OOP in C++

Object-Oriented
Programming in C++
Jan-Feb 2014
Lecture 02-

PROGRAMMING IN C++
Object-Oriented Modeling & Programming
OOP in C++ 2Jan 2014

Topics
 Procedural Enhancements in C++ over C
 Classes
 Overloading
 Inheritance
 Type Casting
 Exceptions
 Templates
3OOP in C++Jan 2014

PROCEDURAL
ENHANCEMENTS IN C++
Object Oriented Programming in C++
Jan 2014 4OOP in C++

TOPICS
 Const-ness
 Reference Data Type
 Inline Functions
 Default Function Parameters
 Function Overloading & Resolution
 Dynamic Memory Allocation
5OOP in C++Jan 2014

const Quantifier
 const qualifier transforms an object into a
constant.
 Example: const int capacity = 512;
 Any attempt to write a const object is an error
 Const object must be initialized.
 Pointer to a non-constant object can not point to
a const object;
const double d = 5.6;
double *p = &d; //error
 Pointer to a constant object vs. constant pointer
to an object.
const double * pConstantObject;
double * const *pConstantPointer;
6OOP in C++Jan 2014

References
 A reference is an additional name /
alias / synonym for an existing variable
 Declaration of a Reference
<type> & <name> = <existing variable>;
 Examples of Declaration
int j = 5;
int& i = j;
7OOP in C++Jan 2014

References
 Wrong declarations
 int& i; // must be initialized
 int& j = 5; // may be declared as const reference
 int& i = j+k; // may be declared as a const reference
 Often used as function parameters :
 called function can change actual argument
 faster than call-by-value for large objects
8OOP in C++Jan 2014

References Do not ..
 Cannot have an array of references
 No operator other than initialization are valid on a
reference.
 Cannot change the referent of the reference (Reference
can not be assigned)
 Cannot take the address of the reference
 Cannot compare two references
 Cannot do arithmetic on references
 Cannot point to a reference
 All operations on a reference actually work on the
referent.
9OOP in C++Jan 2014

Returning a Reference
Returning a reference
 return value is not copied back
 may be faster than returning a value
 calling function can change returned
object
 cannot be used with local variables
10OOP in C++Jan 2014

Returning a Reference
#include <iostream>
using namespace std;
int& max(int& i, int& j) {
if (i > j)
return i;
else
return j;
}
int main(int, char *[]) {
int x = 42, y = 7500, z;
z = max(x, y) ; // z is now 7500
max(x, y) = 1 ; // y is now 1
cout << "x = " << x;
cout << " y = " << y;
cout << " z = " << z << "n";
return 0;
}

Pointers vs. References
 Pointers can point to NULL whereas References
cannot. There is nothing defined as NULL
Reference.
 Pointers can point to different variables at
different times whereas for a reference, its
referent is fixed.
 References make code faster since, unlike
pointers, checks for NULL are not required.
 Reference “refers” to an address but does not
store that address. Pointer does.

Macros
 Macros are expanded at the places of their calls.
 Advantages:
 Speed-wise efficient
 Disadvantages:
 Parameter passing mechanism is not robust and
frequently leads to errors.
 Type checking during parameter passing is not done
 Code size tend to increase
 Typical Use:
 Small code re-use

Inline Functions
 Inline functions act like functions
 They can be class members
 Type checking is performed
 They can be overloaded
 They obey normal parameter passing rules
 But they are implemented like macros
 Code is substituted inline, not called
 Use is faster than calling a function
 Use may take more space
 They are defined in .h files, not in .c/.cxx files

Inline Notes
 inline specification is only a
recommendation.
 A recursive or a large function may not
be inline.
 Unlike a non-inline function, an inline
function must be defined in every text
file where it is called.
 Inline functions must not have two
different definitions.
 May cause unexpected behavior if compiler
does not chose to make the function inline.

Default Function Arguments
 Default arguments are appropriate argument
value of a parameter for a majority of cases.
 Default arguments can be supplied to one or
more parameters.
 Default arguments may be expressions also.
 All parameters to the right of a parameter with
default argument must have default arguments.
 Default arguments cannot be re-defined.
 Default parameters should be supplied only in a
header file and not in the definition of a function.

Default Arguments: Example
 Following are some examples of
functions with default arguments.
Void ff (int, float = 0.0, char *); // Error
Void ff2(int, float = 0, char *=NULL); // OK
Void ff2(int float = 1, char *= NULL); // Error – Redefinition
Assume that ff.h contains the following declaration
ff(int, float, char = ‘a’);
Wrong example:
#include “ff.h”
ff(int i, float f = 0.0, char ch = ‘a’); //Error
However, the following are correct.
#include <ff.h>
ff(int i, float f = 0.0, char ch); //OK
ff(int i = 0, float f, char ch); //OK

Function Overloading
 The same function name may be used in several definitions.
 Functions with the same name must have different number of
formal parameters and/or different types of formal
parameters.
 Function selection based on number and types of the actual
parameters at the places of invocation.
 Function selection (Overload Resolution) is performed by the
compiler
 Two functions having the same signature but different return
types will result in a compilation error due to “attempt to re-
declare”.
 Overloading allows static polymorphism

Overload Resolution
 Steps to resolve overloaded functions
with one parameter
 Identify the set of candidate functions and
the set of viable functions.
 Select the best viable function through
(Order is important)
 Exact Match
 Promotion
 Standard type conversion
 User defined type conversion

Overload Resolution
 Steps to resolve overloaded functions
with one parameter
 Example:
1. void f();
2. void f(int);
3. void f(double, double = 3.4);
4. void f(char, char *);
f(5.6)
Candidate function: 2 & 3
Best function: 3

Exact Match
 lvalue-to-rvalue conversion
 Most common
 Array-to-pointer conversion
Definitions: int ar[10]; void f(int *a);
Call: f(ar)
 Function-to-pointer conversion
Definitions: int (*fp) (int);
void f(int x, fp); int g(int);
Call: f(5, g)
 Qualification conversion
 Converting pointer (only) to const pointer.

Promotion & Conversion
 Examples of Promotion
 char to int; float to double
 enum to int/long/unsigned int/…
 bool to int
 Examples of Standard Conversion
 integral conversion
 floating point conversion
 floating point – Integral conversion
The above 3 may be dangerous!
 pointer conversion
 bool conversion

Examples of Resolution
1. Promotion
enum e1 { a1, b1, c1 };
enum e2 { a2, b2, c2 = 0x80000000000 };
char *f(int); char *f(unsigned int);
int main() {
f(a1); //Which f?
f(a2); //Which f?
}
2. Standard Conversion
void print(int); void print(char *);
void set (int *);
void set (const char *);
int main() {
print (0); //Which print?
set (0); //Which set?
}
3. Conversion Sequence
int arr[3];
void putValues(const int *);
int main() {
putValues(arr);
}

new/delete operators
 In C++, the new and delete operators provide built-in
language support for dynamic memory allocation and de-
allocation.
int *pI = new int;
int *pI = new int(102); //new initializes!!
int *pArr = new int[4*num];
Arrays generally cannot be initialized.
const int *pI = new const int(100);
Array of constant cannot be created.
delete pI;
delete [] pArr;
 new is polymorphic
 new does more than malloc!

new/delete & malloc/free
 All C++ implementations also permit use of
malloc and free routines.
 Do not free the space created by new.
 Do not delete the space created by malloc
 Results of the above two operations is memory
corruption.
 Matching operators
 malloc-free
 new-delete
 new[] – delete []
 It is a good idea to use only new and delete in
a C++ program.

CLASS

TOPICS
 Class Members
 Constructor & Destructor
 Friends
 Static Members
 Struct & Union

Class
 C++ Class is an implementation of “type”
 In C++, class is the only way to implement user defined
data types.
 A C++ class contains data members/attributes to specify the
state and operations/methods to specify the behavior.
 Thus, classes in C++ are responsible to offer needed data
abstraction/encapsulation of Object Oriented Programming.
 C++ Classes also provide means (through access specifier)
to enforce data hiding that separates implementation from
interface.

Class
 A Class is defined by “class” keyword.
 Each member in a class has an access specifier.
 private – these members are accessible inside the definition
of the class.
 public – these members are accessible everywhere.
 Objects/instances of classes are to be created statically or
dynamically.
 Members can be accesses by “.” operator on the object.
 The implicit this pointer holds the address of any object.
 this pointer is implicitly passed to methods.

