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Multiset.H) from Behavior to Implementation

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Multiset.H) from Behavior to Implementation Designing and Implementing a class Behavior of MultiSet: methods? ● Consider a simple class to implement sets (no duplicates) or ● What accessor functions do we need (these are const)? multisets (duplicates allowed) ➤ Size of set? Printing set? Element in set? ➤ Initially we’ll store strings, but we’d like to store anything, ➤ What does Print generalize to? how is this done in C++? • For Iterator class, see book, requires friend class ➤ How do we design: behavior or implementation first? ➤ How do we test/implement, one method at a time or all at ● What mutator functions do we need (non const)? once? ➤ Insert an element into the set ➤ Make a set empty ● Tapestry book shows string set implemented with singly ➤ Erase an element (all occurrences or one?) linked list and header node ➤ Templated set class implemented after string set done ● ➤ What constructor(s) and other, similar standard methods are Detailed discussion of implementation provided needed ● We’ll look at a doubly-linked list implementation of a ➤ Copy constructor, assignment operator, destructor MultiSet CPS 100 4.1 CPS 100 4.2 Draft of multiset behavior (multiset.h) From Behavior to Implementation class MultiSet ● We’ll use a linked list { public: ➤ New nodes added to back, but this will change in other MultiSet(); // construct a multiset versions: add to front, move to front, tree, … ➤ // mutators Use a header node (not in set), last node (in set) void insert(const string & word); ➤ To print, count, etc. we’ll use an internal iterator void clear(); • Pass to the set the operation you want to perform on all set elements // accessor void apply(MSApplicant & app) const; • What’s good/bad about this compared to iterators? int size() const; int count(const string & key) const; ● Use doubly-linked list, where is Node declaration found? private: ➤ Private section means clients cannot access, ok? }; ➤ Notice return type of private Find(..) in multiset.cpp ● What’s missing? What is apply(..) CPS 100 4.3 CPS 100 4.4 What’s easy, hard, first? Iterators and alternatives ● Searching the list is straightforward, visit all nodes MultiSet ms; ➤ We need to search for insert and for count, how can we ms.insert("bad"); factor out common code? ms.insert("good"); ms.insert("ugly"); ➤ If we implemented remove/erase, we’d need to search too. ms.insert("good"); ● Once we implement insert, how do we test? MultiSetIterator it(ms); ➤ We need to print, we should implement print() even if for(it.Init(); it.HasMore(); it.Next()) not general, we’ll toss it later, why is this ok? { cout << it.Current() << endl; } ● What’s after insert? We’ll look to the accessor functions ● What’s printed? What does Current() return? Internals? ➤ count is simple, why? ➤ What does iterator class have access to? ➤ What is apply(..) about? ➤ What happens when MultiSet passed to iterator? Stored? CPS 100 4.5 CPS 100 4.6 Iterators and alternatives, continued Using a common interface ● Iterators require class tightly coupled to container class ● We’ll use a function named apply(…) ➤ Friend class often required ➤ It will be called for each element in a MultiSet ➤ Const can be a problem, but surmountable • An element is a (string, count) pair ➤ Encapsulated in a class with other behavior/state ● Alternative, pass function into MultiSet, function is applied to every element of the set void apply(const string& s, int count) const ➤ { How can we pass a function? cout << count << "\t" << s << endl; ➤ What’s potential difference compared to Iterator class? } ● To pass a function, we’ll put the function in a class and pass // alternative an object void apply(const string& s, int count) const { ➤ Must adhere to naming conventions since written in client myTotal += count; code, called in MultiSet code } CPS 100 4.7 CPS 100 4.8 Interfaces and Inheritance Multisets, function objects, inheritance ● Programming to a common interface is a good idea ● Interface used by MultiSet objects, apply function to every ➤ I have an iterator named FooIterator, how is it used? object in the set • Convention enforces function names, not the compiler ➤ string and count of a set element are passed to apply(..) ➤ I have an ostream object named out, how is it used? • Inheritance enforces function names, compiler checks class MSApplicant { public: ● Design a good interface and write client programs to use it virtual ~MSApplicant(){} ➤ Change the implementation without changing client code virtual void apply(const string & word, ➤ Example: read a stream, what’s the source of the stream? int count) = 0; • file, cin, string, web, … }; ● Virtual means “inheritance works”, function called ● C++ inheritance syntax is cumbersome, but the idea is simple: determined at run-time, not