Storage Classes in C/C++ and their tabular comparison

A storage class defines the scope (visibility) and life time of variables and/or functions within a C /C++ Program.

 Types of Storage Classes

Auto

Auto is the default storage class for all local variables.

Register

Register is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can’t have the unary '&' operator applied to it (as it does not have a memory location). Register should only be used for variables that require quick access - such as counters. 

It should also be noted that defining 'register' goes not mean that the variable will be stored in a register. If registers are busy or already occupied then it will be used as auto variables but as soon as the registers are free it will be converted to register class

Static

Static is the default storage class for global variables. Static variables can be 'seen' within all functions in this source file. Static can also be defined within a function. Static variables are initialized with 0 as default.

If this is done the variable is initialized at run time but is not reinitialized when the function is called. This inside a function static variable retains its value during various calls.

Extern

Extern is used to give a reference of a global variable that is visible to ALL the program files. When you use 'extern' the variable cannot be initialized as all it does is point the variable name at a storage location that has been previously defined. 

When you have multiple files and you define a global variable or function which will be used in other files also, then extern will be used in another file to give reference of defined variable or function. Just for understanding extern is used to declare a global variable or function in another file.

A short comparison of storage classes to give you a overview

Names	Automatic(default)	External	Static	Register
Lifetime	Function block	Entire program/global	Entire program/global	Function block
Visibility	local	Global	local	Global
Initial value	Garbage	0	0	Garbage
Storage memory	Stack segment	Data segment	Data segment	CPU registers
Purpose	For local variables in class.	Those variables which we might/or use throughout the program.	Local variables which return their value, and retain the value even if the program is rerun.	Variables used for temporary calculations and stored in cpu registers for faster access.
Keyword used	Auto/(blank)	Extern	Static	register