Variables in C++ Programming

Variables in C++: An Overview

What are Variables in C++?

A variable is a named storage location of data in memory. Data types of variables can be int, float, boolean, char, and double.

Read More - C++ Interview Questions Interview Questions for Freshers

How to Declare a Variable?

Syntax

type variableName;

Example

float f; 

Here we can see that a float type variable f is created. This means that a memory location of name f which can store floating value is being created in the memory.

You can assign the value to the variable f after a declaration like this:

f=15.7;

The variables of the same type can be declared in a single line like this:

Syntax

type var1, var2, ... varn;

Example

int a, b, c;

You can also declare a variable by assigning values to it in the following way:

int a=10,b=20;//declaring 2 variable of integer type
float f=20.8;
char c='A';

Initialization of Variables in C++

• Initialization at the time of declaration

  int a=10;
  float f=20.8;
  char c='A';
  

• Initialization after the declaration

   int a, b, c;
   a = 55; 
   b = 10;
   c = a - b;
  

If we don't assign the value at the time of declaration, the garbage value is assigned by the compiler

What are the rules for naming a variable?

• Name can consist of alphabets (both uppercase and lowercase), digits, and underscore(_).
• The first character must be either an alphabet or an underscore.
• Variable names are case sensitive, i.e., num and Numare two different variable names.
• No white spaces and special characters like !, *, must be there in the variable names.
• A keyword cannot be your variable name.
• The type of the declared variable cannot be changed after declaration.

Operations that can be performed on variables

• Assigning the value of one variable to another variable

Example in C++ Online Editor

#include <iostream>
using namespace std;
int main() {
 int a = 15;
 int b = 20;
 a = b; // Assign the value of 'b' to 'a'
 cout << a << endl; 
 return 0;
}
 

Here we first declared a and b with values. Then we assigned the value of b to a.

Output

20

• Declaring multiple variables and printing them

Example

#include <iostream>
using namespace std;
int main() {
 int x = 5, y = 6, z = 50;
 cout << x << "\t" << y << "\t" << z << endl;
 return 0;
}
 

Here we have declared 3 int type variables in a single line separated by commas. In the cout function \t is used for giving spacing between 2 values in the same line.

Output

5 6 50

• Addition of two variables

Example

#include <iostream>
using namespace std;
int main() {
 int a = 5;
 int b = 10;
 int sum = a + b;
 cout << sum;
 return 0;
}
 

Here we are adding two integer-type variables and getting the result stored in the sum variable.

Output

15

Types of Variables in C++

1. Local Variable
2. Instance Variable
3. Static Variable
4. Automatic Variable
5. External Variable

1. Local Variable in C++

Example in C++ Compiler

#include <iostream>

void local();

int main() {
 int a = 22, b = 44;
 local();
 std::cout << "Values in the main() function a = " << a << " and b = " << b << std::endl;
 return 0;
}

void local() {
 int a = 50, b = 80;
 std::cout << "Values in the local() function a = " << a << " and b = " << b << std::endl;
}
 

In the above code, there are two functions, main() and local(). The variables a and b are local variables declared inside each function with different values. The value of a and b is restricted to the function in which it is declared.

Output

Values in the local() function a = 50 and b = 80
Values in the main() function a = 22 and b = 44

2. Instance Variable in C++

Example

#include <iostream>
using namespace std;

class Student {
public:
 // instance variables
 int roll_no;
 string name;
 float marks;
};

int main() {
 Student obj; //object of Student class

 return 0;
} 

You will understand instance variables in a better way while learning Object Oriented Programming (OOPs) Concepts in C++

3. Static Variable in C++

Example

#include <iostream>
using namespace std;

class Student {
public:
 // static variable 
 static float passing_marks;
};

float Student::passing_marks = 21;

int main() {
 Student obj; //object of Student class

 return 0;
}

4. Automatic Variable in C++

You will get to know the automatic variable in detail in the section, Storage Classes in C++.

Example in C++ Editor

#include <iostream>

int main() {
 int x = 10; // local variable (also automatic)
 auto y = 20; // automatic variable

 return 0;
}

5. External Variable in C++

External variables specify an external linkage and so such variables are not allocated any memory. The initialization must be done globally and not within any function or block. If an external variable is not initialized anywhere in the program, the compiler will show an error.

Read more: Storage Classes in C++: Types of Storage Classes with Examples

Example

#include <iostream>

extern int a; // Declaration of the global variable 'a'

int main() {
 std::cout << a << std::endl; // Display the value of the global variable 'a'
 return 0;
}

int a; // Definition of the global variable 'a'
 

Summary