[#boost-urls-grammar-CharSet]
= xref:boost/urls/grammar/CharSet.adoc[boost::urls::grammar::CharSet]
:relfileprefix: ../../../
:mrdocs:

Concept for a CharSet

== Synopsis

Declared in `<https://www.github.com/boostorg/url/blob/develop/include/boost/url/grammar/charset.hpp#L114[boost/url/grammar/charset.hpp]>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class T>
concept CharSet = requires (T const t, char c)
{
    { t(c) } ‐> std::convertible_to<bool>;
};
----

== Description

A `CharSet` is a unary predicate which is invocable with this equivalent signature:

[,cpp]
----
bool( char ch ) const noexcept;
----
The predicate returns `true` if `ch` is a member of the set, or `false` otherwise.

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

[,cpp]
----
struct CharSet
{
    bool operator()( char c ) const noexcept;

    // These are both optional. If either or both are left
    // unspecified, a default implementation will be used.
    //
    char const* find_if( char const* first, char const* last ) const noexcept;
    char const* find_if_not( char const* first, char const* last ) const noexcept;
};
----
== Models
* `alnum_chars`
* `alpha_chars`
* `digit_chars`
* `hexdig_chars`
* `lut_chars`

== See Also

xref:boost/urls/grammar/is_charset.adoc[`is_charset`], xref:boost/urls/grammar/find_if.adoc[`find_if`], xref:boost/urls/grammar/find_if_not.adoc[`find_if_not`].


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