CppCon 2026

const is one of the oldest and most widely used features in C++, yet many developers still struggle with the subtle differences between const , constant expressions, constexpr , and consteval .

In this talk, you will learn how compile-time constants evolved in modern C++, starting with const and continuing through the features introduced since C++11. You will see what constant evaluation really means, what the compiler is allowed to execute during compilation, and where the limits still are today.

Using practical examples, we will explore when const is enough, when constexpr is needed, and how compile-time and runtime execution interact in modern C++. We will also look at common pitfalls, surprising behavior, and cases where compile-time programming can either improve code or make it harder to understand.

Finally, you will see how features such as consteval and std::is_constant_evaluated influence correctness, testing, and maintainability.

By the end of this talk, you will have a deeper understanding of constant evaluation and know how to use modern compile-time features more effectively in your code.