Embed Size (px)
Citation preview
C++ &
Object-Oriented Programming
The Class
2
A class is a unit of encapsulation
Public operations, and private implementation.
3
A class is an abstraction
StringString abstracts the char * of C, stackstack -- the canonical abstraction List Student, etc.
4
Classes and C structures
default access for class: private default access for struct:public
5
Classes
Example of a bad class declaration
class Student {public:
float gpa;
char * name;
};
6
Classes and structures Correct example of Class Declaration
class Student {private:
float gpa;
char * name;
}; do we need the “private” designation?
7
Classes: writing setset and getget functions class Student {
public: void setGpa(float g) { gpa = g; } void setName(char * n) { name = new char[strlen(n)+1]; strcpy(name, n); } private: float gpa; char * name;};
8
Classes and Objects Declaration of a class variable (i.e. object)
Student s, t;
Student * v;
int i;
float x;
The syntax is identical to conventional variable declarations.
9
Classes Since class declarations will likely be
used by many program modules, put them in a header file that can be included as required
#include “Student.h”
10
Classes and Objects Access to class data members
s.setGpa(3.5);
s.setName(“Dana”);
v = new Student(4.0, “Fox”);
t = s;
The Student class also needs constructors
11
Classes: constructors
class Student {public:
Student(); //default
Student(char*, float); //conversion
Student(const Student&); //copy
friend ostream & operator<<(ostream &,
const Student &);
private:
char * name;
float gpa;
};
12
Constructor parameters:
Default: no parameters Conversion: parameters to make one Copy: the parameter is the copy instance
13
Constructors/destructors
Guarantee that data is initialized allow classes to manage memory
constructors allocate: new destructors deallocate: delete
14
Prefer initialization to assignment
class Student {public: Student(int a) : age(a), iq(age+100) {} Student(int a) { age = a; iq = age + 100; }private: int age; int iq;};
15
Prefer init to assign
Initialization is more efficient for data members that are objects: init: uses copy constructor assign: uses default constructor and
assignment only way to pass a parameter to base
class
16
Passing param to base classclass Person {public: Person(int a) : age(a) {}private: int age;};
class Student : public Person {public: Student(int age, float g) : Person(age), gpa(g) {}private: float gpa;};
17
List members in init list in order that they are declared
class members are initialized in the order ofclass members are initialized in the order oftheir declaration in the classtheir declaration in the class! What’s the value of iqWhat’s the value of iq?
class Student {public: Student(int a) : age(a), iq(age+100) {}private: int iq; int age;};
18
Exercise to learn constructors Write 3 constructors and a destructor for
Student class. Test it with:
void fun(Student stu) {}int main() { Student a, b(“Darth Maul”, 3.5), c = b; Student * d = new Student(“Anakin”, 4.0); cout << *d << endl; fun(a); return 0;}
19
Why is this a bad class definition?at least 2 member functions missing
class Student {public:
Student(); //default
Student(char*, float); //conversion
Student(const Student&); //copy
friend ostream & operator<<(ostream &,
const Student &);
private:
char * name;
float gpa;
};
20
Q: what output should we get:
int main() {
Student x(“Count Dooku”), y(“Anakin”);
x = y;
y.setName(“Darth Maul”);
cout << x << endl;
}
Soln: need deep copy: operator=
21
Copying a class object
Original Object
ShallowCopy
22
Copying a class object
Original Object
DeepCopy
23
Canonical orthodox class form J. Coplien
Default constructor copy constructor destructor assignment operator
24
What functions does C++silently write?
class Empty{};
class Empty {public: Empty(); Empty(const Empty &); ~Empty();
Empty& operator=(const Empty &); Empty * operator&(); const Empty * operator&() const;};
25
How would you need them?
const Empty e1; // need default constructor, and// destructor
Empty e2(e1); // copy constructore2 = e1; // assignment operator
Empty * pe2 = &e2; // address-of operator (non const)const Empty *pe1 = &e1; // address-of operator (const)
26
What do the compiler generated functions look like?
inline Empty::Empty() {}inline Empty::~Empty() {}
inline Empty * Empty::operator&() { return this; }
inline const Empty * Empty::operator&() const { return this; }
The copy constructor and assignment operator simply do a member wise copy, i.e., shallow. Note that thedefault assignment induces a memory leakmemory leak in Student!
27
When do we need to write a copy constructor, destructor & assign operator?
