CodeBlanks Logo Icon CodeBlanks
Login Register

constexpr

Learn to define compile-time constants with constexpr.

constexpr

constexpr indicates that a variable's value can be computed at compile time. When you write constexpr int width = 800;, the compiler computes 800 during compilation and embeds it in the executable.

The difference between const and constexpr: const only guarantees immutability (the value may be determined at runtime); constexpr guarantees both immutability and compile-time evaluability.

constexpr is used where constant expressions are required (array sizes, template parameters). The compiler can optimize better.

Using constexpr

A variable declared with constexpr must have a value the compiler can compute. So the right-hand side must be a constant expression: constexpr int area = 20 * 30; is valid because 20 * 30 is computed at compile time.

But constexpr int x = readValue(); is invalid because readValue() is called at runtime. In this case use const.

constexpr functions can also be defined: when called in a constant expression context, they run at compile time. This is common in modern C++ for performance.

Practice Define sizes with constexpr

Above main (in the global scope), declare two constexpr int values: width with value 800 and height with value 600. Inside main, add two std::cout lines: the first prints width, the second height; end each line with std::endl.

Starter code

main.cpp
#include <iostream>

// Define two sizes with constexpr

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

Sample solution

main.cpp
#include <iostream>

// Define two sizes with constexpr
constexpr int width = 800;
constexpr int height = 600;

int main() {
    // Print with std::cout
    std::cout << width << std::endl;
    std::cout << height << std::endl;
    return 0;
}

Expected output

800
600

Questions and answers

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

1 What is the difference between constexpr and const?

  • constexpr is evaluated at compile time Correct answer
  • const is faster
  • constexpr is only for double
  • const is only used in classes

Explanation: const may hold a runtime value.

2 Match constexpr concepts

Term Meaning
constexpr Compile-time constant
20 * 30 Constant expression
readValue() Runtime
array size Requires constexpr

3 Compile-time constant

Question 
____ int width = 800;
Answer 
constexpr int width = 800;

Explanation: constexpr int width is a compile-time constant.

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