A Simple Class
class Employee {
public:
void setName (const char *x)
{ name = strdup(x); }
void setSalary (float y)
{ salary = y; }
char *getName ( ) { return
name; }
float getSalary ( ) { return
salary; }
private:
char *name;
float salary;
};
int main ()
{
Employee e1; Employee *e2;
e2 = new Employee;
e1.setName(“Amit”); e2->name =
strdup(“Ashis"); // Error
e2.setSalary(29100); e2-
>setSalary(29100);
}
Re-look at
void setName (const char *x) {
name = strdup(x); }
Whose name?
void setName (const char *x) {
this->name = strdup(x); }
Jan 2014 OOP in C++ 30

More on this
 Type of this
 X * const
 Necessity of this to the programmer
 Distinguishing data member from non-member
variable
 Explicit Use
class DoublyLinkedNode DoublyLinkedNode::append(DoublyLinkedNode *x)
{ {
DoublyLinkedNode *prev, *next; next = x;
int data; x->prev = this;
public: }
void append(DoublyLinkedNode *x);
}

Constructor Functions
 Constructor functions:
 are member functions with the same name as
the class
 are automatically called when an object is
created, just after memory allocation
 In case of auto/static variables, objects are created in the
stack/static Store when their definition is encountered.
 Objects can be created in the Free store with a pointer storing
the address.
 allow the class to initialise the state of an object

Constructor Functions
 Constructor functions:
 Constructors also allocate additional memory
space from the Free store (if) required for the
object.
 must not have any return type even void
 Default constructors are those constructors
which either do not have an argument or have
default arguments for all parameters.
 If users do not define any constructor then the
compiler provides a default constructor.

Additional Constructor Functions
 Constructor function can be overloaded
 Constructor functions can be directly
called to create anonymous objects
 Calling a constructor from a constructor
does not have the desired effect.
 If there is at least one definition of
constructor but no default constructor
then the following statements are
incorrect
 X a;
 X *pX = new X();

Destructor Functions
 Destructor function:
 is a member function named “~ <class-name>” (e.g. ~
String ( ) )
 is automatically called when an object is destroyed,
just before memory is freed
 For auto variables when the variable goes out of scope
 For objects created in the Free store, destructor is called after
“delete” or “delete[]” is explicitly invoked by the programmer.
 must not have any argument or return value
 If destructor is not called then there could be
memory leaks for objects which calls “new” in
the constructor.

Copy Constructor
 Copy constructor is a special constructor
which is used to initialize an object with
another object of the same type.
 Copy constructor of class X takes an
argument of type X &.
 If the type of argument is X in stead of X&, an
infinite loop results.

Copy Constructor
 Situations where copy constructor is used:
 Actual parameter passing in call-by-value
 Return Value passing
 Initializing an object with an object of the same
type as shown in the following example.
 The compiler provides a default
implementation of the copy constructor.

A Sample Class : String
class String {
public:
String();
String(const String&); // Copy Constructor
String(const char *);
~ String();
int length();
int read();
void print();
private:
char *data;
int len;
}

Class String::Constructor &
Destructor
String::String() {
data = NULL;
len = 0;
}
String::String(const char *s) {
data = new char[strlen(s)+1];
len = strlen(s);
strcpy(data, s);
}
void ~String() {
if (data != NULL)
delete [] data;
}
int main()
{
String s1; //default constructor
String s11(); //Error
String s1(“test”);
char str[6];
strcpy(str, “Hello”);
String s2(str);
String s3(s2); //Copy Constructor
String s4 = new String(“one”);
String s5 = new String();
delete s4;
delete s5;
}

Arrays
 Using Default constructor while
creating an array of objects
 String arrayOfString[100]; // 100 objects created using the default
constructor
 Using specialized constructor while
creating an array of objects.
 String arrayOfString[2] = { String(“abc”), String(“def”) };
 Using different constructor for creating
array objects dynamically.
 String *pArrayOfString[2] = { new String(“abc”), new
String(“def”) };

Object Layout
 Simplistically, an object
of a class must have
enough space to store all
members of that class.
 No space is required per
object for storing function
pointers for the methods
declared in the class.
 Methods on objects are
translated by the
compiler to C-like
function calls by passing
this pointer.
A String Object
data
len
H e l l o n

Members as Objects
 Sometimes a class may have a data attribute
which is an object of a class.
 Employee class may have a member “name” whose
type is String.
 When an Employee object is initialized then
name must also be initialized.

Members as Objects
 Initialization of member objects can be arranged
through the use of initializer lists
 Initializer lists appear as a comma separated list
 following a colon, after a constructor’s parameters
 and before the constructor definition
 where each list item is a named member object followed by its
initializer value in parenthesis
 Initializer lists are required for a member object that
must be passed initial arguments for construction
 Initializer lists are required for const members and
reference members

Class Member: Notes
 It is always a good idea to define data members as
“private”.
 Composition through objects are preferred over
Composition through pointers
 Saves time and life cycle management
 Not possible in a number of situations
 Contained object is shared by many containers.
 Contained object may not always be present.
 Methods should not return non-const reference or
pointer to less accessible data
 Defeats basic data hiding philosophy.
 May also lead to stunning consequences.

Const Member Functions
 Constant Member Functions
are not allowed to change
the state of the object on
which they are invoked.
 Const Functions are declared
with keyword const following
member function parameter
list.
 const must also be present
at the definition.
 Type of this pointer passed
to a const function is
const X* const this
class X {
private:
int ipriv;
public:
int ipub;
int f() const;
};
int X::f() const
{
int temp;
temp = ipriv + ipub; //accessing members OK
ipriv += temp; // cannot modify any member
ipub -= temp; // cannot modify any member
}

Friend Functions
Friend functions
 are declared in one or more classes
 have access to the private members of
those classes
 are distinguished from members with
the keyword friend
 are not called with an invoking object
of those classes

Friend Functions: Example
class String {
public :
String();
String(const char *);
String(int len);
friend String concat(String *, String *)
friend String concat(String *, char *);
friend String concat(char *, String *);
private :
char *data;
int len;
} ;
String::String(int len)
{
this->len = len;
data = new char[len+1];
data[len] = ‘0’;
}
String concat(String *left, String *right)
{
String both[left->len + right->len + 1];
strcpy(both.data, left->data);
strcat(both.data, right-> data) ;
return both;
}
String concat(char *left, String *right)
{
String both[strlen(left) + right->len +
1];
strcpy(both.data, left);
strcat(both.data, right->data);
return both;
}

Friends of More Than One class
class Matrix; // Forward declaration to make
// declaration of crossprod legal
class Vector {
public :
Vector(int n) ;
friend Vector prod(Matrix *pM, Vector *pV);
private :
int elements[10] ;
int n;
};
class Matrix {
public :
Matrix(int m, int n) ;
friend Vector prod(Matrix *pM, Vector *pV);
private :
int elements[10][10];
int m, n;
};
Vector prod(Matrix *pM, Vector *pV)
{
int V(pM->m);
for (i = 0; i < pM->m; i++)
for (j = 0; j < pM->n; j++)
{
v[i] += pm->elements[i][j]*
pV->elements[i];
}
}

Friend Members & Class
 Member of a different class
may be a friend function.
 A class may be declared as a
friend implying all member
functions of that class are
friends.
 Friend-ship is neither
commutative nor transitive.
 Friends tend to break data
hiding.
 It is best to avoid friend in
an ideal OO Design.
class Matrix;
class Vector {
public:
void Vector(int n);
Vector prod(Matrix *pM);
int elements[10];
int n;
} ;
class Matrix {
public:
void Matrix(int m, int n);
friend Vector Vector::prod(Matrix *pM);
private:
int elements[10][10];
int m, n;
} ;

Static Data
 A static data member is
shared by all the objects
of a class.
 Static data member may
be public or private.
 Static data member must
be initialized in a source
file.
 It can be accessed
 with the class-name
followed by the scope
resolution operator “::”
 as a member of any
object of the class
class X {
public:
static int count; // This is a
declaration
X() { count++; }
}
X::count = 0; //Definition & Initialization
int main()
{
X a, b, c;
printf(“%d %d %d %dn”, X::count, a.count,
b.count, c.count);
}
The output will be 3 3 3 3

Static Data: A List
class ListNode {
public:
static ListNode *first;
private:
ListNode *next;
int data;
public:
ListNode(int x);
print();
}
List Node *ListNode::first = NULL;
ListNode::ListNode(int x) {
data = x;
next = NULL;
if (first == NULL)
first = this;
else
{
next = first;
first = this;
}
}
void ListNode::print() {
ListNode *x;
x = ListNode::first;
while (x != NULL)
{
printf(“%d “, x->data);
x = x->next;
}
printf(“n”);
}
int main() {
ListNode x(5);
ListNode x(7);
ListNode x(8);
x.print();
}
The output will be 8 7 5

Static Member Functions
 Static member functions
 May be invoked
 with the class-name::
 class_name::static_function (args)
 as part of any objects interface
 any_object.static_function (args);
 this pointer is not passed to a static
function
 must not refer to non-static members of
its “invoking object”
 Philosophy of static members.