compile-time ➤ Design an interface and make classes use the interface ➤ The =0 syntax means this that subclasses must implement ➤ Client code knows interface only, not implementation the function --- subclass implements the interface CPS 100 4.9 CPS 100 4.10 What is a function object? How does interface inheritance help? ● Encapsulate a function in a class, enforce interface using ● MultiSet code uses interface only to process all set elements inheritance or templates ➤ Class has state, functions don’t (really) void MultiSet::apply(MSApplicant & app) const ➤ Sorting using different comparison criteria as in extra- // postcondition: app.apply called for all credit for Anagram assignment // elements in the set { Node * current = myFirst->next; // skip header ● In C++ it’s possible to pass a function, actually use pointer to while (current != 0) function { ➤ Syntax is awkward and ugly app.apply(current->myKey, current->myCount); ➤ Functions can’t have state accessible outside the function current = current->next; (how would we count elements in a set, for example)? } ➤ Limited since return type and parameters fixed, in classes } can add additional member functions ● How do we count # elements in a set? # distinct elements? CPS 100 4.11 CPS 100 4.12 Why inheritance? Guidelines for using inheritance ● Add new shapes easily without ● Create a base/super/parent class that specifies the behavior shape changing code that will be implemented in subclasses ➤ Shape * sp = new Circle(); ➤ Functions in base class should be virtual ➤ Shape * sp2 = new Square(); • Often pure virtual (= 0 syntax), interface only mammal ● abstract base class: ➤ Subclasses do not need to specify virtual, but good idea ➤ interface or abstraction • May subclass further, show programmer what’s going on MultiSetBack, MultiSetTable ➤ pure virtual function ➤ Subclasses specify inheritance using : public Base MultiSet ● concrete subclass • C++ has other kinds of inheritance, stay away from these ➤ implementation User’s eye view: think and ➤ Must have virtual destructor in base class ➤ provide a version of all program with abstractions, realize pure functions different, but conforming ● Inheritance models “is-a” relationship, a subclass is-a parent- ● “is-a” view of inheritance implementations class, can be used-as-a, is substitutable-for ➤ Substitutable for, usable in ➤ Standard examples include animals and shapes all cases as-a CPS 100 4.13 CPS 100 4.14 Inheritance guidelines/examples See students.cpp, school.cpp ● Virtual function binding is determined at run-time ● Base class student doesn’t have all functions virtual ➤ Non-virtual function binding (which one is called) ➤ What happens if subclass uses new name() function? determined at compile time • name() bound at compile time, no change observed ➤ Can’t change which function called if compile-time determined ● How do subclass objects call parent class code? ➤ Small overhead for using virtual functions in terms of speed, design flexibility replaces need for speed ➤ Use class::function syntax, must know name of parent class • Contrast Java, all functions “virtual” by default ● In a base class, make all functions virtual ● Why is data protected rather than private? ➤ Allow design flexibility, if you need speed you’re wrong, ➤ Must be accessed directly in subclasses, why? or do it later ➤ Not ideal, try to avoid state in base/parent class: trouble ● In C++, inheritance works only through pointer or reference • What if derived class doesn’t need data? ➤ If a copy is made, all bets are off, need the “real” object CPS 100 4.15 CPS 100 4.16 Consider a modification to MultiSet How do we print all values in a tree? ● Instead of using prev and next to point to a linear ● When is root printed? arrangement, use them to divide the universe in half ➤ After left subtree, before right subtree. ➤ Similar to binary search, everything less goes left, everything greater goes right void Visit(Node * t) “llama” { if (t != 0) “giraffe” “tiger” { Visit(t->prev); cout << t->info << endl; “jaguar” ➤ How do we search? “elephant” “monkey” Visit(t->next); “llama” ➤ } How do we insert? “giraffe” “hippo” “leopard” “pig” } “tiger” “elephant” “jaguar” “monkey” ➤ How are lists and trees related? ● Inorder traversal “hippo” “leopard” “pig” CPS 100 4.17 CPS 100 4.18 Insertion and Find? Complexity? Binary Trees ● How do we search for a value in a tree, starting at root? ● Linked lists have efficient insertion and deletion, but ➤ Can do this both iteratively and recursively, contrast to inefficient search printing which is very difficult to do iteratively ➤ arrays: search is efficient, insertion and deletion are not ➤ How is insertion similar to search? ● Binary trees are structures that can be used to yield efficient insertion/deletion and search ● What is complexity of print? Of insertion? ➤ trees
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