Classes and Objects

A class is a way to bind data and its associated functions together. It allows data to be hidden from external use. When defining a class, we are creating a new abstract data type that can be treated like any other built-in data type. 

Class declaration 

The class declaration describes the type and scope of its members.  

The general form of class declaration is:  

class class_name  

{ 

private: 

    // variable declarations 

    // function declarations 

public: 

    // variable declarations 

    // function declarations 

}; 

The collection of data and functions in a class is data abstraction. The private members of class being hidden from outside world is data hiding and forming a capsule of data and functions is called data encapsulation. It protects from intentional misuse of important data. 

Data abstraction 

1. Abstraction refers to the act of representing essential features without including the background details of Explanations.
2. Classes are the concept of abstraction and are defined as a list of abstract attributes and functions to operate on these attributes.
3. They encapsulate all the essential properties of the object that is to be created.

Defining Member Functions 

Member functions can be defined in two places  

1. Outside the class definition: Member functions not declared inside a class must be defined separately outside the class. The general form of member function definition is :

 return - type class - name : : function - name (argument declaration)  

2. Inside the class definition: A member function can be defined inside a class. 

class item 

 { 

    void getdata(---------)  

    { 

        --------- 

    } 

    void putdata(---------) 

      { 

        --------- 

      } 

}; 

Smaller functions are declared inside the class and are considered inline, a function defined outside can be made inline by the qualifier inline. 

Memory Allocation for objects 

The memory space for member functions is allocated when they are defined as a part of class specification. All objects belonging to the class use the same member functions. Separate memory locations for the objects are allocated when they are declared. Separate memory locations for the objects are essential because the member variables will hold different data values for different objects.  

Static data members 

A data member of a class can be qualified as static. A static member variable has following characteristics:  

1.  No other initialization is permitted except zero when the first object is created. 
2.  Only one copy of that member is created for the entire class and is shared by all the objects of that class, no matter how many objects are created.  
3.  It is visible only within the class, but its runtime is the entire program. 

Static Member functions   

A member function which is declared as static has the following properties:  

1.  A static function can have access to only other static members (functions or variables) declared in same class. 
2.  A static member function can be called using the class name (instead of its objects). class - name:: function - name; 

Friendly Functions 

The private members cannot be accessed to the private data of a class. However, there could be a situation where we would like two classes to share a particular function. C++ allows a function to be made friendly to both the classes without necessarily being part of any class. To make an outside function "Friendly" to a class, we must declare this function as a friend of the class. 

Pointers to members 

It is possible to take the address of a member of a class and assign it to a pointer. The address of a member can be obtained by applying the operator & to a class member name. A class member pointer can be declared using the operator ::* with class name. The dereferencing operator ->* is used to access a member when we use pointers to both the object and the member. The operator .*is used when the object itself is used with the member pointer.