Static Members in Inline
Functions
 Not Safe
X.hxx
Class X
{
public:
static void f();
static String g();
private:
static String s;
}
inline String X::g()
{
// do some operation on s
return s;
}
X.cxx
#include “X.hxx”
void X::f()
{
X::g();
}
The above code will not fail;
The code in the following
may fail however.
X1.cxx
#include “X.hxx”
int main()
{
X::g();
}
Data members are
guaranteed to be
initialized before any non-
inline function is called.

OPERATOR OVERLOADING

Overloading Operators
 Semantics of an operator is
represented by a function named
operator op, where op is an operator
symbol (+,*, - , [ ], etc. )
 These functions can either be
implemented as global friend functions
and/or methods.

Overloading Operators
 Example
 Let ‘+’ denote concatenation for Strings.
 “s1 + s2” denote concatenation of strings s1
and s2.
 An expression of the form “s1+s2” is
converted to s1.operator+(s2) if there is a
function named “operator+” defined in the
class String.
 “s1+s2” is also equivalent to operator+(s1,
s2) if a global function named “operator+” is
properly defined; typically such global
functions are friends to the class String.

Example of overloading
operator +
/ * string .h * /
const int max_string_length = 128 ;
class String {
public :
String operator + (char *text) ;
String operator + (String &s1) ;
String (char *data);
String () { data = NULL; len = 0; }
private :
char *data;
int len;
} ;
String operator+(char * text)
{
char *save = new char[len+1];
strcpy(save, data);
data = new char[len +strlen(text)
+ 1];
strcpy(data, save);
stcat(data, text);
delete[]save;
return (*this);
}

Overloading Assignment
Operator
String & String: :operator=( char *rhs)
{
data = new char [strlen(rhs) + 1];
strcpy ( data, rhs);
return *this ;
}
Bug:
 Memory Leak as previous
data of this is not
deleted.
.
.
.
String String: :operator=( String &rhs)
{
data = new char [rhs.len + 1];
strcpy ( data, rhs.data);
return *this ;
}
Bug:
 The following
assignment cannot be
made
 String x(“abc”), y(“def”),
z(“ghi”);
 x = y = z;
 (x.operator=((y).operator=(z)
));
Solution
 Change the return
type to String &

Overloading Assignment
Operator
String & String: :operator (String &rhs)
{
delete [] data;
data = new char [rhs.len + 1];
strcpy ( data, rhs.data);
return *this ;
}
Bug:
 Self Assignment will cause problem
Solution:
 Check the following condition and return if
false.
if (this != rhs) .

Reference & Overloading
 Suppose that we have a
class Integer which has a
private data member as
int. The following
function is declared as a
friend of Integer.
Integer & operator*(Integer &lhs,
Integer &rhs) {
Integer result = lhs.data *
rhd.data;
return result;
}
Problem: Returning
reference to a local
object. The code fails.
Integer & operator*(Integer &lhs,
Integer &rhs)
{
Integer *result = new
Integer( lhs.data * rhd.data);
return *result;
}
Who deletes?
The caller.
What about the following
use?
Integer a(1), b(2), c(3),
d(3);
Integer e =
a*b*c*d;

More On “=” Operator
Overloading
 There is a system defined implementation of
assignment operator. In absence of user defined
assignment operator function, systems’ function is
used.
 System defined function makes a shallow copy.
 Some times shallow copying may be dangerous
 If the constructor uses new, assignment operator
may have to be overloaded.
 If there is a need to define a copy constructor then
there must be a need to overload assignment
operator and vice-versa.

Overloading Unary Operators
 Typically methods implementing unary
operators will not have any argument.
 Exceptions:
 post increment and post decrement operators.
 As there are two operators with the same symbol “+
+”, the name of the function is the the same.
 Prototype for pre increment function:
 void operator ++ ( );
 Prototype for post increment function:
 void operator ++ (int a);

Operator Overloading Facts
 Some operators can not be implemented as
global(friend) functions.
 =, [] etc.
 Some Operators cannot be overloaded
 ::,.*.?:,., sizeof() etc.
 Precedence or arity of an operator cannot be
changed by overloading an operator.
 Conditional Operators like &&, ||, comma
operator should not be overloaded.

Friends Vs Methods
 Members are better in terms of restriction of
scope which results in efficient searching for
best function to resolve overloading.
 Members will not help if the left hand side of the
operator is not of the class type.
 String s1 = “abc” + s2; // may be wrong
 In case of overloading stream operators, we
need friend due to the reason stated above.
 Resolving in case of a conflict between friend
and method.

Returning const from a function
 Consider the following definition
const Rational & operator * (const Rational & lhs,
const Rational & rhs);
 If the function returns a non-const, the
following is valid.
Rational a, b, c;
(a * b) = c;
 Retuning const ensures that overloaded “*” is
compatible with the same operator on built-in
types.

Overloading using const
Class String {
public:
char & operator [ ] (int pos)
{ return data[pos]; }
const char & operator [ ] (int pos)
{ return data[pos]; }
private:
char * data;
……
}
String s1 = “Hello”;
cout << s[0]; // fine
s[0] = ‘x’; // fine
const String cs = “World”;
cout << s[0]; // fine
s[0] = ‘x’; // Error!!!

Bit-wise const vs. Conceptual
const
 What should a constant member
function preserve?
 Bit-wise const-ness
 Conceptual const-ness
 Bit-wise const member functions may
become unsafe.
 Conceptual const member function may
need to change some bits of the object
 mutable keyword.

INHERITANCE
Object-Oriented Programming in C++

Topics
 Fundamentals of Inheritance
 protected Access Specifier
 Initialization
 Virtual Functions

Reusability
 Reuse an already tried and tested code
 Advantages:
 Reduces cost & development time.
 Improves quality
 C Style Code Reuse
 Library Function
 Disadvantage: Reuse cannot be customized
 C++ Style Reuse:
 Inheritance
 Composition

Basics of C++ Inheritance
 If a class A is derived from another class B then
A is called the derived/sub class and B is called
the base/super class.
 All (?) data members and methods of the base
class are immediately available to the derived
class.
 Thus, the derived class gets the behavior of the
base class
 The derived class may extend the state and
behavior of the base class by adding more
attributes and methods.

Accessibility of Base Class
Members
 What happens to the access specifier of the
members of the base class when they are
derived?
 Depends on the mode of derivation.
 In public inheritance, private members of the
base class become private members of the
derived class and public members of the base
class become public members of the derived
class
 However, private members of the base class
are not directly accessible to the members in
the derived class.

Assume the following
class hierarchy
class C: public B
{ .. };
class B: public A
{ .. };
class A
{ .. };
Object Layout in Inheritance
Layout for an object
of type C
A-Part Data Member
B-Part Data Member
C-Part Data Member

protected Members
 private data members of the base class
cannot be directly accessed by the
methods of the derived class.
 However, it is important for the derived
class to have more accessibility to the
members of the base class than other
classes or functions.
 If a member is protected then it is
directly accessible to the methods of the
derived class.

Syntax of Inheritance
 An example
class Employee {
protected:
float basic;
long id;
public:
Employee(long id);
float getSalary();
};
class Manager : public Employee {
protected:
Employee *supervised[10];
int numberOfPeopleManaged;
public:
Manager(Id, n);
float getSalary();
void printSupervisedEmployeeId();
}

Order of Constructor Calls
 The constructor of the derived class is
responsible for initializing the state of the
derived object.
 The derived object contains attributes which
are inherited by the derived class.
 The constructor of the derived class calls an
appropriate constructor of the base class
 Therefore, the constructor of the base class is
executed first and then the constructor of the
derived class is executed.

Example of Derived Class
Constructor
Employee::Employee(long id)
{
this->id = id;
}
Manager::Manager(long id, int n) : Employee(id)
{
numberOfPeopleManaged = n;
}

Order of Destructor Calls
 The destructor of the derived class is
responsible for cleaning up the state of
the derived object.
 The derived object contains attributes
which are inherited by the derived class.
 The destructor of the derived class calls
the destructor of the base class
 Therefore, the destructor of the base
class is executed first and then the
destructor of the derived class is
executed.

