Lecture1 classes3

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Lecture 1 – Introduction to
C++ Classes (Part 3)
TTTK1924 – Program Design and Problem
Solving

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Using Classes
1. Define a class according to the following syntax:
class <classIdentifier> {
private:
<declaration of member variables>
public:
<definition of member functions>
};

2. Declare a variable of the class – object variable
<classIdentifier> <object-variable>;

3. Access or modify the data by calling member functions with the dot (.)
operator
<object-variable>.<member-function>;
or
<variable> = <object-variable>.<member-function>;

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Class Definition

 Includes
 declaration of member variables(may contain different types
of data)
 definition of member functions – used to manipulate data
items

 Member variables are declared private to the class
– can only be accessed or modified by its own class

 Member functions are declared public ( can be used
by other than its own class)

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Class Definition - Example
class timeType { :
Name of class
private: string getZone() {
int hour;
(class identifier)
return zone;
int minute; }
int second;
string zone; void printTime() const {
if (hour < 10)
public: cout << "0";
void setTime(int h, int m, int s, string z) { cout << hour << ":";
hour = h; if (minute < 10)
minute = m; cout << "0";
second = s; cout << minute << ":";
zone = z; if (second < 10)
} cout << "0";
cout << second << ":";
int getHour() { cout << zone << endl;
return hour; }
}
timeType(){
int getMinute() { hour = 0;
return minute; minute = 0;
} second = 0;
zone = "Kuala Lumpur";
int getSecond() { }
return second; };
}
:

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class timeType { :
private: string getZone() {
int hour; return zone;
int minute; }
int second;
if (hour < 10)
public: Declaration"0"; member
cout <<
of variables
} cout << "0";
return hour; }
}
timeType(){
} second = 0;
int getSecond() { }
return second; };
}
:

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class timeType { :
Definition of string getZone() {
private:
int hour; member functions return zone;
int minute; }
int second;
if (hour < 10)
public: cout << "0";
} cout << "0";
return hour; }
}
timeType(){
} second = 0;
int getSecond() { }
return second; };
}
:

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Using Classes

 Defined classes can be used by
 The main program, or
 Other classes

 Only public member functions can be called or used by the
main program or other classes

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Using Classes - Example

int main (int argc, const char * argv[])
{
timeType currentTime;
currentTime.printTime();
currentTime.setTime(5, 13, 0, "Singapore");
cout << "The time in " << currentTime.getZone() << " now is "
<< currentTime.getHour() << ":" << currentTime.getMinute()
<< endl;
return 0;
}

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{
timeType currentTime; Declaration of
currentTime.printTime(); object variable
<< endl;
return 0;
}

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{
<< endl;
return 0;
}

Accessing member variables using
member functions

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{
<< endl;
return 0;
}

Modifying member variables
using member functions

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Using Classes – Example 7

{
currentTime.printTime(); :
timeType(){
hour = 0;
<< currentTime.getHour() << ":" << currentTime.getMinute() = 0;
minute
<< endl; second = 0;
return 0; zone = "Kuala Lumpur";
} }

currentTime };

hour 0
minute 0
first
second 0
zone Kuala Lumpur

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Using Classes – Example 7

{
cout << "The time in " << currentTime.getZone() << " now is " printTime() const {
void
if (hour < 10)
<< currentTime.getHour() << ":" << currentTime.getMinute() << "0";
cout
<< endl; cout << hour << ":";
return 0; if (minute < 10)
} cout << "0";
cout << minute << ":";
currentTime if (second < 10)
cout << "0";
hour 0
cout << zone << endl;
minute 0 }
first
second 0 :
zone Kuala Lumpur };

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Using Classes - Example 7
:
void printTime() const {
if (hour < 10)
cout << "0";
cout << hour << ":";
if (minute < 10)
cout << "0";
if (second < 10)
cout << "0";
}
:
};

currentTime

00:
hour 0
minute 0
first
second 0
zone Kuala Lumpur

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:
if (hour < 10)
cout << "0";
if (minute < 10)
cout << "0";
if (second < 10)
cout << "0";
}
:
};

currentTime

00:00:
hour 0
minute 0
first
second 0
zone Kuala Lumpur

+
:
if (hour < 10)
cout << "0";
if (minute < 10)
cout << "0";
if (second < 10)
cout << "0";
}
:
};

currentTime

00:00:00:
hour 0
minute 0
first
second 0
zone Kuala Lumpur

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:
if (hour < 10)
cout << "0";
if (minute < 10)
cout << "0";
if (second < 10)
cout << "0";
}
:
};

currentTime

00:00:00:Kuala Lumpur
hour 0
minute 0
first
second 0
zone Kuala Lumpur

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{
currentTime.setTime(5, 13, 0, "Singapore"); void setTime(int h, int m, int s, string z)
currentTime.printTime(); {
cout << "The time in " << currentTime.getZone() << " now is " = h;
hour
<< currentTime.getHour() << ":" << currentTime.getMinute() = m;
minute
<< endl; second = s;
return 0; zone = z;
} }
currentTime :
};
hour 0
minute 0
first
second 0
zone Kuala Lumpur

