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

Match one of a set of rules

== Synopsis

Declared in `<https://www.github.com/boostorg/url/blob/develop/include/boost/url/grammar/variant_rule.hpp#L105[boost/url/grammar/variant_rule.hpp]>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    xref:boost/urls/grammar/Rule.adoc[Rule] R0,
    xref:boost/urls/grammar/Rule.adoc[Rule]... Rn>
constexpr
/* implementation-defined */
variant_rule(
    R0 const& r0,
    Rn const&... rn) noexcept;
----

== Description

Each specified rule is tried in sequence. When the first match occurs, the result is stored and returned in the variant. If no match occurs, an error is returned.

== Value Type
[,cpp]
----
using value_type = variant< typename Rules::value_type... >;
----
== Example
Rules are used with the function xref:boost/urls/grammar/parse-09.adoc[`parse`]&period;

[,cpp]
----
// request-target = origin-form
//                / absolute-form
//                / authority-form
//                / asterisk-form

system::result< variant< url_view, url_view, authority_view, core::string_view > > rv = grammar::parse(
    "/index.html?width=full",
    variant_rule(
        origin_form_rule,
        absolute_uri_rule,
        authority_rule,
        delim_rule('*') ) );
----
== BNF
[,cpp]
----
variant     = rule1 / rule2 / rule3...
----
== Specification
* https://datatracker.ietf.org/doc/html/rfc5234#section-3.2[]
* https://datatracker.ietf.org/doc/html/rfc7230#section-5.3[]

== Return Value

The variant rule

== Parameters

[cols=2]
|===
| Name
| Description
| *r0*
| The first rule to match
| *rn*
| A list of one or more rules to match
|===

== See Also

xref:boost/urls/absolute_uri_rule.adoc[`absolute&lowbar;uri&lowbar;rule`], xref:boost/urls/authority_rule.adoc[`authority&lowbar;rule`], xref:boost/urls/grammar/delim_rule-0a.adoc[`delim&lowbar;rule`], xref:boost/urls/grammar/parse-09.adoc[`parse`], xref:boost/urls/origin_form_rule.adoc[`origin&lowbar;form&lowbar;rule`], xref:boost/urls/url_view.adoc[`url&lowbar;view`]&period;


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