---
title: "constexpr in C++"
description: "How constexpr moves work to compile time in C++11/14/17: constexpr functions, compile-time vs runtime evaluation, and why it beats template metaprogramming."
date: 2023-04-06
last_modified_at: 2026-10-02
keywords: [C++, constexpr, compile-time computation, C++14, C++17]
---

# Constexpr in C++

The `constexpr` keyword was introduced in C++11 and enhanced in C++14 and C++17. It enables computations to be performed at compile time.

## Basic Usage

A `constexpr` function can be evaluated at compile time if its arguments are compile-time constants:

```cpp
constexpr int square(int x) {
    return x * x;
}

// Used at compile time
constexpr int result = square(5); // Computed during compilation

// Used at runtime
int input;
std::cin >> input;
int dynamic_result = square(input); // Computed at runtime
```

## Constexpr vs Template Metaprogramming

While template metaprogramming also enables compile-time computation, `constexpr` provides a more readable and maintainable approach:

```cpp
// Fibonacci with constexpr (C++14 and later)
constexpr int fibonacci(int n) {
    if (n <= 1) return n;
    return fibonacci(n-1) + fibonacci(n-2);
}

// Usage
constexpr int fib10 = fibonacci(10); // Computed at compile time
```

The advantage of `constexpr` over template metaprogramming is that it uses familiar function syntax rather than recursive template instantiations, making the code easier to understand and maintain.