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const

Learn to declare immutable variables with the const keyword.

const

The const keyword makes a variable immutable. After declaring const int maxUsers = 100;, writing maxUsers = 200; gives a compile error.

const tells the compiler and the reader that this value will not change. This improves reliability and helps the compiler optimize.

Using const for constants is a good habit: const double pi = 3.14159;. Add const wherever you do not want a value to change.

Using const

A const variable must be given a value when declared; it cannot be assigned later. const int x; alone is invalid; const int x = 5; is correct.

The value of a const variable can be determined at runtime. You can hold a function result as a constant: const int y = readValue(); — the value is computed when the program runs but never changes afterward.

If the value is known at compile time, constexpr may be more appropriate; you will see it in the next lesson.

Practice Define constants and print

Inside main, declare two const variables: maxUsers with value 100 (int) and taxRate with value 0.18 (double). Print both with std::cout using std::endl.

Starter code

main.cpp
#include <iostream>

int main() {
    // Define constants with const
    // Print with std::cout
    return 0;
}

Sample solution

main.cpp
#include <iostream>

int main() {
    // Define constants with const
    const int maxUsers = 100;
    const double taxRate = 0.18;
    // Print with std::cout
    std::cout << maxUsers << std::endl;
    std::cout << taxRate << std::endl;
    return 0;
}

Expected output

100
0.18

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Defining a constant

Question 
____ double pi = 3.14159;
Answer 
const double pi = 3.14159;

Explanation: const double pi is an immutable variable.

2 What happens if you reassign a const variable?

main.cpp
const int x = 5;
x = 10;
  • The value changes
  • The program crashes
  • A warning is given but it changes
  • A compile error is raised Correct answer

Explanation: Reassigning a const variable is a compile error.

3 Match const concepts

Term Meaning
const Immutable
reassignment Compile error
declaration time Value given
optimization Helps compiler

All lessons in Beginner

1 C++ basics

  1. What is C++?
  2. Your first program
  3. std::cout
  4. std::cin
  5. Comments
  6. Summary - C++ basics

2 Variables

  1. Declaring variables
  2. auto
  3. const
  4. constexpr
  5. Type conversion
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and char
  4. std::string
  5. sizeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 Strings

  1. std::string basics
  2. String concatenation
  3. String length
  4. Substring
  5. String search
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range-based for
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. C-style arrays
  2. Array access
  3. std::array
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 std::vector

  1. Declaring vectors
  2. push_back
  3. size and at
  4. Vector access
  5. Vector loop
  6. Summary - std::vector

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function overloading
  5. Default parameters
  6. Summary - Functions

13 References

  1. What is a reference?
  2. Declaring references
  3. Reference parameters
  4. const references
  5. Dangling references
  6. Summary - References

14 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. nullptr
  5. Pointer arithmetic
  6. Summary - Pointer basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. String streams
  2. stoi and stod
  3. to_string
  4. String comparison
  5. String modification
  6. Summary - Advanced strings

17 Structs

  1. Defining structs
  2. Using structs
  3. Struct and function
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Exception basics

  1. try/catch
  2. throw
  3. std::exception
  4. Custom exceptions
  5. catch all
  6. Summary - Exceptions

19 Namespaces

  1. Defining namespaces
  2. using
  3. std namespace
  4. Nested namespaces
  5. Namespace alias
  6. Summary - Namespaces

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together