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:
void setTime(int h, int m, int s, string z)
{ h m s
hour = h;
minute = m; 5 13 0

second = s;
zone = z; z
}
Singapore
:
};

currentTime

hour 0
minute 0
first
second 0
zone Kuala Lumpur

+

:
{ h m s
hour = h;
minute = m; 5 13 0

second = s;
zone = z; z
}
Singapore
:
};

currentTime

hour 5
minute 0
first
second 0
zone Kuala Lumpur

+

:
{ h m s
hour = h;
minute = m; 5 13 0

second = s;
zone = z; z
}
Singapore
:
};

currentTime

hour 5
minute 13
first
second 0
zone Kuala Lumpur

+

:
{ h m s
hour = h;
minute = m; 5 13 0

second = s;
zone = z; z
}
Singapore
:
};

currentTime

hour 5
minute 13
first
second 0
zone Singapore

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{
cout << "The time in " << currentTime.getZone() << " now is " printTime() const {
void
if (hour < 10)
<< currentTime.getHour() << ":" << currentTime.getMinute() << "0";
cout
<< endl; cout << hour << ":";
return 0; if (minute < 10)
} cout << "0";
currentTime if (second < 10)
cout << "0";
hour 5
minute 13 }
first
second 0 :
zone Singapore };

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{
<< endl;
return 0;
}
currentTime

hour 5 05:13:00:Singapore
minute 13
first
second 0
zone Singapore

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{
<< endl;
return 0;
}
currentTime

minute 13 The time in
first
second 0
zone Singapore

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{ :
timeType currentTime; string getZone() const {
currentTime.printTime(); return zone;
}
:
<< endl;
return 0;
}
currentTime

minute 13 The time in Singapore
first
second 0
zone Singapore

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{
<< endl;
return 0;
}
currentTime

minute 13 The time in Singapore now is
first
second 0
zone Singapore

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{ :
timeType currentTime; int getHour() const {
currentTime.printTime(); return hour;
}
:
<< endl;
return 0;
}
currentTime

minute 13 The time in Singapore now is 5
first
second 0
zone Singapore

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{
<< endl;
return 0;
}
currentTime

minute 13 The time in Singapore now is 5:
first
second 0
zone Singapore

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{ :
timeType currentTime; int getMinute() const {
currentTime.printTime(); return minute;
}
<< endl;
return 0;
}
currentTime

minute 13 The time in Singapore now is 5:13
first
second 0
zone Singapore

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Class Definition Variations
 A class in C++ is more commonly defined using the following
syntax:
public:
<member functions prototype>
private:
};
:
<member functions implementation>

 In this case, we need to use the class name and scope resolution
operator (::) in member functions implementation

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:
 Eg. void timeType::setTime(int h, int m, int s, string z) {
hour = h;
class timeType { minute = m;
public: second = s;
void setTime(int h, int m, int s, string z); zone = z;
}
int getHour();
int getMinute();
int timeType::getHour() {
int getSecond(); return hour;
string getZone(); }
void printTime() const;
timeType(); int timeType::getMinute() {
return minute;
private: }
int hour; :
int minute; timeType:: timeType(){
Member functions
int second; hour = 0;
declaration
string zone; minute = 0;
}; second = 0;
: zone = "Kuala Lumpur";
}

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Member functions
:
 Eg. implementation
void timeType::setTime(int h, int m, int s, string z) {
hour = h;
public: second = s;
void setTime(int h, int m, int s, string z); zone = z;
}
int getHour();
int getMinute();
int getSecond(); return hour;
string getZone(); }
timeType(); int timeType::getMinute() {
return minute;
private: }
int hour; :
int minute; timeType:: timeType(){
int second; hour = 0;
string zone; minute = 0;
}; second = 0;
}
Definition ends here

