[/
    Copyright (c) 2021 Vinnie Falco (vinnie.falco@gmail.com)

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

    Official repository: https://github.com/boostorg/url
]

[section:charset CharSet]

A ['CharSet] is a unary predicate which is invocable with
this equivalent signature:

```
bool( char ch ) const noexcept;
```

The predicate returns `true` if `ch` is a member of the
set, or `false` otherwise.

[heading Related Identifiers]

[link url.ref.boost__urls__grammar__is_charset `is_charset`],
[link url.ref.boost__urls__grammar__find_if `find_if`],
[link url.ref.boost__urls__grammar__find_if_not `find_if_not`].

[heading Requirements]

In this table:

* `T` is a type meeting the requirements of ['CharSet]
* `t` is a `const` value of type `T`
* `c` is a value of type `char`
* `first`, `last` are values of type `char const*`

[table Valid expressions
[[Expression] [Type] [Semantics, Pre/Post-conditions]]
[
    [`t(c)`]
    [`bool`]
    [
        This function returns `true` if `c` is a member of
        the character set, otherwise it returns `false`.
    ]
][
    [
    ```
    t.find_if(first,last)
    ```
    ]
    [`char const*`]
    [
        This optional member function examines the valid
        range of characters in `[first, last)` and returns
        a pointer to the first occurrence of a character 
        which is in the set, or returns `last` if no such
        character.

        The implementation of
        [link url.ref.boost__urls__grammar__find_if `find_if`]
        calls this function if provided by the character
        set, allowing optimized or otherwise performant
        implementations to be developed. If this member
        function is not provided, a default implementation
        is used which calls `operator()`.
    ]
][
    [
    ```
    t.find_if_not(first,last)
    ```
    ]
    [`char const*`]
    [
        This optional member function examines the valid
        range of characters in `[first, last)` and returns
        a pointer to the first occurrence of a character 
        which is not in the set, or returns `last` if no
        such character.

        The implementation of
        [link url.ref.boost__urls__grammar__find_if_not `find_if_not`]
        calls this function if provided by the character
        set, allowing optimized or otherwise performant
        implementations to be developed. If this member
        function is not provided, a default implementation
        is used which calls `operator()`.
    ]
]]

[heading Exemplar]

For best results, it is suggested that all constructors and
member functions for character sets be marked `constexpr`.

[code_charset_1]

[heading Models]

* [link url.ref.boost__urls__grammar__alnum_chars `alnum_chars`]
* [link url.ref.boost__urls__grammar__alpha_chars `alpha_chars`]
* [link url.ref.boost__urls__grammar__digit_chars `digit_chars`]
* [link url.ref.boost__urls__grammar__hexdig_chars `hexdig_chars`]
* [link url.ref.boost__urls__grammar__lut_chars `lut_chars`]

[endsect]