---
title: "1.8 Compile-Time Constants (constexpr)"
description: "The `constexpr` keyword declares values that can be evaluated during compilation. A compile-time constant can be used wherever a constant expression is required, such as array sizes, template arguments, and `switch` case labels. This example demonstrates how to declare a…"
url: "https://edcret.in/docs/cpp-programming/basic-refreshers-cplusplus/1-8-compile-time-constants-constexpr/"
updated: "2026-07-28"
category: C++ Programming / Basic Refreshers
---

# 1.8 Compile-Time Constants (constexpr)

The `constexpr` keyword declares values that can be evaluated during compilation. A compile-time constant can be used wherever a constant expression is required, such as array sizes, template arguments, and `switch` case labels.

This example demonstrates how to declare a compile-time constant using `constexpr`.

## Source Code

```
#include <iostream>

int main()
{
    constexpr int days_in_week = 7;

    std::cout << "Days in a week: " << days_in_week << '\n';

    return 0;
}
```

## Output

```
Days in a week: 7
```

## Explanation

- `constexpr int days_in_week = 7;` declares a compile-time constant named `days_in_week`.
- The value of a `constexpr` object must be known during compilation.
- A `constexpr` object is implicitly `const` and cannot be modified after initialization.
- Unlike a regular `const` object, a `constexpr` object is guaranteed to be a constant expression when initialized with a compile-time value.
- `std::cout << "Days in a week: " << days_in_week << '\n';` prints the value of the compile-time constant.
- `return 0;` terminates the program successfully.