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 Eg. Name of the class

:
void timeType::setTime(int h, int m, int s, string z) {
hour = h;
minute = m;
second = s;
zone = z;
}
Scope resolution operator
:

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 The order of declarations can also be changed, like in the
following syntax:
private:

public:
};
:

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:
hour = h;
private: second = s;
int hour; zone = z;
}
int minute; private member variables
int second; aretimeType::getHour() {
int declared first
string zone; return hour;
}
public:
void setTime(int h, int m, int s, string z); int timeType::getMinute() {
int getHour(); return minute;
int getMinute(); }
int getSecond(); :
string getZone(); timeType:: timeType(){
void printTime() const; hour = 0;
timeType(); minute = 0;
}; second = 0;
}

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 The default specifier for a class member is private, therefore, if
private members are placed first, the keyword private can be
omitted.

 The following syntax is the same as the previous one:
public:
};
:

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:
hour = h;
int hour; second = s;
private keyword is omitted
int minute; zone = z;
}
int second;
string zone;
return hour;
public: }
void setTime(int h, int m, int s, string z);
int getHour(); int timeType::getMinute() {
int getMinute(); return minute;
int getSecond(); }
string getZone(); :
void printTime() const; timeType:: timeType(){
timeType(); hour = 0;
}; minute = 0;
: second = 0;
}

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 Member variables and member functions can also be declared
as protected

 For further explanation, please refer the class inheritance topic

+ Accessor, Mutator and Constant Functions

 Accessor functions – member functions that are used to (and only) access
(do not modify) the value/s of member variables

 Eg.
getHour(), getMinute(), getSecond(), getZone(), printTime()

 Mutator functions – member functions that are used to modify the value/s
of member variables

 Eg.
setTime()

 Constant functions – accessor functions that have the keyword const in its
heading – as a safeguard not to modify member variables

 Eg.
void printTime() const

 A class definition must have accessor and mutator functions to let other
classes use them

+ Accessor, Mutator and Constant Functions

class timeType {
private: Mutator function
:

public:
int getHour(); Accessor functions
int getMinute();
int getSecond();
string getZone();
void printTime() const; Constant function
timeType();
};
:

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Reference Parameters
 Let’s say we want to define a member function that calculates the time
difference in hours between two cities.

 We can do this by passing an object variable as a parameter of a
function as follows:
int timeType::timeDifference(timeType otherTime){
if (hour > otherTime.hour)
return (hour - otherTime.hour);
else if (hour < otherTime.hour)
return (otherTime.hour - hour);
else
return 0;
}

 Therefore, we can call the function like this:

cout << "The time difference between " << time1.getZone() << " and " << time2.getZone() << " is "
<< time1.timeDifference(time2) << " hours " << endl;

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 However, this is not a good programming practice, as we it
requires memory space to copy the value of the actual
parameters

 Assuming we have the following statements :

timeType time1, time2;
time2.setTime(5, 13, 0, "Paris");

 We will have memory space allocated as follows:
time1 time2

hour 0 hour 5
minute 0 minute 13
first first
second 0 second 0
zone Kuala Lumpur zone Paris

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 When we call the timeDifference() function, the values of
member variables of time2 are copied into otherTime

 So, additional memory space is allocated for otherTime :

otherTime

hour 5
minute 13
first
second 0
zone Paris

 This can be improved by using reference parameters.

+

 The following member function uses a reference parameter:

int timeType::timeDifference (const timeType& otherTime){
if (hour > otherTime.hour)
return (hour - otherTime.hour);
else if (hour < otherTime.hour)
return (otherTime.hour - hour);
else
return 0;
}

 Similarly, we can call the function:
cout << "The time difference between " << time1.getZone() << " and " << time2.getZone()
<< " is " << time1.timeDifference(time2) << " hours " << endl;

+
 otherTime refers to the memory allocated for time2:

time2
otherTime
hour 5
minute 13
first
second 0
zone Paris

 No additional memory needed

 Can the values of the member variables be modified during the
function call?