Casting
 Derived class pointer can be implicitly
cast to a base class pointer
Manager m;
Employee *e = &m; // Employee *e = (Employee *)(&m);
 Only base class part of the derived object
can be seen through the base pointer.
e-> printSupervisedEmployeeId(); //error

Casting
 A Base class pointer cannot be implicitly
cast to a derived class pointer
Manager *pM; pM = e; //error
pM = (Manager *)e; //ok
Down casting may be dangerous

Static vs. Dynamic Binding
 Binding refers to associate a type to a
name.
 Consider the following example:
Manager m; Employee *e = &m;
e->getSalary(); //Which getSalary? Employee or Manager?
 “e” is declared as a pointer to Employee

Static vs. Dynamic Binding
 Continue on the example:
Manager m; Employee *e = &m;
e->getSalary(); //Which getSalary? Employee or Manager?
 In the example however, it makes more sense to
mean “getSalary” of the Manager class.
 We need a dynamic binding of “e” so that the type
of “e” may be set at run time by pointer to the
type of the actual object
 This is also called late binding

Virtual Functions
 In C++, dynamic binding is made
possible only for pointer & reference
data types and for methods that are
declared as virtual in the base class.
 If a method is declared as virtual, it
can be overridden in the derived class.
 If a method is not virtual and it is re-
defined in the derived class then the
latter definition hides the former one.

Virtual Function: Example
class X{ class Y: public X{
public: public:
int f(){ return 2; } int f(){ return 4;}
virtual int g(){ return 3;} int g(){ return 6;}
}; };
main() {
Y a;
int i, j , k, m;
X *b = &a;
i = b->f(); j = a.f();
k = b->g(); m = a.g();
printf(“%d %d %d %dn”, i, j, k, m);
}
Output will be 2 4 6 6

Redefining a Non-Virtual
Function
 Simply do not do that.
class X() { class Y : public X {
protected: protected:
void f(); void f();
}; };
int main() {
Y y1;
Y *pY; X *pX;
pX = &y1;
pX->f(); // f as defined in X will be called
pY = &y1;
pY->f(); // f as defined in Y will be called
}

Virtual Function Table
 If a class has a virtual
function, then each
instance of that class
contains a space to store
a pointer to the actual
definition of that function.
 During object creation,
the actual address of the
function is assigned to
the function pointer.
 Y is derived from X,
which has a virtual
function.
X-part-data
X-part-virtual-function-ptr
Y-part-data
Actual definition of
the virtual function

Abstract Class
 Pure Virtual Function
 A virtual function may be assigned to NULL meaning that
this function is declared but not defined in a class.
 Definition of such a class is incomplete.
 A class with one or more pure virtual function is
called an abstract class.
 Abstract class cannot be instantiated.
 Abstract class define a contract or interface to be
used by the user of the class library and to be
implemented by the developer of the class library.

Virtual Destructor
 Constructors cannot be virtual
 For a base class which has been derived from,
the destructor must be declared virtual.
 Occasionally we create a derived object and store
it using a pointer to Base class such as
 Base *pBase = new Derived(/*arguments*/);
 If we destroy this object using “delete pBase”
then two destructors need to be called.
 If the destructor in the Base class is not declared
virtual then the destructor of the Derived class
will not be automatically called in this example.

Inheritance Example:
Polymorphic Array
 Consider an abstract base class Shape which
contains a pure virtual function “CalculateArea”.
 Suppose three classes Triangle, Rectangle and
Circle derived from Shape.
 Consider a main function that creates different
Shape objects and store them in an array.
 If in a for loop the function calculateArea is
called on all objects in the array, we see
dynamic binding in use.

Polymorphic Array: Class
Definitions
class Shape {
public:
virtual double calculateArea() =
0;
};
class triangle : public Shape() {
private:
Point a, b, c;
Triangle(double x_a, double y_a,
double x_b, double y_b,
double x_c, double y_c);
public:
double calculateArea();
};
class Circle : public Shape() {
private:
Point centre;
double radius;
Circle(double x_centre,
double y_centre,
double r);,
public:
double calculateArea();
};

Polymorphic Array: main
function
int main() {
Shape *pArr = NULL;
int n = getInput(pArr);
for (int i = 0; i < n; i++) {
double area =
Shape[i]->calculateArea();
printf (“%d %lf n”, i, area);
}
}
int getInput(Shape *pArr) {
printf(“Which Shape do you want to
create?n”);
printf(“Write 1 for triangle, 2 for
rectangle, 3 for circle and 0 to
quitn”);
int i,
double x_a, x_b, x_c, y_a, y_b, y_c;
while (1) { int j = 0;
scanf(“%d”, &i);
switch (i) {
case 0: break;
case 1:
scanf(“%f%f%f%f%f%f”, &x_a,
&y_a, &x_b, &y_b, &x_c, &y_c);
pArr[j++] = new Triangle(&x_a,
&y_a, &x_b, &y_b, &x_c, &y_c);
break;
……..
}
}
}

Inheritance: Benefits
 Code Sharing/Reuse
 Consistency of Interface
 Construction of Software Components
 Rapid Prototyping
 Information Hiding

Inheritance: Cost
 Execution Speed
 Program Size
 Message Passing Overhead
 Program Complexity

Inheritance: Limitations
 operator= cannot be inherited
 Can be used to assign objects of the same
type only
 Copy Constructor cannot be inherited
 Static members are inherited in a
derived class
 Static members cannot be “virtual”
 If you redefine a static member function, all
other overloaded functions in the base class
are hidden

Inheritance Notes
 Constructors cannot be virtual
 Calling a virtual function from within a
constructor does not have the desired effect.
 The following code is buggy. Tell why.
void f(Base *b) { int main() {
b[0].f(); b[1].f(); Derived d[10];
} f(d);
}
 Derived is publicly derived from Base.
 Class Base has a virtual function “f” which is
redefined in Derived.

Default Parameter & Virtual
Function
 Do not change the default parameter in a
redefined virtual function
class X() { class Y : public X {
protected: protected:
virtual void f(int i = 10); virtual void f(int i =20);
}; };
int main() {
Y y1;
Y *pY; X *pX;
pX = &y1;
pX->f(); // f with value of i as 10 will be called
pY = &y1;
pY->f(); // f with value of i as 20 will be called
}

Is an Ostrich a Bird
 Suppose there is a base class Bird
 a virtual method fly returns altitude > 0.
 A class Ostrich is derived from Bird.
 fly method has to be redefined as an
empty function.
 Leads to a logical dilemma.
 Can an overridden method be empty?
 Can an overridden method throw
exceptions?

Is a Circle an Ellipse?
 Circle is a special type of ellipse.
 Let Circle be derived from Ellipse.
 Suppose that Ellipse has a method
setSize(x,y).
 Also suppose that there is a function sample as
defined below.
sample (Ellipse &e) { e. setSize(10,20); ……. }
 If sample is called on a circle, strange things
happen!
 Subset is not substitutable!!

Should a Stack inherit from a
List?
 Probably Not!
 If List is the base class of Stack
 Methods such as push, pop etc. are to be
defined (at least as pure virtual) in the
List class.
 All members of List must have (even a
trivial) implementation in Stack.
 A Stack has a List.

Multi-level Inheritance
 Suppose that C is derived from B and B is
derived from A.
 Suppose that a method, f, in A is virtual.
 If f is redefined in B then f is virtual even if the
keyword “virtual” does not precede the
declaration/definition in the derived class.
 It is advisable to explicitly write “virtual” in
front of the definition of f in B as, otherwise,
an implementer of C may think that f is not a
virtual method.

Inheritance & Code Reuse
 Suppose that C and B and are derived from A.
 Both C and B contain a function f ; therefore, f
is made a virtual (not pure) function in A.
 This is bad.
 A new class D is required to be derived from A
later.
 f in D is different than A.
 Interfaces should not have implementation.

private Inheritance
 If B is privately derived from A then private,
protected and public members of A become
private members of B. However, private
members of A are not directly accessible to B.
 Thus, even if C is publicly derived from B then
no member of A is accessible to C.
 Functions which may access members of A in B
are
 Methods of class B
 Friend functions of class B.

protected Inheritance
 If B is protectedly derived from A then,
protected and public members of A become
protected members of B. However, private
members of A remain private in B and are not
directly accessible to B.
 Functions which may access members of A in B
are
 Methods of class B
 Friend functions of class B.
 Methods in classes publicly derived from B
 Friend functions of classes publicly derived from B

Private Inheritance: Implications
 public Inheritance models “is a”
 private inheritance models “is implemented in
terms of ”
 Assume two classes, Set and List.
 Set contains unique elements while List may contain
duplicate elements.
 Thus Set is not a List
 But a Set can use the code of the List class as a Set
can be implemented in terms of a list.
 Users of the class Set should not have an access to
the List behavior even to create further derived
classes

TYPE CASTING
Jan 2014 OOP in C++ 105

Type Casting
 Why casting?
 Casts are used to convert the type of an object,
expression, function argument, or return value to
that of another type.
 (Silent) Implicit conversions.
 The standard C++ conversions and user-defined
conversions
 Explicit conversions.
 type is needed for an expression that cannot be
obtained through an implicit conversion
 more than one standard conversion creates an
ambiguous situation
Jan 2014 OOP in C++ 106

