Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Match a rule, or the empty string.
Defined in header <boost/url/grammar/optional_rule.hpp>
template< class Rule> constexpr implementation-defined optional_rule( Rule 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 ]
|
Name |
Description |
|---|---|
|
|
The rule to match |
alpha_chars,
parse,
optional,
token_rule.
Convenience header <boost/url/grammar.hpp>