C++ Variables, Literals and Constants

In this tutorial, we will learn about variables, literals, and constants in C++ with the help of examples.
Content from https://www.programiz.com/cpp-programming/variables-literals.
C++ Variables
In programming, a variable is a container (storage area) to hold data.
To indicate the storage area, each variable should be given a unique name (identifier). For example,
int age = 14;
Here, 
age is a variable of the 
int data type, and we have assigned an integer value 14 to it.
Note: The 
int data type suggests that the variable can only hold integers. Similarly, we can use the 
double data type if we have to store decimals and exponentials.
We will learn about all the data types in detail in the next tutorial.
The value of a variable can be changed, hence the name variable.
int age = 14;   // age is 14
age = 17;       // age is 17
Rules for naming a variable
A variable name can only have alphabets, numbers, and the underscore 
_.
A variable name cannot begin with a number.
It is a preferred practice to begin variable names with a lowercase character. For example, 
name is preferable to 
Name.
A variable name cannot be a keyword. For example, 
int is a keyword that is used to denote integers.
A variable name can start with an underscore. However, it's not considered a good practice.
Note: We should try to give meaningful names to variables. For example, 
first_name is a better variable name than 
fn.
C++ Literals
Literals are data used for representing fixed values. They can be used directly in the code. For example: 
1, 
2.5, 
'c' etc.
Here, 
1, 
2.5 and 
'c' are literals. Why? You cannot assign different values to these terms.
Here's a list of different literals in C++ programming.
Integers
An integer is a numeric literal(associated with numbers) without any fractional or exponential part. There are three types of integer literals in C programming:
decimal (base 10)
octal (base 8)
hexadecimal (base 16)
For example:
Decimal: 0, -9, 22 etc
Octal: 021, 077, 033 etc
Hexadecimal: 0x7f, 0x2a, 0x521 etc
In C++ programming, octal starts with a 
0, and hexadecimal starts with a 
0x.
Floating-point Literals
A floating-point literal is a numeric literal that has either a fractional form or an exponent form. For example:
-2.0
0.0000234
-0.22E-5
Note: 
E-5 = 10-5
Characters
A character literal is created by enclosing a single character inside single quotation marks. For example: 
'a', 
'm', 
'F', 
'2', 
'}' etc.
Escape Sequences
Sometimes, it is necessary to use characters that cannot be typed or has special meaning in C++ programming. For example, newline (enter), tab, question mark, etc.
In order to use these characters, escape sequences are used.
Escape Sequences
Characters
\b
Backspace
\f
Form feed
\n
Newline
\r
Return
\t
Horizontal tab
\v
Vertical tab
\\
Backslash
\'
Single quotation mark
\"
Double quotation mark
\?
Question mark
\0
Null Character
String Literals
A string literal is a sequence of characters enclosed in double-quote marks. For example:
"good"
string constant
""
null string constant
" "
string constant of six white space
"x"
string constant having a single character
"Earth is round\n"
prints string with a newline
We will learn about strings in detail in the C++ string tutorial.
C++ Constants
In C++, we can create variables whose value cannot be changed. For that, we use the 
const keyword. Here's an example:
const int LIGHT_SPEED = 299792458;
LIGHT_SPEED = 2500 // Error! LIGHT_SPEED is a constant.
Here, we have used the keyword 
const to declare a constant named 
LIGHT_SPEED. If we try to change the value of 
LIGHT_SPEED, we will get an error.
A constant can also be created using the 
#define preprocessor directive. We will learn about it in detail in the C++ Macros tutorial.