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grammar::variant_rule

Match one of a set of rules.

Synopsis

Defined in header <boost/url/grammar/variant_rule.hpp>

template<
    class... Rules>
constexpr implementation-defined
variant_rule(
    Rules... rn);

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

using value_type = variant< typename Rules::value_type... >;

Example

Rules are used with the function parse.

// request-target = origin-form
//                / absolute-form
//                / authority-form
//                / asterisk-form

result< variant< url_view, url_view, authority_view, string_view > > rv = grammar::parse(
    "/index.html?width=full",
    variant_rule(
        origin_form_rule,
        absolute_uri_rule,
        authority_rule,
        delim_rule('*') ) );

BNF

variant     = rule1 / rule2 / rule3...

Specification

See Also

absolute_uri_rule, authority_rule, delim_rule, parse, origin_form_rule, url_view, variant.

Convenience header <boost/url/grammar.hpp>