boost::urls::grammar::optional_rule

Match a rule, or the empty string

Synopsis

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

template<class Rule>
constexpr
    _implementation-defined_
optional_rule(Rule const& r);

Description

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 ]

Parameters

Name Description

r

The rule to match

See Also

alpha_chars , parse , optional , token_rule .

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