Synopsis

template <class P, class Default = /* unspecified */>
struct optional;

This is a parser combinator.

Description

It tries parsing the input with P. When P succeeds, the rsult of parsing is the result of P. Otherwise no characters are consumed and the result of parsing is Default.

Header

#include <boost/metaparse/optional.hpp>

Expression semantics

For any p parser and d template metaprogramming value

optional<p, d>

is equivalent to

one_of<p, return_<d>>

Example

#include <boost/metaparse/optional.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/int_.hpp>
#include <boost/metaparse/middle_of.hpp>
#include <boost/metaparse/sequence.hpp>
#include <boost/metaparse/lit_c.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/get_result.hpp>

#include <boost/mpl/int.hpp>
#include <boost/mpl/equal.hpp>
#include <boost/mpl/equal_to.hpp>
#include <boost/mpl/vector_c.hpp>

using namespace boost::metaparse;

using complex_number =
  sequence<
    // Real
    int_,

    // Imaginary
    optional<
      middle_of<lit_c<'+'>, int_, lit_c<'i'>>,
      boost::mpl::int_<0>
    >
  >
;

static_assert(
  boost::mpl::equal<
    boost::mpl::vector_c<int, 1, 0>,
    get_result<
      complex_number::apply<BOOST_METAPARSE_STRING("1"), start>
    >::type,

    boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
  >::type::value,
  "No imaginary"
);

static_assert(
  boost::mpl::equal<
    boost::mpl::vector_c<int, 1, 0>,
    get_result<
      complex_number::apply<BOOST_METAPARSE_STRING("1+0i"), start>
    >::type,

    boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
  >::type::value,
  "0 as imaginary"
);

static_assert(
  boost::mpl::equal<
    boost::mpl::vector_c<int, 0, 1>,
    get_result<
      complex_number::apply<BOOST_METAPARSE_STRING("0+1i"), start>
    >::type,

    boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
  >::type::value,
  "Non-null imaginary"
);