Type Casting
To perform a type cast, the compiler
 allocates temporary storage
 Initializes temporary with value being cast
float f (int i,int j) {
return (float ) i / j;
}
// compiler generates:
float f (int i, int j) {
float temp_I = i, temp_j = j;
return temp_i / temp_j;
}

Automatic (Implicit) Conversion
 Automatic conversions from one type to
other may be extremely convenient.
 Automatic conversions (either
conversion operators or single argument
non-explicit constructors) are unsafe as
well.
 can interfere with overload resolutions.
 can silently let wrong code compile cleanly.
String s1, s2, s3;
s3 = s2 – s1; // Though “-” is not overloaded in String
Jan 2014 OOP in C++ 108

Casting to User Defined Type
 Constructors help
 The following statement is not an error
even when an appropriate assignment
operator is not defined but an
appropriate constructor is defined
(which constructor?)
 String s1; s1 = “example”;

Casting to User Defined Type
 The assignment statement is
converted to the following
 s1 = String(“example”);
 Lots of temporary objects are created.
 Even the following statement is
correct:
 s1 = s2 + “abc” + “def”;

Ambiguities: Example
 Overuse of such
casting may lead to
ambiguities as
illustrated in the
following example
/* ambig.cpp */
#include “string.h”
class example {
public:
example(const char *) { } ;
} ;
void
f1 (const String & )
{
}
void
f1 (const example & )
{
}
int
main ( )
{
// f1 (“hello, world”) ; is ambiguous
f1 ((String) “hello world” );
f1 ((example) “hello world” );
// or provide void f1 (const char *)
return 0;
}

Ambiguity: Example
class B;
class A {
public:
A (const B &);
...
};
class B {
public:
operator A () const;
};
void f(const A &);
B b;
f(b); //Error - Ambiguous
 Which one to use for
casting?
 Constructor in A OR
 Cast Operator in B

Casting from Class Types
 To cast a user defined type to some other type,
type casting operator is to be defined explicitly.
 To convert to a type, say <type>, the following
function operator <type> is to be defined.
 Such functions do not have an explicit return
type and do not take any argument
(NATURALLY!).
 The following example illustrates how a string
object can be cast to a char * by defining a
function “operator char *()” .

Example of user defined cast
const int max_string_length = 128;
class String {
public:
String ( ) :
String (const String & ) ;
String (const char *);
~String ( ) ;
String & operator = (const String & )
;
operator const char * ( ) const ;
int length ( ) const ;
int read ( ) ;
void print ( ) const ;
.
.
private:
char text [max_string_length+1]
};
void operator-(const String &, const
String &);
int main ( ) {
int fd;
String filename = “tmp/test”;
// cast filename to type const char *
fd = open (filename, O_WRONLY |
O_CREAT, 0666);
write (fd, “test”, 4);
close (fd);
// not legal, since we can cast only
to const char *
// strcpy (filename, “zbc”);
String name = “Zaphod
Beeblebrox”;
name – “Beeblebrox”; // is now
ambiguous.
return 0 ;
}

Avoiding Ambiguities
const int max_string_length = 128;
class String {
public:
String ( ) ;
String (const String & ) ;
String (const char *);
~ String ( ) ;
String & operator = (const String
& );
const char *as_char_pointer ( )
const;
int length ( ) const;
int read ( );
void print ( ) const;
...
private:
char text [max_string_length+1];
};
void operator-(const String &, const
String &);
int main ( ) {
int fd;
String filename = “/tmp/test”;
// convert filename to type char *
fd = open
(filename.as_char_pointer ( ),
O_WRONLY | O_CREAT, 0666);
write (fd, “test”, 4);
close (fd);
// not legal
// strcpy (filename.as_char_pointer (
), “zbc”);
String name = “Zaphod
Beeblebrox”;
name – “Beeblebrox”; // is no longer
ambiguous
return 0;
}

Casting Pointers & References
 Casting a value:
 float f_var = 3.14;
 cout << (int) f_var;
 creates a temporary object
 does not violate data encapsulation
 Casting a pointer or reference
 cout << *(( int *) &f_var);
 cout << (int &) f_var;
 Re-interprets representation of object
 Violates data encapsulation

C++ casts
 There are four cast operators in C++
 const_cast
 static_cast
 reinterpret_cast
 dynamic_cast
 Only dynamic_cast is not equivalent
to a C-style cast

Prefer C++ casts to C-style
casts
 Type conversions of any kind (either explicit via
casts or implicit by compilers) often lead to
code that is executed at runtime.
 Easier to identify (can “grep”)
 C-style casts do not have a usage semantics
that compilers may use.
 More narrowly specified purpose for each
specified cast; compilers can diagnose user
errors.
 Some casts are performed safely at run-time.
Jan 2014 OOP in C++ 118

const_cast operator
 Syntax:
 const_cast < type-id > ( expression )
 The const_cast operator can be used to remove the
const, volatile attribute(s) from a class.
 A pointer to any object type can be explicitly converted
to a type that is identical except for the const, volatile
qualifiers. Applying on pointers and references, the
result will refer to the original object.
 The const_cast operator cannot be used to directly
override a constant variable's constant status.
Jan 2014 OOP in C++ 119

const_cast operator: Example
Example
#include <iostream>
class CCTest {
public:
void setNumber( int );
void printNumber() const;
private:
int number;
};
void CCTest::setNumber( int num )
{ number = num; }
void CCTest::printNumber() const {
cout << "nBefore: " << number;
const_cast< CCTest * >( this )-
>number--;
cout << "nAfter: " << number;
}
int main() {
CCTest X;
X.setNumber( 8 );
X.printNumber();
}
 On the line containing the
const_cast, the data type of
the this pointer is const CCTest
*. The const_cast operator
changes the data type of the
this pointer to CCTest *,
allowing the member “number”
to be modified. The cast lasts
only for the remainder of the
line on which it appears.

Usage of “const”
 The example of the previous slide is not
the best usage of const.
 The member “number” should be “mutable”
instead.
 When one has a const object and has to
pass it to some function that takes a
non-const parameter and it is known
that the function does not modify that
parameter then casting away const-ness
is both useful and safe.

Guidelines for const
 const is a powerful tool for writing safer code;
use it as much as possible but no more.
 Do not use const pass-by-value parameters in
function declarations. This is not useful and
misleading at best.
 When returning a user defined type, prefer
returning a const value.
 Do not return handles to internal data from a
const member function.
Jan 2014 OOP in C++ 122

static_cast operator
 Syntax:
 static_cast < type-id > ( expression )
 The static_cast operator converts expression to
the type of type-id based solely on the types
present in the expression.
 static_cast has basically same power, meaning
and restrictions as C-style cast. It cannot be
used to convert a struct into an int or a double
into a pointer.
Jan 2014 OOP in C++ 123

static_cast operator
 In general, a complete type can be converted
to another type so long as some conversion
sequence is provided by the language.
 may also use static_cast to convert any
expression to a void, in which case the value of
the expression is discarded.
 It cannot change the const-ness of an
expression.
Jan 2014 OOP in C++ 124

Example:
class B { ... };
class D:public B { ... };
void f(B* pb, D* pd){
D* pd2 =
static_cast<D*>(pb);
// not safe, pb
may point to just B
B* pb2 = static_cast<B*>(pd);
// safe conversion
...
}.
static_cast: Example
 The static_cast operator
can be used for
operations such as
converting a base class
pointer to a derived class
pointer . Such
conversions are not
always safe since no run-
time type check is made
to ensure the safety of
the conversion.For such
conversions dynamic_cast
should be used.

C-style casts VS static-cast
class Base {
public: Base() : _data(999) {}
int Data() const {
return _data;
}
private: int _data;
};
class Derived : private Base
{ public: Derived () : Base() {}
};
Derived *d1 = new Derived;
 The following C-style
code works even if .
int i = d1->Data(); //Error!
Base* b1 = (Base*) d1;
int i = b1->Data(); // works!
Base *b2 = static_cast<Base
*>(d1); //Error!
 The old C-style casts let
us cast from one
incomplete type to
another! static_cast does
not let you do that.
Jan 2014 OOP in C++ 126

reinterpret_cast operator
 Syntax:
 reinterpret_cast < type-id > ( expression )
 The reinterpret_cast operator allows any
pointer to be converted into any other pointer
type. It also allows any integral type to be
converted into any pointer type and vice versa.
 The reinterpret_cast operator can be used for
conversions such as char* to int*, or
One_class* to Unrelated_class*, which are
inherently unsafe.

reinterpret_cast operator
 The result of a reinterpret_cast cannot safely be used for
anything other than being cast back to its original type.
Other uses are, at best, non-portable.
 The reinterpret_cast operator cannot cast away the
const, volatile attributes.
 One practical use of reinterpret_cast is in a hash
function, which maps a value to an index in such a way
that two distinct values rarely end up with the same
index.
 Reinterpret_cast should rarely be used in a C++
program
Jan 2014 OOP in C++ 128

reinterpret_cast: Example
Example
#include <iostream>
unsigned short Hash( void *p ){
// Returns a hash code based on an
address
unsigned int val =
reinterpret_cast<unsigned
int>( p );
return ( unsigned short )( val ^ (val
>> 16));
}
int main(){
int a[20];
for ( int i = 0; i < 20; i++ )
cout << Hash( a + i ) << endl;
}
 The reinterpret_cast
allows the pointer to be
treated as an integral
type. The result is then
bit-shifted and XORed
with itself to produce a
unique index (unique to
a high degree of
probability). The index is
then truncated by a
standard C-style cast to
the return type of the
function.

