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