.end());

int A[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; reverse(A, A+8); for (int i = 0; i < 10; ++i) cout << "A[" << i << "] = " << A[i]; //reverse.cpp

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Iterators Basic problem - how do you allow

access to elements of collection, without knowing how the collection is organized?

Solution, define a new data type specifically for creating loops

A large number of algorithms are provided by the standard library, all built using iterators.

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How do you describe a range of values

Notice how a range of values is often described by a starting value and a past-the-end value.

The past the end value is not part of the collection, but just a marker.

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Begin and End By convention, containers return a starting

value in response to begin(), and a past-the-end value in response to end().

For example, to shuffle a vector of values: random_shuffle

(aVector.begin(), aVector.end(), randomInteger);

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Iterators are generalized pointers

Vectors

List Dequeue

begin

end

begin

end

begin

end

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Iterator organization Iterators are classified into five

categories: forward, bidirectional, random access, input, and output.

The description of container classes includes the category of the iterator types they provide.

The description of generic algorithms includes the iterator categories they work with.

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What must iterators do To see what iterators must do, consider a typical algorithm: template <class InputIterator, class T> InputIterator find (InputIterator first, InputIterator last, T & value) { while (first != last && *first != value) ++first; return first; } Could be used to find values in an array, or in a list: int data[100]; ... int * where = find(data, data+100, 7); list<int> aList; list<int>::iterator where = find(aList.begin(), aList.end(), 7); //iter.cpp

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Iterator Operations An iterator can be compared for equality to another iterator. They

are equal when they point to the same position, and are otherwise not equal.

An iterator can be dereferenced using the * operator, to obtain the value being denoted by the iterator. Depending upon the type of iterator and variety of underlying container, this value can also sometimes be used as the target of an assignment in order to change the value being held by the container.

An iterator can be incremented, so that it refers to the next element in sequence, using the operator ++.

What makes iterators possible is that all of these can be overloaded.

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Faction objects Function objects are STL's way of

representing "executable data". In C++, the function call operator

(parenthesis operator) can be overloaded. Allows for creation of objects that can be used like functions.

In a function object, at least one parenthesis operator () is defined.

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A function is an object Objects can take arguments computed at run-time, specialize

functions in a way that simple functions cannot: class biggerThan { public: biggerThan (int x) : testValue(x) { } const int testValue; bool operator () (int val) { return val > testValue; } };

list<int>::iterator firstBig = find_if (aList.begin(), aList.end(), biggerThan(12)); In functional languages, this kind of object is sometimes known as a

curry. //bigthan.cpp

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Function Object Examples #include <iostream> class threshold { double val; public: threshold( double v): val(v) {} bool operator()( double y){ return y < val; } } reallySmall( 1.e-8 ), small(1.e-3);

int main() { double x; while ( std::cin >> x ) std::cout << x << " " << (reallySmall(x) ? "tiny" : small(x) ? "small" : "big") << '\n'; }//funobj.cpp

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The Visitor in C++ in the STL

class printingObject { public: void operator () (int x) { cout << "value is " << x << endl; } }; printingObject printer;

// create an instance of the function object

for_each (aList.begin(), aList.end(), printer); The function for_each passes the object to element

element in the collection. //funo.cpp

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C++ List C++ provides many class templates of

containers including vector, list, set, deque, map, queue, priority_queue, and stack. They can be parameterized into any class of these to deal with a sequence of objects.

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The interface of ListMember function

nameDescriptions

back() Get the last element

begin() Get the pointer to the first element

clear() Erase all the elements

end() Get the pointer to one past the last element

erase(first, last) Erase from first to last

erase(p) Remove the element at p

remove(e) Remove the element equal to e

front() Get the first element

insert(p, x) Add x before p

push_back(e) Adds an element e to the end of a list

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An Example of List#include <list>//… list<int> L; L.push_back(0); // Insert a new element at the end L.push_front(0); // Insert a new element at the beginning L.insert(++L.begin(),2);

// Insert "2" before position of first argument // (Place before second argument) L.push_back(5); L.push_back(6); list<int>::iterator i; for(i=L.begin(); i != L.end(); ++i) cout << *i << " "; cout << endl;

//list.cpp

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The algorithm library The header file for these functions is

<algo.h> or <algorithm>. The STL algorithms are generalized

function templates. //alg.cpp

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Function name Descriptions

binary_search Search an object with a binary search algorithm. If the object is in the sequence return true. Otherwise, return false.

copy Copy one sequence of objects to another.

count Returns the number of objects in a sequence.

equal Returns true if the objects positioned in a subsequence are equal to the objects of another subsequence. Otherwise, returns false.

find_if Returns true if there is an object that satisfy the predicate provided. Other wise, return false.

for_each Apply the function provided to each object in a sequence. The function is as the returned value.

lower_bound Returns the iterator pointing the first object equal to the object provided. If there is no such object, an iterator pointing to the first object that is grater than the object provided. If there is no such an object, a pointer to a sentinel is returned.

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Max Returns the maximum object of the two objects provided.

Merge Merger two subsequences to the 3rd place.

Min Returns the minimum object of the two objects provided.

random_shuffle Rearrange the object sequence randomly.

remove Removes an object in the sequence that is equal to the object provided.

replace Replace occurrences of the object with another object.

reverse Reverse the sequence of objects.

sort Sorts the sequence of objects.

swap Swaps the two objects provided.

upper_bound Returns the iterator pointing the last object equal to the object provided. If there is no such object, an iterator pointing to the first object that is grater than the object provided. If there is no such an object, a pointer to a sentinel is returned.

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Map A map is a sequence of <key, value> pair that

provides for fast retrieval based on the key.

….

… …

A map

map<T1,T2>

Key Value

… …

pair<T1,T2>

Pair objects

Key Value

… …

pair<T1,T2>

Key Value

pair<T1,T2>

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Set Set is a Sorted Associative Container

that stores objects of type Key. Set is a Simple Associative Container,

meaning that its value type, as well as its key type, is Key.

It is also a Unique Associative Container, meaning that no two elements are the same.

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Map examples int main() { string as[] = {"A+","A","A-","B+","B","B-","C"}; //passing grades typedef set<string> SSet; // set of strings SSet acceptable( as, as + sizeof(as)/sizeof(as[0]) ); // construct set using all elements of the array typedef map<string, SSet> SMap; // map strings into SSets SMap m; ifstream infile("Source.txt"); for(string n, c, g; infile >> n >> c >> g ; ) // name course grade if ( acceptable.count( g ) /* > 0 */ ) // if grade is in passing set m[n].insert( c ); for(SMap::iterator it = m.begin(), stop=m.end(); it != stop; it++) { cout << it->first << ": "; // output person's name for(SSet::iterator p = it->second.begin(), q=it->second.end(); p != q; p++) cout << *p << ' '; // output each course passed cout << '\n'; infile.close(); } }//map.cpp

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Summary General Principles on Containers

Substitution/Downcast/Overriding/Generic Object-Based/Template based

STL Generic algorithms http://www.sgi.com/tech/stl/table_of_cont

ents.html

Container classes List Map

Everything is an object!