Usage of casts
class A { public: virtual ~A(); …};
class B : private virtual A { …};
class C : public A { … };
class D : public B, public C { …};
A a1; B b1; C c1; D d1;
const A a2;
const A& ra1 = a1;
const A& ra2 = a2;
char c;
 A *pa = (A *)&ra1;
 Use const_cast
 B * pb = (B*)&c1;
 Use reinterpret_cast
 A *pa = (A*)&a2;
 Cannot be expressed
in a new-style cast.
 void *pv = &b1;
 B *pb = (B*)(pv);
 Use static_cast
instead;
Jan 2014 OOP in C++ 130

Avoid Unnecessary Cast
 Look at the cast and comment
class SpecialWindow: public Window { // derived class
public:
virtual void onResize() { // derived onResize impl;
static_cast<Window>(*this).onResize(); // cast *this to Window,
// then call its onResize;
// this doesn’t work!
... // do SpecialWindow-
} // specific stuff
...
};
Jan 2014 OOP in C++ 131

type_info class
 The type_info class describes type information
generated within the program by the compiler. The
entire definition of this class is implementation
dependent but the following features of this class is
standardized.
 Objects of this class effectively store a pointer to a
name for the type. The type_info class also stores an
encoded value suitable for comparing two types for
equality or collating order.
 The operators “==“ and “!=“ are overloaded and can
be used to compare for equality and inequality with
other type_info objects, respectively.
Jan 2014 OOP in C++ 132

type_info class
 Features of type_info class (contd):
 The “name” member function returns a const char*
to a null-terminated string representing the human-
readable name of the type. The memory pointed to is
cached and should never be directly deallocated.
 Type information is generated for polymorphic
classes only if the /GR (Enable Run-Time Type
Information) compiler option is specified.
Jan 2014 OOP in C++ 133

typeid operator
 Syntax:
typeid( type-id ) OR typeid(expression)
 The typeid operator allows the type of an object
to be determined at run time.
 The result of typeid is a const type_info&.
 The typeid operator does a run-time check when
applied to an l-value of a polymorphic class
type, where the true type of the object cannot
be determined by the static information
provided. Such cases are: a reference/ Pointer

typeid operator
 When the operand of typeid is of a non-
polymorphic class type, the result is the type of
the operand not the type of the underlying
object.
 type-id may be used with operands of built-in
types.

 The expression usually
points to a polymorphic
type (a class with virtual
functions).
 The pointer must be de-
referenced so that the
object it points to is used.
Without de-referencing the
pointer, the result will be
the type_info for the
pointer, not pointee
Example
#include <iostream>
#include <typeinfo.h>
class Base {
public: virtual void vvfunc()
{} }
class Derived : public Base {};
typeid:Example
int main(){
Derived* pd = new Derived;
Base* pb = pd;
cout << typeid( pb ).name() << endl;
//prints "class Base *”
cout << typeid( *pb ).name() <<
endl; //prints "class Derived"
cout << typeid( pd ).name() << endl;
//prints "class Derived *"
cout << typeid( *pd ).name() <<
endl; //prints "class Derived"
delete pd;
}

dynamic_cast operator
 Syntax:
 dynamic_cast<type-
id> (expression)
 The expression
dynamic_cast<type-
id>( expression)
converts the operand
expression to an
object of type type-
id.
 It is used for down-
casting.
Example
class B { ... };
class C : public B { ... };
class D : public C { ... };
void f(D* pd){
C* pc = dynamic_cast<C*>(pd);
// ok: C is a direct base class, pc
points to C subobject of pd
B* pb = dynamic_cast<B*>(pd);
// ok: B is an indirect base
class , pb points to B subobject of pd
...
}

dynamic_cast: Example
Example:
class B { ... };
class D : public B { ... };
void f(){
B* pb = new D; // unclear but ok
B* pb2 = new B;
D* pd = dynamic_cast<D*>(pb); // ok: pb actually points to a D
...
D* pd2 = dynamic_cast<D*>(pb2); // pb2 points to a B not a D the cast is bad
so pd2 is NULL
...
}

dynamic_cast
 If dynamic_cast to a pointer type fails, the result of the
dynamic_cast is null; if dynamic_cast to a reference type
fails, an exception is thrown.
 dynamic_cast is performed at run-time.
 dynamic_cast can be used only for pointer or reference
types to navigate a class hierarchy. The dynamic_cast
operator can be used to cast from a derived class pointer
to a base class pointer, cast a derived class pointer to
another derived (sibling) class pointer, or cast a base
class pointer to a derived class pointer. Each of these
conversions may also be applied to references.

dynamic_cast
 dynamic_cast operator cannot be used for built-in types.
 All of the derived-to-base conversions are performed
using the static (compile-time) type information. These
conversions may, therefore, be performed on both non-
polymorphic and polymorphic types. These conversions
will produce the same result if they are converted using a
static_cast. However, even these results may be wrong in
the presence of multiple inheritance.
 dynamic_cast is strongly recommended to be applied on
polymorphic types.

Cost of dynamic_cast
 The pointer to the type_info object of a class is
stored in the vtbl array of the class.
 Thus, the space cost for RTTI is an additional
entry in each class vtbl plus the cost of storage
for the type_info object for each class.
 Size of the objects do not increase for RTTI
operations.
 Cost of RTTI at run time is similar to the cost of
calling a virtual function; cost of calling a
virtual function is the same as the cost of
calling a function through a function pointer.

EXCEPTIONS

Topics
 Basic Concept of Exceptions
 try-catch block in C++
 Semantics of throw

Error Handling in C++
 Error Condition Handling - C Style
 via return value
 return statement is dedicated for passing
error conditions
 by output parameter
 normal and abnormal control flow
tend to mix
 Reusing code for error handling is
difficult.

Error Handling in C++
 Error Condition Handling - C++
Style
 On error condition an exception object
is created and thrown.
 A function catches exception objects
generated from the function it calls in
a distinct control flow.
 Similar Exception objects can enjoy
benefits of inheritance.

C-Style Error Handling
int
Calculator::divide (int i)
{
if (i == 0)
{
// what do we do?
}
else
{
value /= i;
}
return value;
}
 A Calculator need to
handle divide by zero
 Could set value to NAN
 But, program would need
to check for special value
(and might ignore)
 Could return –1
 Again program could
ignore
 Might be a valid return
value

“try” and “catch”
 A function has its usual prototype and it may
throw a number of exceptions on detecting
several error condition.
 “try” block encloses code that has usual flow of
control and can potentially throw exceptions
 “catch” block can occur after a “try” block or
another “catch” block
 catch blocks can catch and handle exceptions
of different types

Exception Object and throw
 Exception object is just another object having
members (attributes and methods) suitable to
model the state and behavior of an object
representing an error condition.
 Whenever an error condition is detected, a
suitable Exception object is thrown. Semantics
of throw is as follows.
 Creation of an object (function of new)
 passing control from this function to the caller
function (similar to return)
 Unlike return, throw initiates unwinding of the call
stack till the exception is handled.

