Variables are containers for storing data values. In C++, there are different types of variables. Some of them are as follows:

• An integer variable defined with the keyword int stores integers (whole numbers), without decimals, such as 63 or -1.
• A floating point variable defined with the keyword float stores floating point numbers, with decimals, such as 79.97 or -13.26.
• A character variable defined with the keyword char stores single characters, such as 'A' or 'z'. Char values are bound to be surrounded by single quotes.
• A boolean variable defined with the keyword bool stores a single value 0 or 1 for false and true respectively.

Declaration

We cannot declare a variable without specifying its data type. The data type of a variable depends on what we want to store in the variable and how much space we want it to hold. The syntax for declaring a variable is simple:

data_type variable_name;

OR

data_type variable_name = value;

The tutorial will go over data types later on. They will be dealt with in great detail.

Naming a Variable

There is no limit to what we can call a variable. Yet there are specific rules we must follow while naming a variable:

• A variable name in C++ can have a length of range 1 to 255 characters.
• A variable name can only contain alphabets, digits, and underscores (_).
• A variable cannot start with a digit.
• A variable cannot include any white space in its name.
• Variable names are case sensitive.
• The name should not be a reserved keyword or any special character.

Variable Scope

The scope of a variable is the region in a program where the existence of that variable is valid. Based on its scope, variables can be classified into two types:

Local variables:

Local variables are declared inside the braces of any function and can be accessed only from that particular function.

Global variables:

Global variables are declared outside of any function and can be accessed from anywhere.

An example that demonstrates the difference in applications of a local and a global variable is given below.

#include <iostream>
using namespace std;
 
int a = 5; // global variable
 
void func()
{
    cout << a << endl;
}
 
int main()
{
    int a = 10; // local variable
    cout << a << endl;
    func();
    return 0;
}

Output:

10
5

Explanation: A local variable a was declared in the main function, and when printed, gave 10. This is because, within the body of a function, a local variable takes precedence over a global variable with the same name. But since there was no variable declared in the func function, it considered the global variable a for printing, and hence the value 5.

A variable, as its name is defined, can be altered, or its value can be changed, but the same is not true for its type. If a variable is of integer type, it will only store an integer value throughout a program. We cannot assign a character type value to an integer variable. We cannot even store a decimal value into an integer variable.