Variables (Links to an external site.) are arbitrary names given to a memory location in the system. These memory locations addresses in the memory. Suppose we want to save our marks in memory. Now, these marks will get saved at a particular address in the memory. Now, whenever these marks will be updated, they will be stored at a different memory address. Thus, to facilitate the fetching of these memory addresses, variables are used. Variables are names given to these memory locations. The memory location referred to by this variable holds a value of our interest. Now, these variables once declared, are assigned some value. This assignment of value to these variables is called initialization of variables.

Initialization of a variable is of two types:

• Static Initialization: Here, the variable is assigned a value in advance. This variable then acts as a constant.
• Dynamic Initialization: Here, the variable is assigned a value at the run time. The value of this variable can be altered every time the program is being run.

Different ways of initializing a variable in C

Method 1 (Declaring the variable and then initializing it)

int a;   
a = 5; 

Method 2 (Declaring and Initializing the variable together):

int a = 5;   

Method 3 (Declaring multiple variables simultaneously and then initializing them separately)

int a, b;       
a = 5;         
b = 10;       

Method 4 (Declaring multiple variables simultaneously and then initializing them simultaneously)

int a, b;     
a = b = 10;  

int a, b = 10, c = 20;   

Method 5 (Dynamic Initialization : Value is being assigned to variable at run time.)

int a;
printf("Enter the value of a");
scanf("%d", &a);   

Different ways of initializing a variable in C++

Method 1 (Declaring and Initializing a variable)

int a = 5;  

Method 2 (Initializing a variable using parenthesis)

int a (5) ;   

Yes, they’re the same. On the other hand, for a class type they’re different. For example :

struct A {
    A(int);
};
A a(5);   
// This statement is to construct a;

Method 3 (Initializing a variable using braces)

int a{5} ;   

Method 4 (Declaring a variable using auto class)

auto a = 5;   

‘auto (Links to an external site.)’ is a keyword which tells the compiler the type of the variable upon its initialization.

Method 5 (Declaring and Initializing a variable through ‘auto’ keyword with parenthesis)

auto a (5);  

Method 6 (Declaring and Initializing a variable through ‘auto’ keyword with braces)

auto a{5};   

Method 7 (Dynamic Initialization)

int a;
cin>>a;           

These are all the different ways in which a variable can be defined in C or C++. The ways are similar for all fundamental variable but the way to initialize a variable of derived data type changes accordingly. Different derived data types (Links to an external site.) have an altogether different way of getting their variable initialized and hence can be explored in detail while diving in the all about of that particular data type.

C++ Reference variable

C++ introduces a new kind of variable known as Reference Variable. It provides an alias (alternative name) for a previously defined variable.

A reference variable must be initialized at the time of declaration.

This establishes the correspondences between the reference and the data onject which it name.

When a reference is created, you must tell it which variable it will become an alias for. After you create the reference, whenever you use the variable, you can just treat it as though it were a regular integer variable. But when you create it, you must initialize it with another variable, whose address it will keep around behind the scenes to allow you to use it to modify that variable.

Declaration

    [data_type] & [reference_variable]=[regular_variable];

regular_variable is a variable that has already initialized, and reference_variable is an alternative name (alias) to represent the variable regular_variable.

Consider the example

#include <iostream.h>
int main()
{
	int student_age=10;
	int &age=student_age;	// reference variable

	cout<< " value of student_age :"<< student_age << endl;
	cout<< " value of age :"<< age << endl;

	age=age+10;
	cout<<"\nAFTER ADDING 10 INTO REFERENCE VARIABLE \n";
	cout<< " value of student_age :"<< student_age << endl;
	cout<< " value of age :"<< age << endl;
	return 0;
}

Output

    value of student_age :10
    value of age :10

    AFTER ADDING 10 INTO REFERENCE VARIABLE
    value of student_age :20
    value of age :20

Reference Variables as Function Parameter (Call by reference)

A Reference Variable can also be used in function parameter, this is known as call by reference.

The call by reference mechanism is useful in object oriented programming due to following reasons:

• It permits the manipulation of objects by reference.
• It eliminates the copying of object parameters.
• Reference variables can be created for user defined data types such as structures and classes.

Consider the example

//Program to illustrate call by references.
#include <iostream.h>

// function call by reference
void fun(int &x)
{
	x+=10;
}

int main()
{
	int m=100;
	fun(m);
	cout<<"\n Value of m is : "<< m << endl;
	return 0;
}

Output

    Value of m is : 110

Difference between Pointer and Reference

• It is not possible to refer directly to a reference object after it is defined; any occurrence of its name refers directly to the object it references.
• Once a reference is created, it cannot be later made to reference another object; it cannot be reseated. This is often done with pointers.
• References cannot be null, whereas pointers can; every reference refers to some object, although it may or may not be valid.
• References cannot be uninitialized. Because it is impossible to reinitialize a reference, they must be initialized as soon as they are created. In particular, local and global variables must be initialized where they are defined, and references which are data members of class instances must be initialized in the initializer list of the class's constructor.