C++ classes tutorials

C++ Classes
&
Object Oriented Programming

 Programmer thinks about and defines the
attributes and behavior of objects.

 Often the objects are modeled after real-
world entities.

 Very different approach than function-based
programming (like C).

 Object-oriented programming (OOP)
– Encapsulates data (attributes) and functions
(behavior) into packages called classes.
 So, Classes are user-defined (programmer-
defined) types.
– Data (data members)
– Functions (member functions or methods)
 In other words, they are structures +
functions

Classes in C++
 A class definition begins with the keyword
class.
 The body of the class is contained within a
set of braces, { } ; (notice the semi-colon).
class class_name Any valid
} identifier
.…
.…
Class body (data member
.…
+ methods)
methods
;{

++Classes in C
 Within the body, the keywords private: and
public: specify the access level of the
members of the class.
– the default is private.

 Usually, the data members of a class are
declared in the private: section of the class
and the member functions are in public:
section.

Classes in C++

class class_name
}
:private private members or
… methods
…
…
public:
… Public members or methods
…
…
;{

Classes in C++
 Member access specifiers
– public:
 can be accessed outside the class directly.
– The public stuff is the interface.
– private:
 Accessible only to member functions of class
 Private members and methods are for internal use
only.

Class Example
 This class example shows how we can
encapsulate (gather) a circle information
into one package (unit or class)
No need for others classes to
class Circle access and retrieve its value
directly. The
{
class methods are responsible for
private: that only.
double radius;
public:
void setRadius(double r); They are accessible from outside
double getDiameter(); the class, and they can access the
double getArea(); member (radius)
double getCircumference();
};

Creating an object of a Class
 Declaring a variable of a class type creates an
object. You can have many variables of the same
type (class).
– Instantiation
 Once an object of a certain class is instantiated, a
new memory location is created for it to store its
data members and code
 You can instantiate many objects from a class
type.
– Ex) Circle c; Circle *c;

Special Member Functions
 Constructor:
– Public function member
– called when a new object is created
(instantiated).
– Initialize data members.
– Same name as class
– No return type
– Several constructors
 Function overloading

Special Member Functions

class Circle
Constructor with no
{
argument
private:
double radius;
public: Constructor with one
Circle(); argument
Circle(int r);
void setRadius(double r);
double getDiameter();
double getArea();
};

Implementing class methods
 Class implementation: writing the code of class
methods.
 There are two ways:
1. Member functions defined outside class
 Using Binary scope resolution operator (::)
 “Ties” member name to class name
 Uniquely identify functions of particular class
 Different classes can have member functions with same
name
– Format for defining member functions
ReturnType ClassName::MemberFunctionName( ){
…
}

Implementing class methods
2. Member functions defined inside class
– Do not need scope resolution operator, class
name;
class Circle
{ Defined
private: inside
double radius; class
public:
Circle() { radius = 0.0;}
Circle(int r);
void setRadius(double r){radius = r;}
double getDiameter(){ return radius *2;}
double getArea();
};

class Circle
{
private:
double radius;
public:
Circle(int r);
double getArea();
};
Circle::Circle(int r)
{
Defined outside class
radius = r;
}
double Circle::getArea()
{
return radius * radius * (22.0/7);
}
double Circle:: getCircumference()
{
return 2 * radius * (22.0/7);
}

Accessing Class Members
 Operators to access class members
– Identical to those for structs
– Dot member selection operator (.)
 Object
 Reference to object
– Arrow member selection operator (->)
 Pointers

class Circle
{
private:
double radius;
public: The first
Circle() { radius = 0.0;} The second
constructor is
Circle(int r); constructor is
called
void setRadius(double r){radius = r;} called
double getArea();
void main() Since radius is a
{ private class data
}; Circle c1,c2(7); member
Circle::Circle(int r)
{ cout<<“The area of c1:”
radius = r; <<c1.getArea()<<“n”;
}
double Circle::getArea() //c1.raduis = 5;//syntax error
{ c1.setRadius(5);
return radius * radius * (22.0/7);
cout<<“The circumference of c1:”
} << c1.getCircumference()<<“n”;
double Circle:: getCircumference()
{ cout<<“The Diameter of c2:”
return 2 * radius * (22.0/7); <<c2.getDiameter()<<“n”;
} }

class Circle
{
private:
double radius;
public:
Circle(int r);
double getArea();
};
Circle::Circle(int r) void main()
{
{
Circle c(7);
radius = r; Circle *cp1 = &c;
} Circle *cp2 = new Circle(7);
double Circle::getArea()
{ cout<<“The are of cp2:”
return radius * radius * (22.0/7); <<cp2->getArea();
}
double Circle:: getCircumference() }
{
return 2 * radius * (22.0/7);
}

Destructors
 Destructors
– Special member function
– Same name as class
 Preceded with tilde (~)
– No arguments
– No return value
– Cannot be overloaded
– Before system reclaims object’s memory
 Reuse memory for new objects
 Mainly used to de-allocate dynamic memory locations

Another class Example
 This class shows how to handle time parts.
class Time
{
private:
int *hour,*minute,*second;
public:
Time();
Time(int h,int m,int s);
void printTime();
void setTime(int h,int m,int s);
int getHour(){return *hour;}
int getMinute(){return *minute;}
Destructor int getSecond(){return *second;}
void setHour(int h){*hour = h;}
void setMinute(int m){*minute = m;}
void setSecond(int s){*second = s;}
~Time();
};

Time::Time()
{
hour = new int;
minute = new int;
second = new int;
*hour = *minute = *second = 0;
}

Dynamic locations Time::Time(int h,int m,int s)
should be allocated {
to pointers first
hour = new int;
minute = new int;
second = new int;
*hour = h;
*minute = m;
*second = s;
}

void Time::setTime(int h,int m,int s)
{
*hour = h;
*minute = m;
*second = s;
}

void Time::printTime()
{
cout<<"The time is : ("<<*hour<<":"<<*minute<<":"<<*second<<")"
<<endl;
}
Destructor: used here to de-
Time::~Time() allocate memory locations
{
delete hour; delete minute;delete second;
}

void main()
Output:
{
Time *t; The time is : (3:55:54)
t= new Time(3,55,54); The time is : (7:17:43)
t->printTime(); Press any key to continue

t->setHour(7);
t->setMinute(17);
t->setSecond(43);

t->printTime(); When executed, the
destructor is called
delete t;
}

Reasons for OOP
1. Simplify programming
2. Interfaces
 Information hiding:
– Implementation details hidden within classes themselves

3. Software reuse
 Class objects included as members of other
classes

C++ classes tutorials

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