A class is a user defined abstract data type that further expands upon the concept of a structure but instead of containing just data, it also includes functions.

 

In OO parlance, functions within a class are referred to as methods.

 

The concept of enclosing data and functions together within a single entity is referred to as Encapsulation.

 

Strictly speaking, as far as the compiler is concerned, the only difference between a structure and a class is that of accessibility (aka visibility). A structure's members are all public by default, whereas a class's members are all private by default. However, it is normal programming convention not to mix the two up and therefore structures are generally used for data (or POD: Plain Old Data), and classes are used for objects.

 

A class consists of three elements:

 

• Identity
  • Name for the class
• Attributes
  • Data members
• Methods
  • Function members

 

A class is akin to a blueprint or plan of how to make an object, that declares its attributes (member data types) and methods.

 

A class is declared by use of the keyword class

• followed by the user defined identity name for the class (akin to a primitive data type, e.g. int)
• followed by the class members enclosed within a set of { curly braces }
• followed by optional object identifiers
• followed by a semi-colon ;

 

Syntax:

class identity {

public:

members ;

private:

members ;

protected:

members ;

} objects ;

 

The keywords public, private and protected are known as the access specifiers, which define the level of visibility of a class's members.

 

Once the class has been declared, an object is declared just like any other data type (e.g. int x;).

 

Extremely simple Class example:

#include <iostream>
using namespace std ;

class Triangle {

public:
	float width ;
	float height ;

	float area(){
		return ((width * height) / 2) ;
	}
} ;

int main () {

	Triangle myTriangle ;

	myTriangle.width = 5 ; //member access using the .dot operator
	myTriangle.height = 6 ;

	cout << "Triangle area: " << myTriangle.area() << endl ;

	return 0;
}

Compile & Run:

Triangle area: 15

 

The above example is overly simplified to show a function being used within a class. Normally all attributes (in this case the two floats on lines 7 and 8) should be private to ensure that only the object itself is responsible for any changes of its own data.

 

An object is said to be instantiated, by declaring a new identifier against the class's identity (i.e. the abstract data type), as per line 17 above.