optional.qbk
2.31 KB
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[#optional]
[section optional]
[h1 Synopsis]
template <class P, class Default = /* unspecified */>
struct optional;
This is a [link parser_combinator parser combinator].
[table Arguments
[[Name] [Type]]
[[`P`] [[link parser parser]]]
[[`Default`] [[link metaprogramming_value template metaprogramming value]]]
]
[h1 Description]
It tries parsing the input with `P`. When `P` succeeds, the result of parsing is
the result of `P`. Otherwise no characters are consumed and the result of
parsing is `Default`.
[h1 Header]
#include <boost/metaparse/optional.hpp>
[h1 Expression semantics]
For any `p` [link parser parser] and `d`
[link metaprogramming_value template metaprogramming value]
optional<p, d>
is equivalent to
one_of<p, return_<d>>
[h1 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"
);
[endsect]