Storage Classes In C++

Storage class is used to define the lifetime and visibility or scope of a variable and function within a C++ program.There are five types of storage classes in C++. They are,

1.     Automatic

2.     External

3.     Static

4.     Register

5.     Mutable

Storage class	Keyword	Lifetime	Visibility	Initial Value
Automatic	auto	Function Block	Local	Garbage
External	extern	Whole Program	Global	Zero
Static	static	Whole Program	Local	Zero
Register	register	Function Block	Local	Garbage
Mutable	mutable	Class	Local	Garbage

Automatic Storage Class

Automatic storage class assigns a variable to its default storage type. Auto keyword is used to declare automatic variables. However, if a variable is declared without any keyword inside a function, it is automatic by default.

Syntax

Datatype variable_name= value;
Or
auto datatype variable_name= value;

Example

auto  int  x;
float  y =5.6; //by default ,storage class is auto

Register  Storage Class
The register variable allocates memory in register than RAM. Its size is same of register size. It has a faster access than other variables.
It is recommended to use register variable only for quick access such as in counter.

Syntax
register datatype  variable_name=value;

Example:
register int x;

Example 1:
#include<iostream>
using namespace std;

    int z;      //global variable which initially holds 0 and it can access anywhere in the                          program

void sample_function()

{

    static int x;    //static variable which initially holds 0

    register int y;    //register variable

    x=7;

    y=x*3;

    cout<<"Inside sample_function"<<endl;

    cout<<"x = "<<x<<endl;

    cout<<"y = "<<y<<endl;

    cout<<"z = "<<z<<endl;

}

int main()

{

    int a;    //automatic variable

    z=89;

    a=50;

   sample_function();

    cout<<"Inside main"<<endl;

    cout<<"a = "<<a<<endl;

    cout<<"z = "<<z<<endl;

   sample_function();

    return 0;

}

Output:

Inside sample_function                                                                                                         

x = 7                                                                                                                          

y = 21                                                                                                                         

z = 89                                                                                                                         

Inside main                                                                                                                    

a = 50   

y = 21                                                                                                                         

z = 89   

Static Storage Class:

The static variable is initialized only once and exists till the end of a program. It retains its value between multiple functions call. The static variable has the default value 0 which is provided by the compiler.
Syntax

static datatype variable_name=value;

Example

static  int  x = 10;

Example 1:

#include <iostream>

using namespace std;

void display()

{

    static int x = 0;

    x++;

    cout <<"x is:"<< x << endl;

}

int main()

{

    display();

    display();

    display();

 return 0;

}

Output:

x is:1

x is:2

x is:3

External Storage Class

The extern variable is visible to all the programs. extern keyword is used to declare external variables. They are visible throughout the program and its lifetime is same as the lifetime of the program where it is declared.

Syntax

extern  datatype  variable_name;

Example,

extern float x;

Example 1:

#include<iostream>

using namespace std;

int x=5;

void add(int y)

{

int c=x+y;

cout<<"sum is:"<<c<<endl;

}

extern int x;

int main()

{

   

    add(6);

 

    return 0;

}

Output:

sum is: 11