# <numbers>

**[<numbers>](https://en.cppreference.com/w/cpp/header/numbers)** provides mathematical constants (C++20): `std::numbers::pi`, `std::numbers::e`, `std::numbers::sqrt2`, etc. The constants are exact as `double` by default, but you can specialize them for other floating-point types.

Use these instead of hardcoding `3.14159` or defining your own.

## Example

This example uses the pi and sqrt2 constants to calculate the area of a circle and compose other mathematical expressions.

```cpp
// compile: g++ -std=c++20 -o numbersexample numbersexample.cpp
// run: ./numbersexample
// description: use mathematical constants

#include <numbers>
#include <iostream>
#include <cmath>

int main() {
    double area = std::numbers::pi * 5.0 * 5.0;
    std::cout << "area of circle (r=5): " << area << "\n";
    
    double result = std::numbers::e * std::numbers::sqrt2;
    std::cout << "e * sqrt(2): " << result << "\n";
    
    return 0;
}
```