boost::urls::grammar::optional_rule
Match a rule, or the empty string
Synopsis
Declared in header <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
system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );optional = [ rule ]See Also
alpha_chars , parse , optional , token_rule .