C++ Storage Classes - Learn With AVRK

Storage Classes in C++ Programming Language

In the C++ programming language, there are variables and each variable has a data type. When we declared the variable, the compiler assigned the storage class by default and that storage class is used to define the lifetime and visibility of a variable or function within the program.

Lifetime refers to the period during which the variable remains active whereas visibility refers to the module of a program during which the variable is accessible.

We used five types of storage in C++ program, which are as follows -

1. Automatic
2. Register
3. Static
4. External
5. Mutable

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

Automatic Storage Class

All local variables have a default storage class which is an automatic storage class. The automatic storage class has a pre defined keyword that is auto. The auto keyword is automatically applied to all local variables.

{
    auto int c; 
    float c= 1.24; 
}

The above example defines two variables with the same storage class, auto can only be used within functions.

Register Storage Class

Register storage class is employed to declare register variables. Register variable is similar to auto variable. It allocates memory in registers as compared to RAM. It's size is same as the size of register.  This makes operations on register variables much faster than other variables stored in memory during runtime. The point to note is that we will not get the address of the register variable.

register int counter=0; 

Static Storage Class

Static variables are initialized only once and exist until the end of the program. It retains it's value between multiple function calls.

The default value of a static variable is 0 which is provided by the compiler.

#include <iostream>
using namespace std;
void func() 
{    
    static int i=0; /*static variable */
    int j=0; /*local variable */
    i++;
    j++;
    cout<<"i=" << i<<" and j=" <<j<<endl;
}
int main()
{
    func();
    func();
    func();
}

Output -

i= 1 and j= 1
i= 2 and j= 1
i= 3 and j= 1

External Storage Class

The extern variable is visible to all programs. It is used when two or more files are sharing the same variable or function.

extern int counter=0;    

Also check out this : Call by value and call by reference - Learn With AVRK