310.md

<details open><summary>Info</summary><p>

* **Did you know that C+23 permitts static constexpr variables in constexpr functions?**

  * https://wg21.link/P2647

</p></details><details open><summary>Example</summary><p>

```cpp
constexpr auto foo() {
  static constexpr auto value = 42; // error in C++20, okay in C++23
  return value;
}
```

> https://godbolt.org/z/6zK6vj91z

</p></details><details open><summary>Puzzle</summary><p>

> **Can you implement `to_string_view` which converts in-place given characters to string_view?**

```cpp
template<char... Cs>
[[nodiscard]] constexpr auto to_string_view(); // TODO

static_assert(std::string_view{""} == to_string_view<>());
static_assert(std::string_view{"42"} == to_string_view<'4', '2'>());
static_assert(std::string_view{"foo"} == to_string_view<'f', 'o', 'o'>());
static_assert(std::string_view{"bar"} == to_string_view<'b', 'a', 'r'>());
```

> https://godbolt.org/z/4fbYGGxco

</p></details><details><summary>Solutions</summary><p>

```cpp
template<char... Cs>
[[nodiscard]] constexpr auto to_string_view(){
    static constexpr char p[sizeof...(Cs)] = {Cs...};
    return std::string_view(p, sizeof...(Cs));
}
```

> https://godbolt.org/z/r31M7hhno

```cpp
template<char... Cs>

[[nodiscard]] constexpr auto to_string_view(){
    static constexpr std::array<char, sizeof...(Cs)> chars{Cs...};
    return std::string_view{chars};
}

template<>
[[nodiscard]] constexpr auto to_string_view<>(){
    return std::string_view{};
}
```

> https://godbolt.org/z/eTWasjTTr

```cpp
[[nodiscard]] constexpr auto to_string_view()
{
    constexpr static std::size_t N = sizeof...(Cs);
    if constexpr (N >0)
    {
        constexpr static std::array<char,N> chars{Cs...};
        return std::string_view(chars.data(),N);
    } else
        return std::string_view("");
}
```

> https://godbolt.org/z/9MMKv98dE

```cpp
template<char... Cs>
[[nodiscard]] constexpr auto to_string_view() {
  static constexpr char cs[]{Cs..., 0};
  return std::string_view{cs};
}
```

> https://godbolt.org/z/PPdxETv76