Example of Exception Handling
in C++
class DivideByZero {
private:
int dividend;
public:
print() {
cout << dividend << “is divided by zero”
<<endl;
}
DivideByZero(int d) { dividend = d; }
};
int Calculator::divide(int i) throws DivideByZero
{
if (i ==0)
throw DivideByZero(value);
value /= i;
return value;
}
int main (int argc, char **argv) {
int i = 0;
Calculator c;
try {
c.divide (0);
cout << c.getValue ();
}
catch (DivideByZero& ext) {
ex.print();
return 1;
}
return 0;
}

Details of throw
 Normal program control flow is halted
 At the point where an exception is thrown
 The program call stack “unwinds”
 Stack frame for each function up call chain is popped
 Stack variables in each popped frame are destroyed
 Until an enclosing try/catch scope is reached where
the type of exception thrown is caught.
 Control passes to first matching catch block
 Can handle the exception and continue
 Can free resources and re-throw

More on “try” and “catch”
 Whenever a function is called in a try block, the
“catch” blocks to be examined after the try
block are known as the extended prototype of
a function includes the throw clauses.
 catch blocks after a try block are examined in
order when an exception is thrown from a
function called in the try block.
 Parentheses for each catch block has semantics
of a “function argument declaration”

Exception Specifications
// can throw anything
void
Calculator::subtract
(int i);
// promises not to throw
void
Calculator::add
(int i) throw ();
// promises to only throw int
void
Calculator::divide
(int i) throw (int);
 Make promises to the caller
 Allow stronger type checking
enforced by the compiler
 By default, a function can throw
anything it wants
 A throw clause in the signature
 Limits what a function can throw
 A promise to the calling function
 A throw clause with no types
 Promises nothing will be thrown
 Can list multiple types
 Comma separated

Stack Frame
 g++ -s gives assembler output that
can be used to deduce exact structure
for a given platform
 In general, the overall structure is
common
 A chunk of memory representing a
function call
 Pushed on program call stack at run-
time. Contains:
 The frame pointer
 The return address for the call (i.e., where
it was called from)
 Parameters passed to the function
 Automatic (stack) variables for the function
previous frame pointer
return address
parameters
automatic variables

Illustrating the Call Stack
int i = 0;
Calculator c;
try {
c.divide (0);
cout <<
c.get_value ();
}
catch (int) {
return 1;
}
return 0;
}
 Stack frame for function main
 Pushed on program call stack
 With stack variables i and c
 With parameters argc and argv
 Note that parameters are
initialized at frame creation
 Variables are not necessarily
initialized at frame creation
 May occur later in called
function
argc argv
i c value_
main

Illustrating the Call Stack, cont.
int i = 0;
Calculator c;
try {
c.divide (0);
cout <<
c.get_value ();
}
catch (int) {
return 1;
}
return 0;
}
 Enter function main
 Stack variable initialized to
0
argc argv
i c value_
main

int i = 0;
Calculator c;
try {
c.divide (0);
cout <<
c.get_value ();
}
catch (int) {
return 1;
}
return 0;
}
 Call Default Constructor for c
 Push a new stack frame
 No parameters or automatic variables for
that specific function
 Params depend on function signature
declaration
 Automatics depend on function body
definition
argc argv
i c value_
main
Calculator::
Calculator( ) this

Calculator::Calculator ()
: value_ (0)
{}
void
Calculator::divide (int i) throw (int)
{
if (i == 0) {
throw i;
} else {
value_ /= i;
}
cout << value_;
}
 Enter function Calculator::Calculator ( )
 Member variable value_ of stack variable c
is initialized to zero
 How do we know which value_ to set?
 There may be multiple Calculator instances
 Answer: implicit “this” parameter in stack
frame
argc argv
i c value_
main
Calculator::
Calculator() this

: value_ (0)
{
}
void
Calculator::divide (int i) throw
(int) {
if (i == 0) {
throw i;
} else {
value_ /= i;
}
cout << value_;
}
 Return from function
Calculator::Calculator ( )
 Pop the stack frame, return to
previous
argc argv
i c value_main

int main (int argc, char **argv)
{
int i = 0;
Calculator c;
try {
c.divide (0);
cout <<
c.get_value ();
}
catch (int) {
return 1;
}
return 0;
}
 Call divide method on c
 Push a new stack frame
 Contains parameters this and i
 Copy address of current instance into
this
 Copy value 0 into i
void Calculator::
divide (int )
main
argc argv
i c value_
this i

 Enter function Calculator::divide
(int)
 Test i equal to 0 and take the true
branch
 Throw integer i
main
void Calculator::
Calculator (int )
argc argv
i c value_
this i
: value_ (0)
{
}
void
Calculator::divide (int i) throw (int)
{
if (i == 0) {
throw i;
} else {
value_ /= i;
}
cout << value_;
}

: value_ (0)
{
}
void
Calculator::divide (int i) throw
(int)
{
if (i == 0) {
throw i;
} else {
value_ /= i;
}
cout << value_;
}
 Thrown exception unwinds call stack
 Notice control skips over cout statement
to end
 Pop the stack frame, return to previous
 Return from function void
Calculator::divide ( )
argc argv
i c value_
main

int main (int argc, char **argv)
{
int i = 0;
Calculator c;
try
{
c.divide (0);
cout << c.get_value ();
}
catch (int)
{
return 1;
}
return 0;
}
 We’ve reached an enclosing
try/catch scope
 So stack unwinding stops
 Control jumps to first matching
catch block
 Again, skips over intervening cout
statement
argc argv
i c value_
main

More on catch
try {
// can throw an exception
}
catch (Derived &d) {
// ...
}
catch (Base &b) {
// ...
}
catch (...) // catch all...
{
// ...
}
 Control jumps to first
matching catch block
 Order matters with
multiple possible matches
 Especially with
inheritance-related
exception classes
 Hint: put catch blocks for
derived exception classes
before catch blocks for
their respective base
classes
 More specific catch before
more general catch
 catch (…) catches any
type

A Few More on catch
try
{
// can throw an exception
}
catch (MyClass &e)
{
// ...
throw; // rethrows e
}
catch (int)
{
// ...
}
catch (...) // catch all...
{
// ...
}
 Notice catch-by-reference
for user defined types
 More efficient
 Only a reference propagates
 Rather than entire object
 More correct
 Avoids class-slicing problem
if catch as base type, rethrow
 Preserves polymorphism
 More on this in later lectures
 Can leave off variable
name
 Unless catch block needs
to do something with the
instance that was caught

TEMPLATES
Jan 2014 OOP in C++ 165

What is a Template?
 Templates are specifications of a
collection of functions or classes which
are parameterized by types.
 Examples:
 Function search, min etc..
 The basic algorithms in these functions are the same
independent of types.
 But, we need to write different versions of these
functions for strong type checking in C++.
 Classes list, queue etc.
 The data members and the methods are almost the
same for list of numbers, list of objects.
 We need to define different classes, however.

template<class X> void swap (X &one, X
&other)
{
X temp;
temp = one;
one = other;
other = temp;
}
Main() {
int I=10, j=20;
swap(I, j);
String s1(“abc”), s2(“def”);
swap(s1, s2);
}
Function Template: An Example
void swap(int &i, int &j)
{
int temp;
temp = i;
i = j;
j = temp;
}
void swap(String &i, String &j)
{
String temp;
temp = i;
i = j;
j = temp;
}
Type parameter
Type parameter list
Template instantiation

Parameterized Functions
 A function template
 describes how a function should be built
 supplies the definition of the function using some
arbitrary types, (as place holders),
 a parameterized definition
 can be considered the definition for a set of
overloaded versions of a function
 is identified by the keyword template
 followed by parameter identifiers
 enclosed between < and > delimiters
 noting they are class, (i.e. type), parameters

Template Non-type Parameter
 It is an ordinary parameter
 template <class T, int size> T min (T
(&x[size]));
 When min is called, size is replaced with
a constant value known at compile time
 The actual value for a non-type
parameter must be a constant
expression.

typename
 The key words class and typename have
almost the same meaning in a template
parameter
 typename is also used to tell the compiler that
an expression is a type expression
template <class T> f (T x) {
T::name * p; // Is this a pointer declaration or multiplication?
}
template <class T> f (T x) {
typename T::name * p; // Is this a pointer declaration or
multiplication?
}

Template Argument Deduction
 Each item in the template parameter list is a
template argument
 When a template function is invoked, the
values of the template arguments are
determined by seeing the types of the function
arguments
template <class T, int size> Type min( T(&x[size]));
int pval[9]; min(pval); //Error!!