Anytime the class has heap-based data membersheap-based data members all base classes should have a virtual destructor Studies have shown that, if you don’t need
copy/assign, the compiler generated functions run 10-20% faster than programmer generated ones.
28
How do we overload assignment?
Student & operator=(const Student & stu) { if (*this == stu) return * this; delete [] name; name = new char[strlen(stu.name)+1]; strcpy(name, stu.name); gpa = stu.gpa; return *this;}
(1) Why the comparison on the first line?(1) Why the comparison on the first line?(2) Could the first line be: if (this == &stu)?(2) Could the first line be: if (this == &stu)?(3) Why return *this? What does it enable?(3) Why return *this? What does it enable?(4) Why not return stu, rather than *this?(4) Why not return stu, rather than *this?
29
Formula for overloaded assignment
Check for equality of lhs & rhs delete storage for lhs create new storage for lhs, that’s size of
rhs copy rhs “stuff” to lhs return *this
30
an overloaded binary operator
can be written in infix form a = b; cout << stu;
or can be written in prefix form a.operator=(b) cout.operator<<(stu)
31
Principle of least privilege
Can reduce debugging and provide documentation by only allowing a function the least amount of privilege
can prevent a function from modifying a parameter
can prevent a member function from modifying data attributes
allow a function enough data access to accomplish its task – and no more!
32
Explicitly disallow any implicit functions you don’t want!
Suppose you want to write a class template array that behaves like C++ arrays, except that it does bounds check
C++ does not allow array assignment; e.g. int a[10], b[10]; a = b; //error
class Array {private: Array & operator=(const Array & );};
don’t define theoperator= to keepmember & friendfunctions from using it!
33
Overloading operators
almost all operators can be overloaded operators are binary or unary have the same precedence as their
compiler counterpart can be members or friends, usually overloaded output operator cannot be a
member of any user defined class
34
overloading output operator:as a friend function
class Student {public: getName() { return name; } getGpa() { return gpa; } friend ostream & operator<<(ostream &, const Student &); private: char * name; float gpa;};
ostream & operator<<(ostream & out, const Student & s) { out << s.getName() << “\t” << s.getGpa(); return out;}
35
Overloading output operator:as stand-alone function
class Student {public: getName() { return name; } getGpa() { return gpa; } private: char * name; float gpa;};
ostream & operator<<(ostream & out, const Student & s) { out << s.getName() << “\t” << s.getGpa(); return out;}
36
Put prototype in header file,and body in implementation file
class Student {public: getName() const { return name; } getGpa() const { return gpa; } private: char * name; float gpa;};ostream & operator<<(ostream &, const Student &);
ostream & operator<<(ostream & out, const Student & s) { out << s.getName() << “\t” << s.getGpa(); return out;}
37
Exercise with classes:
Complete the definition of Student with all 3 constructors, overloaded assignment, overloaded output, and get and set
Write a main program that contains 2 functions: a function to init the list with 3 Students a function that prints the list
38
Using students
In actual practice, we want to store lots of students
need an array of students
Student list[10];
Q: How many times does a constructor get called for list?
Q: which constructor?
39
What’s missing:
Need an easier way to print a student overload the output operator << operator<< must be a friend function;
cannot be a member function cuz it’s already a member of class ostream
Need a class to manage the list of Students
40
Friends
Can access private member functions and private data attributes!
Can be functions or classes Might not always be best solution!
41
Overloading the output operator
Class Student {public: friend ostream & operator<<(ostream & out,
const Student & student){ out << student.name; return out; }};
42
Class StudentManager {public: StudentManager() : count(0) { } void insert(const Student &); friend ostream & operator<<(ostream &, const Student &);
private: int count; Student list[100];};
4242
43
Class StudentManager {public: StudentManager() : count(0) {} void insert(const Student & student) { list[count++] = student; } friend ostream & operator<<(ostream & out, const Student & student) { for (int i = 0; i < count; ++i) out << student[i] ;<< endl; return out; }
private: int count; Student list[100];};
4343
44
Class exercise:
Complete the StudentManager Class; rewrite main to use the List Class. Write a new main program that allows the
user to choose among commands: insert student delete student print the list find a student exit
45
Discussion of Manager class:
Advantages: encapsulates functionality into a single
class simplifies the main program
Disadvantages: there is a limit (100) on number of Students Student’ default constructor called 100
times!