 No, because of the keyword const in the function header

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Constructors
 A constructor is a special member function that is
called during the declaration of an object variable

 A constructor is used to initialise the member
variables of a class

 Notice that it has neither a return type nor void

 A class
can have more that one constructors, and
which constructor is called depends on the number
and type of the actual parameters (if any)when it is
called

 A constructorwithout any parameters is called the
default constructor

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Constructors
 Let’s say we have the following constructors in a class:
timeType::timeType(){
hour = 0;
minute = 0;
second = 0; Default constructor
}

timeType::timeType(int h, int m, int s, string z) {
hour = h;
minute = m;
second = s;
zone = z;
}

 And we have the following statements in the main program:
:
timeType time1;
timeType time2(5, 13, 0, "Paris");
time1.printTime();
time2.printTime();
:

+
Constructors
: timeType::timeType(){
timeType time1; hour = 0;
timeType time2(5, 13, 0, "Paris"); minute = 0;
time1.printTime(); second = 0;
time2.printTime(); zone = "Kuala Lumpur";
: }

hour = h;
minute = m;
second = s;
zone = z;
}

time1

hour 0
minute 0
first
second 0
zone Kuala Lumpur

+
Constructors
: timeType::timeType(){
timeType time1; hour = 0;
timeType time2(5, 13, 0, "Paris"); minute = 0;
time1.printTime(); second = 0;
time2.printTime(); zone = "Kuala Lumpur";
: }

hour = h;
minute = m;
second = s;
zone = z;
}

time1 time2

hour 0 hour 5
minute 0 minute 13
first first
second 0 second 0

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Constructor With Default Values

 We can also simplify the previous definition using a constructor with
default values:

class timeType2 {
public: Default values
int getHour();
int getMinute();
int getSecond();
string getZone();
void getTime(int& h, int& m, int& s, string z) const;
int timeDifference(const timeType2& otherTime);
timeType2(int = 0, int = 0, int = 0, string = "Kuala Lumpur");

private:
int hour;
int minute;
int second;
string zone;
};

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 And use only one constructor implementation:
hour = h;
minute = m;
second = s;
zone = z;
}

 Assuming we have the following statements in the main
program:
:
timeType time1;
:

+

 We get the same result as before:

:
timeType time1;
:

time1

hour 0
minute 0
first
second 0
zone Kuala Lumpur

+

 We get the same result as before:

:
timeType time1;
:

time1 time2

hour 0 hour 5
minute 0 minute 13
first first
second 0 second 0

+
Destructors

 A class can only have one destructor

 Justlike a constructor, it has no type nor return
value, and marked with tilda (~)

 Called automatically when the program has finished
using an object

 Ifno destructor is declared, the compiler will
implicitly insert one

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Destructors - Example
class timeType { Destructor declaration
public:
void setTime(int h, int m, int
s, string z);
int getHour(); Destructor
int getMinute(); implementation
int getSecond();
string getZone(); :
timeType(); timeType:: timeType(){
~timeType(); hour = 0;
minute = 0;
private: second = 0;
int hour; zone = "Kuala Lumpur";
int minute; }
int second;
string zone; timeType::~timeType() {
}; }
:

+
Array of Class Objects (Variables)

 Just like other types of data, we can declare an array of object
variables

 Eg.

timeType2 time[5];
time[1].setTime(1, 0, 0, "Tokyo");
time[2].setTime(5, 0, 0, "Paris");
time[3].setTime(11, 0, 0, "New York");
time[4].setTime(3, 0, 0, "Sydney");
for (int i=0; i<5; i++)
time[i].printTime();

+
timeType2 time[5];
for (int i=0; i<5; i++)

time time[0]
time[1]
hour time[2] 0
hour time[3] 0
minute 0
minute first
secondhour time[4] 0
0 0
second hour first0 0
zone minute 0
Kuala Lumpur
hour first 0
zone minute 0 0
second Kuala Lumpur
first
zone minute 0 0
second Kuala Lumpur
first
0
second Kuala Lumpur
zone
zone Kuala Lumpur

+
timeType2 time[5];
for (int i=0; i<5; i++)

time
time[1]

hour 1
minute 0
first
second 0
zone Tokyo

+
timeType2 time[5];
for (int i=0; i<5; i++)

time

time[2]

hour 5
minute 0
first
second 0
zone Paris

+
timeType2 time[5];
for (int i=0; i<5; i++)

time

time[3]

hour 11
minute 0
first
second 0
zone New York

+
timeType2 time[5];
for (int i=0; i<5; i++)

time

time[4]

hour 3
minute 0
first
second 0
zone Sydney

+

timeType2 time[5];
for (int i=0; i<5; i++)

01:00:00:Tokyo
05:00:00:Paris
11:00:00:New York
03:00:00:Sydney

+
References:

 D.S. Malik (2012). C++ Programming: Program Design
Including Data Structures (5th ed), Thomson Course
Technology.
 Chapter 12 – Classes and Data Abstraction

Lecture1 classes3

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