Match a rule, or the empty string.

Synopsis

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

template<
    class Rule>
constexpr implementation-defined
optional_rule(
    Rule 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.

Value Type

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

Example

Rules are used with the function grammar::parse.

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

BNF

optional     = [ rule ]

Specification

Parameters

See Also

alpha_chars, parse, optional, token_rule.

Convenience header <boost/url/grammar.hpp>