107 lines
2.3 KiB
Plaintext
107 lines
2.3 KiB
Plaintext
[#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]
|
|
|