Objects and classes in cpp

Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box

When the above code is compiled and executed, it produces the following result −

Volume of Box1 : 210 Volume of Box2 : 1560

It is important to note that private and protected members can not be accessed directly using direct member access operator (.). We will learn how private and protected members can be accessed.

Classes and Objects in Detail

So far, you have got very basic idea about C++ Classes and Objects. There are further interesting concepts related to C++ Classes and Objects which we will discuss in various sub-sections listed below −

A member function of a class is a function that has its definition or its prototype within the class definition like any other variable.

A class member can be defined as public, private or protected. By default members would be assumed as private.

A class constructor is a special function in a class that is called when a new object of the class is created. A destructor is also a special function which is called when created object is deleted.

The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously.

A friend function is permitted full access to private and protected members of a class.

With an inline function, the compiler tries to expand the code in the body of the function in place of a call to the function.

Every object has a special pointer this which points to the object itself.

A pointer to a class is done exactly the same way a pointer to a structure is. In fact a class is really just a structure with functions in it.

Both data members and function members of a class can be declared as static.

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C++ Classes and Objects

Everything in C++ is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

Attributes and methods are basically variables and functions that belongs to the class. These are often referred to as «class members».

A class is a user-defined data type that we can use in our program, and it works as an object constructor, or a «blueprint» for creating objects.

Create a Class

To create a class, use the class keyword:

Example

Create a class called » MyClass «:

class MyClass < // The class
public: // Access specifier
int myNum; // Attribute (int variable)
string myString; // Attribute (string variable)
>;

Example explained

• The class keyword is used to create a class called MyClass .
• The public keyword is an access specifier, which specifies that members (attributes and methods) of the class are accessible from outside the class. You will learn more about access specifiers later.
• Inside the class, there is an integer variable myNum and a string variable myString . When variables are declared within a class, they are called attributes.
• At last, end the class definition with a semicolon ; .

• Structures are much simpler than objects. Unlike objects, structures cannot do encapsulation, inheritance or polymorphism, which you will learn more about in the next chapters. If all you need is a collection of variables, a structure is easier to use than an object.

Create an Object

In C++, an object is created from a class. We have already created the class named MyClass , so now we can use this to create objects.

To create an object of MyClass , specify the class name, followed by the object name.

To access the class attributes ( myNum and myString ), use the dot syntax ( . ) on the object:

Example

Create an object called » myObj » and access the attributes:

class MyClass < // The class
public: // Access specifier
int myNum; // Attribute (int variable)
string myString; // Attribute (string variable)
>;

int main() MyClass myObj; // Create an object of MyClass

// Access attributes and set values
myObj.myNum = 15;
myObj.myString = «Some text»;

Multiple Objects

You can create multiple objects of one class:

Example

// Create a Car class with some attributes
class Car public:
string brand;
string model;
int year;
>;

int main() // Create an object of Car
Car carObj1;
carObj1.brand = «BMW»;
carObj1.model = «X5»;
carObj1.year = 1999;

// Create another object of Car
Car carObj2;
carObj2.brand = «Ford»;
carObj2.model = «Mustang»;
carObj2.year = 1969;

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C++ Classes and Objects

In previous tutorials, we learned about functions and variables. Sometimes it’s desirable to put related functions and data in one place so that it’s logical and easier to work with.

Suppose, we need to store the length, breadth, and height of a rectangular room and calculate its area and volume.

To handle this task, we can create three variables, say, length , breadth , and height along with the functions calculateArea() and calculateVolume() .

However, in C++, rather than creating separate variables and functions, we can also wrap these related data and functions in a single place (by creating objects). This programming paradigm is known as object-oriented programming.

But before we can create objects and use them in C++, we first need to learn about classes.

C++ Class

A class is a blueprint for the object.

We can think of a class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc. Based on these descriptions we build the house. House is the object.

Create a Class

A class is defined in C++ using keyword class followed by the name of the class.

The body of the class is defined inside the curly brackets and terminated by a semicolon at the end.

class Room < public: double length; double breadth; double height; double calculateArea()< return length * breadth; >double calculateVolume() < return length * breadth * height; >>;

Here, we defined a class named Room .

The variables length , breadth , and height declared inside the class are known as data members. And, the functions calculateArea() and calculateVolume() are known as member functions of a class.

C++ Objects

When a class is defined, only the specification for the object is defined; no memory or storage is allocated.

To use the data and access functions defined in the class, we need to create objects.

Syntax to Define Object in C++

className objectVariableName;

We can create objects of Room class (defined in the above example) as follows:

// sample function void sampleFunction() < // create objects Room room1, room2; >int main()< // create objects Room room3, room4; >

Here, two objects room1 and room2 of the Room class are created in sampleFunction() . Similarly, the objects room3 and room4 are created in main() .

As we can see, we can create objects of a class in any function of the program. We can also create objects of a class within the class itself, or in other classes.

Also, we can create as many objects as we want from a single class.

C++ Access Data Members and Member Functions

We can access the data members and member functions of a class by using a . (dot) operator. For example,

This will call the calculateArea() function inside the Room class for object room2 .

Similarly, the data members can be accessed as:

In this case, it initializes the length variable of room1 to 5.5 .

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