A storage class defines the scope (visibility) and the life time of the variable in the program or in the class.

There are following storage classes:

• auto

• register

• static

• extern

The auto Storage Class

It is the default storage class for all the variables.

{
   int mount;
   auto int month;
}

The register Storage Class

The register storage class store the variable into the registers not in the RAM.

You cannot apply unary operator into it.

{
   register int  miles;
}

The static Storage Class

The static storage class instructs the compiler to keep the variable into the memory until program executes.

#include <iostream>
 
// Function declaration
void func(void);
 
static int count = 10; /* Global variable */
 
main()
{
    while(count--)
    {
       func();
    }
    return 0;
}
// Function definition
void func( void )
{
    static int i = 5; // local static variable
    i++;
    std::cout << "i is " << i ;
    std::cout << " and count is " << count << std::endl;
}

The extern Storage Class

The extern storage class is used to defined a reference of a global variable accessible by every class.

First Program

#include <iostream>
 
int count ;
extern void write_extern();
 
main()
{
   count = 5;
   write_extern();
}


Second Program


#include <iostream>
 
extern int count;
 
void write_extern(void)
{
   std::cout << "Count is " << count << std::endl;
}