46
Suggested Naming Conventions global constants: ALL CAPS! local & global variables: ALL LOWER CASE, USE
UNDERSCORE Class names: BEGIN EACH WORD WITH UPPER
CASE, NO UNDERSCORE Class member functions: BEGIN LOWER CASE,
then BEGIN EACH WORKD WITH UPPER CASE Data members: SAME AS MEMBER FUNCTIONS
47
Operator Overloading:ADT for binary math
Would like variables of type binary to be like any other data type. Thus: overload arithmetic operators overload output to be binary overload input to use decimal
How do we represent internally?
48
Binary should “look like” int
#include “Binary.h”
main() { int sum = 0; Binary number1, number2 = 7; number1 = 15; cout << number1 + number2 << endl;}
49
// This is file Binary.hclass Binary {public: Binary() : number(0) {} Binary(int n) : number(n) {} Binary(const Binary & bin); Binary operator+(const Binary &); Binary& operator++(); Binary& operator=(const Binary &); friend ostream & operator<<(ostream &, const Binary &);
private: int number;};
4949
50
#include “Binary.h” // this is file Binary.cpp
Binary Binary::operator+(const Binary & rhs) { Binary temp(number+rhs.number); return temp;}
Binary& operator++() { ++number; return *this;}
5050
51
// this is a global function; s/b in Binary.cppostream & operator<<(ostream & out, const Binary & bin) { stack<int> stk; int number = bin.number; while (number) { stk.push( number % 2 ); number /= 2; } while ( !stk.empty() ) { out << stk.top(); stk.pop(); } return out;}
5151
52
Exercise for Binary type:
Complete the class definition for Binary construct a main program that uses Binary Discuss return value transmission mode
operator= operator++ operator+ (postfix, prefix)
Discuss merits of friend vs member functions for +, ++, output, etc.
53
Discussion of Binary class Compilation can be made simpler by
using a makefile makefile automatically compiles and
links program modules makefiles are mysterious
54
makefile/Makefile
Consist of definitions, Followed by sequences of 2 line
commands. Begins with id:id:, followed by dependencies
of idid. Second line is the rule to make idid; this line
MUST be preceded by a tab To use the make file type: make {<id>}make {<id>}
55
CCC=g++FLAGS=-Wall
main: main.o Binary.o$(CCC) $(FLAGS) -o main main.o Binary.o
main.o: main.cpp Binary.h$(CCC) $(FLAGS) -c main.cpp
Binary.o: Binary.cpp Binary.h$(CCC) $(FLAGS) -c Binary.cpp
clean:rm -f main *.o core 55
55
56
Discussion of makefile
$(CCC) permits us to easily switch to another compiler; e.g. CC
make clean will ‘clean’ the directory of large files
-o option creates an executable -c option creates .o file
57
C-strings are no fun:
Programmer must manage memory comparison is awkward concatenation is non-standard
strcat(a, b), instead of: a + b
No substring facilities
58
Solution: write our own String classWhat do we need:
Constructors default conversion copy
overloaded operators output concatenation
59
Dynamic storage
Need pointers “never” run out of room ;-) must allocate storage: new must also deallocate storage: delete failure to deallocate storage: memory leak constructors/destructors were designed to
facilitate memory management
60
// This class definition of String s/b in file: String.hclass String {public: String(); String(char *); String(const String &); String operator+(const String &); int len() const ; friend ostream & operator<<(ostream &, const String &);
private: char * buf;};
6060
61
// This class definition of String s/b in file: String.hclass String {public: String(int n = 0) : buf(new char[n+1]) { buf[0] = ‘\0’; } String(char * s) : buf(new char[strlen(s)+1]) { strcpy(buf, s); } String(const String &); ~String() { delete [] buf; } char & operator[](int i) { return buf[i]; } String operator+(const String &); int len() const { return strlen(buf); } friend ostream & operator<<(ostream &, const String &);
private: char * buf;};
6161
62
// This is file String.cpp#include “String.h”
ostream & operator<<(ostream & out, const String & s) { out << s.buf; return out;}
String String::operator+(const String & rhs) { String temp( strlen(buf)+strlen(rhs.buf) ); strcpy(temp.buf, buf); strcat(temp.buf, rhs.buf); return temp;} 62
62
63
We need operator==
Discuss the merits of the following comparison function:
bool String::operator==(const String & rhs) {
return (*this == rhs);
}
64
String exercise:
Discuss ‘what’s missing’ from String spec Complete the class definition and
implementation; write copy constructor Write a main program that uses Strings. Write a makefile Thoroughly test the class Is there one in the standard C++ library?
