boost::urls::grammar::optional_rule

Match a rule, or the empty string

Synopsis

Declared in <boost/url/grammar/optional_rule.hpp>

template<Rule R>
constexpr
/* implementation-defined */
optional_rule(R const& r);

Optional BNF elements are denoted with square brackets. If the specified rule returns any error it is treated as if the rule did not match.


using value_type = optional< typename Rule::value_type >;

Rules are used with the function grammar::parse.


system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );


optional     = [ rule ]

The adapted rule

Name	Description
r	The rule to match