---
title: 6.14 constexpr Functions
description: "A `constexpr` function is a function that can be evaluated at compile time when it is called with arguments that permit constant evaluation. The `constexpr` specifier indicates that the function is intended to be usable in constant expressions. A `constexpr`…"
url: "https://edcret.in/docs/cpp-programming/functions/6-14-constexpr-functions/"
updated: "2026-09-06"
category: C++ Programming / Functions
---

# 6.14 constexpr Functions

A `constexpr` function is a function that can be evaluated at compile time when it is called with arguments that permit constant evaluation. The `constexpr` specifier indicates that the function is intended to be usable in constant expressions.

A `constexpr` function can also be called at runtime when its arguments or context do not permit compile-time evaluation. This allows the same function to be used in both compile-time and runtime contexts.

## Source Code

```
#include <iostream>

constexpr int square(int number)
{
    return number * number;
}

int main()
{
    constexpr int result = square(5);

    std::cout << "Square: " << result << '\n';

    return 0;
}
```

## Output

```
Square: 25
```

## Explanation

- The `constexpr` specifier declares `square()` as a function that can participate in constant evaluation.
- `square()` accepts an `int` parameter and returns an `int`.
- `return number * number;` calculates the square of the supplied value.
- `constexpr int result = square(5);` requires `square(5)` to produce a constant expression because `result` is declared `constexpr`.
- The compiler can evaluate `square(5)` during compilation.
- A `constexpr` function is not restricted to compile-time execution; it can also be called at runtime when compile-time evaluation is not possible or required.
- The rules governing what a `constexpr` function may contain have expanded across successive C++ standards.
- `return 0;` terminates the program successfully.