65
String exercise:
Add operator= to String Q: when are constructors called? Q: when is assignment called? Q: which is better: initialization or
assignment? Consider main.cpp, String.h and
String.cpp
66
#include “String.h”
String & String::operator=(const String & rhs) { if (*this == rhs) return *this; delete [] buf; buf = new char[strlen(rhs.buf) + 1]; strcpy(buf, rhs.buf); return *this;}
6666
67
Discussion of String::operator=
Note the return transmission mode. Why? what happens if we do not delete buf? Why do we need +1 when we new buf ? What does return *this mean? Discuss the following 3 options:
if (*this == rhs) return *this; if (this == &rhs) return *this; if (strcmp(buf, rhs.buf) ==0) return *this;
68
#include “String.h”
int main() { String a, b(“hello”), c(a), d = “world”; }
What constructors are called?
6868
69
// Note that it’s more efficient if constructors// use initialization rather than assignment.// To see: place cout in constructors for String!
class Test {public: Test(char *n) : name(n) {} // initialization Test(char *n) { name = n; } // assignmentprivate: String name;};
main() { Test t(“surprise”);}
6969
70
Making Strings useful:
Sometimes we want to ‘run through’ a string; i.e. iterateiterate through the string.
For example, if we read a line of text from a file, we might want to access each word on the line.
Can write a special class called an iterator
71
Iterator class
Allows user to ‘look at’ each item in the data structure
can be forward or backward can have more than one iterator on a
given data structure
72
iterator
For a linked list: would return each item in the list, starting with the first an proceeding through the list;
for a string: might return each word in the list, or it might return each character
for a tree: would return each item in the tree, in some order (preorder, depth first, etc.)
73
// partial specification for iterator over Stringclass StringIterator {public: StringIterator(const String & s, char d = ‘ ‘) : str(s), index(0) , delim(d) {} void operator++(); String operator() () const; bool atEnd() const { return index == str.len(); }private: String str; unsigned int index; char delim;};
7373
74
StringIterator spec discussion
Constructor must init data members in the order that they are listed in the declaration
default delimiter is a blank overloaded parens looks odd note that str.len() is one past the end! we could overload operator<< for
debugging
75
// implementation of operator++#include “StringIterator.h”
void StringIterator::operator++() { while (index < str.len() && str[index] != delim) ++index; while (index < str.len() && str[index] == delim) ++index; }
7575
76
// implementation of operator()
String StringIterator::operator() () const { char buf[80]; int temp_index = index, i = 0; while (temp_index < str.len() && str[temp_index] != delim) buf[i++] = str[temp_index++];
buf[i] = ‘\0’; return String(buf); }
7676
77
StringIterator exercises:
Complete the specification and implementation overload operator<< for debugging thoroughly test your ADT extend StringIterator to accept a set of
delimiters what happens if you initialize the data
members in different order than they are declared?
78
Stack: a common data structure
Public operations: push pop test empty possibly test full
private data representation
The canonical abstraction
78
79
class IntStack {public: IntStack() : count(EMPTY) {} void push(int n) { items[++count] = n; } void pop() { --count; } int top() const { return items[count]; } bool isEmpty() const { return count == EMPTY; } bool isFull() const { return count == 99; }private: enum {EMPTY = -1}; int items[100]; int count;};
7979
80
Stack exercise:
Implement and test Stack class Discuss alternatives to items[100] modify IntStack so that it accepts a
parameter describing the size of items Discuss drawbacks of IntStack What are solutions to drawbacks?
81
Template classes
Functions accept variables as parameters
template classes accept types as parameters
functions can also use templates
82
template <class T>class Stack {public: Stack() : count(EMPTY) {} void push(const T & n) { items[++count] = n; } void pop() { --count; } const T top() const { return items[count]; } bool isEmpty() const { return count == EMPTY; } bool isFull() const { return count == 99; }private: enum {EMPTY = -1}; T items[100]; int count;};
8282
83
Discussion of template stack
Note the change in parameter transmission from IntStack to Stack<T>
Note that top() passes return by value; can pass-by-value return be avoided?
Can arrays, in C++, be dimensioned dynamically?
What happens if user pops from empty stack?
84
Exceptions
Useful for handling error conditions. Also useful for exceptional cases, such
as eof, end-of-line, etc. keywords: try, catch, throw
85
How to use exceptions
Each try block has corresponding catch block(s)
calls to functions that may throw an exception are placed in a try block.