Template Argument Deduction
 Three kinds of conversions are allowed.
 L-value transformation (e.g., Array-to-pointer
conversion)
 Qualification conversion
 Conversion to a base class instantiation from a class
template
 If the same template parameter are found for
more than one function argument, template
argument deduction from each function
argument must be the same

Explicit Template Arguments
 It is possible to override template argument
deduction mechanism and explicitly specify the
template arguments to be used.
 template <class T> T min(T x, T y);
 unsigned int ui; min (ui, 1024); //Error!!
 min<unsigned int>(ui, 1024); // OK
 Specifying return type generically is often a
problem.
 template <class T, class U> ??? sum(T x, U y);
 sum(ch, ui) returns U; sum (ui, ch) returns T
 template < class R, class T, class U> R sum(T x, U
y)
 min<int>(i, ‘a’);

Template Explicit Specialization
 Some times, a template may not be
suitable for all types.
 The following template is not good for
char *
template <class T> T min(T x, T y)
{ return (x < y) ? x : y); }
 Define the function explicitly for char *
template<> char * min <char *>(char *x, char *y)
{ return ((strcmp(x, y) < 0) ? x : y); }

A List Template Class
template<class T>
class List {
public :
List ();
virtual ~List ();
int put (const T &val);
T *unput ();
T *get ();
int unget (const T &val);
T *find(const T &val);
int length ();
private:
struct Node {
Node *next_item;
T *list_item;
} *beg_ptr; *end_ptr;
int how_many;
};
template<class T> int List<T>:: put (const T
&val)
{
Node *tmp_ptr = new Node;
if (tmp_ptr && (tmp_ptr->list_item = new
T (val) ) ) {
tmp_ptr->next_item = 0;
if (end_ptr)
end_ptr->next_item = tmp_ptr;
else
beg_ptr = tmp_ptr;
end_ptr = tmp_ptr;
how_many++;
return 1;
}
else
return 0;
}

Using a Template Class
int main () {
register int i, *iptr;
String *sptr;
char *somedata [] = {“one”, “two”, “three”, “four”,
“five”, “six”, “seven”, “eight”, “nine”, “ten”};
List<int> ii;
List<String> is;
for (i = 1; i <=10; i++) { ii.put(i); }
cout << “The List<int> contains “ ;
cout << ii.length () << “ itemsn” ;
return 0;
}

Parameterized Class
 A class template
 describes how a class should be built
 Supplies the class description and the
definition of the member functions using some
arbitrary type name, (as a place holder).
 is a parameterized type with
parameterized member functions

Parameterized Class
 can be considered the definition for an
unbounded set of class types
 is identified by the keyword template
 followed by parameter identifiers
 enclosed between < and > delimiters
 noting they are class, (i.e. type),
parameters
 is often used for “container” classes

Parameter Requirements
Parameter Types
 may be of any type, (including user defined
types)
 may be parameterized types, (i.e. templates)
 MUST support the methods used by the
template functions:
 what are the required constructors ?
 the required operator functions ?
 What are the necessary defining
operations ?

Class Template Instantiation
 Class Template is instantiated only when it is
required.
 Matrix is a class template
 Matrix m; //error
 Matrix *pm; // OK
 void inverse (Matrix & m); //OK
 Class template is instantiated before
 An object is defined with class template instantiation
 If a pointer or a reference is de-referenced (e.g., a
method is invoked)
 A template definition can refer to a class
template or its instances but a n non-template
can only refer to template instances.

Friend & Static Declaration
 There are 3 kinds of friend declarations
 Non-template friend.
 A bound friend class template.
 One-to-one mapping between the instance of a class template
and its friend
 An unbound friend class template.
 One-to-many mapping between the instance of a class
template and its friend
 Operators can be overloaded for a class template
 A static data member in a class template is
itself a template.
 Each static data member instantiation
corresponds to a class template instantiation.

Source code organization
 Inclusion Model
 Make the template definition visible to
compiler at the point of instantiation
 Include source files as well.
 Increase in build and link time
 Instantiate the types you need explicitly in a
separate compile unit.
 Cannot use benefits of lazy instantiation.
 Separation Model
 Use keyword ‘export’ in front of definition.
 Not many compiler supports the feature.

Template specialization
 Allows to make specific implementation when
pattern is of determined type.
 The syntax is
template<> class XXX<Type> {..}
 No member is ‘inherited’ from the generic
template to specialized one. So, must define
ALL members equating them to specialization
 Class templates can be partially specialized too.
 A totally disjoint template that the compiler gives a
priority while instantiation.

Templates & Derivation
 A template expresses a commonality across
types
 Two types generated from a common template are
different.
 A Base class expresses commonality of
representation and calling interface.
 Example
 template <class T, class A> class specalContainer :
public Container<T>, A;
 specialContainer<Node, Fast_Allocator> s;
 specialContainer<Node, Shared> s1;
 specialContainer<ProcessDescriptor, Fast_Allocator>
p;Jan 2014 184OOP in C++

Template Inheritance: A Base
Template
template<class T>
class set {
public :
Set ( ) { } ;
virtual void add (const T
&val);
int length ( ) ;
int find (const T &val);
private:
List<T> items;
};
template<class T>
void Set<T> : : add (const T &val)
{
if (items.find (val) ) return;
items.put (val) ;
}
template<class T>
int Set<T> : : length ( )
{
return items.length ( ) ;
}
template<class T>
int Set<T> ::find (const T &val)
{
return (int) items.find(val);
}

Template Inheritance: A
Derived Template
/* boundset.h */
#include “set.h”
template<class T>
class BoundSet : public Set<T>
public:
BoundSet (const T &lower,
const T &upper);
void add(const T &val);
private:
T min;
T max;
};
template<class T>
BoundSet<T>:: BoundSet (const T
&lower, const T &upper)
: min (lower), max (upper)
{
}
template<class T>
void BoundSet<T> : :add(const T
&val)
{
if (find (val) ) return;
if ( (val <= max) && (val
>= min) )
Set<T>: : add
(val) ;
}

Using a Derived Template Class
int main ( ) {
register int i ;
BoundSet<int> bsi (3, 21);
Set<int> *Setptr = &bsi;
for (i = 0; i < 25; i++)
setptr->add( i ) ;
if (bsi.find (4)
cout << “We found an expected valuen”;
if (bsi.find (0) || bsi.find(25 ) ) {
cout << “We found an Unexpected valuen”;
return 23;
}
else
cout << “We found NO unexpected valuen”;
return 0;
}

Inheritance vs Templates
 Inheritance : helps in reusing object code
 Templates : helps in reusing source-code
 object code size of template code is therefore much
less.
 Compiler view
 Constructor of objects containing virtual functions
must initialize the vptr table.
 Template class knows the type of the class at
compile-time instead of having to determine it at
run-time (as in case of virtual function usage)

Inheritance vs Templates:
Example
Implement the following command.
# ECHO infile outfile
main() {
(argc > 2 ? ofstream
(argv[2] ,ios::out) : cout)
<<
(argc > 1 ? ifstream (argv[1],
ios::in) : cin)
.rdbuf();
}
main()
{
fstream in, out;
if (argc > 1) in.open(argv[1],
ios::in);
if (argc > 2) out.open(argv[2],
ios::out);
Process(in.is_open() ? in : cin,
out.is_open() ? out :
cout);
}
 How to implement ‘Process’ ?
 Two ways to get the
polymorphic behavior
 virtual function and templates

Inheritance vs Templates:
Solution
template<typename In, typename
out>
void process(In& in, Out& out)
{
// out << in.rdbuf();
}
 Merely requires that
the passed objects
to have suitable
interface (such as
member function
named rdbuf(). )
void Process (basic_istream& in,
basic_ostream& out) {
// out << in.rdbuf();
}
 Requirement:
 cin and ifstream both derived
from basic_istream
 The common base class
‘basic_istream has suitable
interface ‘rdbuf’.
 Both Methods solve the current
problem.
 But templates provide
extensibility.
 Other streams can be provided
with rdbuf() interface.
 Other streams cannot be
guaranteed to be derived from
the same base class

Another Example
 A collection of classes is required for
 Stack: for ints , strings, floats etc.
 Has operations: push, pop, length of stack.
 transportationMode . For airplane, car, boat etc.
 Has features : move, comfort etc
 each move in diff ways.
 Each provide different level of comfort.
 Analyze if the ‘type’ being manipulated has any
affect on the behavior of class .
 If ‘type’ does not affect the behavior, use templates
 If ‘type’ affects the behavior, use virtual functions
(inheritance)

Jan 2014 OOP in C++ 193
Ranking of Programming Languages in Jan 2014
TIOBE Index for January 2014
Jan-14 Jan-13Change Programming Language Ratings Change
1 1 = C 17.87% 0.02%
2 2 = Java 16.50% -0.92%
3 3 = Objective-C 11.10% 0.82%
4 4 = C++ 7.55% -1.59%
5 5 = C# 5.86% -0.34%
6 6 = PHP 4.63% -0.92%
7 7 = (Visual) Basic 2.99% -1.76%
8 8 = Python 2.40% -1.77%
9 10 + JavaScript 1.57% -0.41%
10 22 + Transact-SQL 1.56% 0.98%
11 12 + Visual Basic .NET 1.56% 0.52%
12 11 – Ruby 1.08% -0.69%
13 9 – Perl 0.92% -1.35%
14 14 = Pascal 0.78% -0.15%
15 17 + MATLAB 0.78% 0.14%
16 45 + F# 0.72% 0.53%
17 21 + PL/SQL 0.63% 0.05%
18 35 + D 0.63% 0.33%
19 13 – Lisp 0.60% -0.35%
20 15 – Delphi/Object Pascal 0.60% -0.32%
http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

OOP in C++

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