Define constant variables in C++ header

Hello readers! In this tutorial, we are going to learn how we can define constant variables in the C++ header file. In C++ constant variables are defined as the types of variables whose value can’t be changed once they are defined, they remain fixed all throughout the program.

In C++ a variable can be declared constant in 2 ways:

  • Using #define keyword method
Syntax:- #define identifier_name value

Example- #define var3 290

  •  By using const keyword method
 Syntax:- const datatype identifier_name = value

Example- const int var8 = 300

Before moving to the explanation of the above two types, first, we should know what is literal?

Literals:  A literal is actually the value that is assigned to a constant variable. Like, if we take const float z= 10.2; In this case, 10.2 is referred to as the constant literal of float type. Now we will see in detail the two types of constant.

C++ Code: constant variables in C++ header

Refer to the code below to see both the ways :

Refer to the comments in the code for a better understanding

#include <iostream>
#define var1 89    
const int var2=340;   //using const keyword method
int main()
   std::cout<<var1<<" "<<var2; //using std::cout for output


89 340

Explanation of the code:

Using  #define declare a constant variable with a constant value. Now using const keyword declares another constant variable. Declare the main function. The output values of both variables will be printed using std::cout.

1. Using the const keyword:  In this case, first, we will look at the syntax of declaring constant in C++ using the const keyword.

                   Syntax:    const datatype  name_of_the_constant = initial_value;

In the above syntax –

1. One should never remove the const keyword from the beginning.

2.  The data type can be float, integer, double, long long, long long int etc.

3.   “name_of_the_contant” should always be like a declared name. You should always remember once you declare a variable as const you should never declare the same variable name inside the programming anywhere.

4.  ” initial_value ” should go with the data type.

using namespace std;

int main()
  const int a = 10 //Valid defining of constant
   Error 1 :
   const int z; //Remember always this code will give you an error
  Error 2:
   const int z; // This will also throw an error

  return 0;

In the above example, we can see two error statements. Those are because of –

1. Error 1: Suppose you are declaring a constant and assigning the value in the next line, that also will throw an error.

2. Error 2: You should never leave a const after declaring it, without assigning a value to it.

3. One should not program in such a way that, a constant can only be initialized with a value according to the user input. In that case, the code will run with an error.


2. Using #define:  In this case, first, we will look to the syntax of declaring constant in C++ using #define.

                Syntax:    #define identifier_name value

In the above syntax –

1. #define should be present in this constant initialization technique.

2.  ” identifier_name ” should not be a data type like int, float. It’s a given name. By which we will access the value of it, later inside the code.

3.   Next is value initialization to the const.

using namespace std;

#define floatVariable 4.35 //Defining the value

int main()
  cout<<"Float constant variable value is - "<<floatVariable; //Printing Value
  return 0;


Remember:  You should not define a constant with #define within the main function or within a class or within a function.

That’s all for the tutorial. Hope you liked it!

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