The called function, when faced with exceptional condition, performs a throw
the exception is thrown back, through the call call chainchain, to matching catch, or
program bombs!
// give the output for the program below:
void fun(int number) { if (number % 2 == 0) throw “error”; cout << “This is fun” << endl;}
main() { try { fun(2); fun(3); } catch (char * msg) { cout << msg << endl; return 1; }} 86
// give the output for the program below:
void fun(int number) { if (number % 2 == 0) throw “error”; cout << “This is fun” << endl;}
main() { int n = 2; while (n) { try { fun(n--); } catch (char * msg) { cout << msg << endl; } }}
87
class Clean { public: Clean() { cout << “constructing Clean” << endl; } ~Clean() { cout << “destroying Clean” << endl; }};
void fun() { Clean c; throw “error”;}
main() { try { fun(); } catch(char * msg) { cout << msg << endl; }} 88
89
Stack exercise
Add exception handling to Stack class write a main program with try/catch
block that uses Stack
90
Dynamic storage
Need pointers “never” run out of room ;-) must allocate storage: new must also deallocate storage: delete failure to deallocate storage: memory leak constructors/destructors were designed to
facilitate memory management
91
Linked list
Dynamic memory used to store data (usually in a nodenode!)
node usually stores data and pointer to next node (at least)
can be singly linked, doubly linked, circularly linked
can have dummy header node and/or dummy footer node
Singly linked list
node node node
92
93
// partial specification & implementation for Node classclass Node {public: friend class List; Node() : data(0), next(NULL) {} Node(int d, Node * n) : data(d), next(n) {} int getData() const; void setData(int d) ; friend ostream & operator<<(ostream &, const Node &);private: int data; Node * next;};
9393
94
// partial specification & implementation for Node classclass Node {public: friend class List; Node(int d = 0, Node * n = NULL) : data(d), next(n) {} int getData() const { return data; } void setData(int d) { data = d; } friend ostream & operator<<(ostream & out, const Node & node) { out << node.data; return out; }private: int data; Node * next;};
9494
95
#include “Node.h”class List{public: List() : head(NULL), current(NULL) {} void insert(int data) { head = new Node(data, head); } void delete() { Node * temp = head; head = temp->next; delete temp; } Node * getFront() { current = head; return current; } Node * getNext() { current=current->next; return current:} friend ostream & operator<<(ostream &, const List &);private: Node * head; Node * current;};
9595
96
Discussion of List class insert actually inserts at the front getFront, getNext and current allow us to iterate
through the list. Is there a better way? Why is List a friend of Node? Can we construct Node and List so that friendship
is not required? Note that deleting an arbitrary element is tricky.
Would a doubly linked list simplify? Should List be templated?
97
List class exercise
Complete specification & implementation, adding insertAtRear, insertAtFront, insert, deleteFront, deleteRear, deleteNth, ...
Write a main program that uses List and test Re-write Node and List so that friendship is
not required. Push the envelope: template List Write an iterator class for List
98
List exercise
Find a solution to the Josephus problem for inputs n and m
describe a solution, using IntList, to the problem of arbitrary precision arithmetic. Compare the speed of your program with bc on unix.
99
Doubly linked list
Easier to delete an arbitrary element more overhead to implement more storage sometimes use a dummy header node to
facilitate inserting into an empty list and deleting from a list of size 1.
Can use a pointer to the front and rear, facilitating insertion at either end.
100
Doubly linked list
dummy node node
number nextprev
#include <iostream.h>class Node {public: friend class List; friend class ListIterator; Node(int i = 0, Node * n = NULL, Node * p = NULL) : number(i), next(n), prev(p) {} friend ostream & operator<<(ostream &, const Node &);
private: int number; Node * next; Node * prev;} 101
#include “Node.h”class List {public: friend class ListIterator; List() { rear = front = new Node; } void insertAtFront(int n); void insertAtRear(int n); bool empty const { return front == rear; } int size() const; friend ostream & operator<<(ostream &, const List &);
private: Node * front; Node * rear;}
102
class ListIterator {public: ListIterator (const List & list) : front(list.front), current(list.front->next) {} void init() { current = front; } void operator++() { current = current->next; } Node * operator() () const { return current; } bool more() const { return current != NULL; }
private: Node * front; Node * current;};
103
104
Doubly linked list exercises
Complete the spec and implementation test the list
