empty.qbk
1.38 KB
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[#empty]
[section empty]
[h1 Synopsis]
template <class Result>
struct empty;
This is a [link parser parser].
[table Arguments
[[Name] [Type]]
[[`Result`] [[link metaprogramming_value template metaprogramming value]]]
]
[h1 Description]
It accepts empty input only. The result of parsing is the `Result`
argument.
[h1 Header]
#include <boost/metaparse/empty.hpp>
[h1 Expression semantics]
For any `c` class the following are equivalent:
empty<c>
except<one_char, c, error::end_of_input_expected>
[h1 Example]
#include <boost/metaparse/empty.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/is_error.hpp>
#include <boost/metaparse/get_result.hpp>
#include <type_traits>
using namespace boost::metaparse;
using want_empty = empty<BOOST_METAPARSE_STRING("result")>;
static_assert(
!is_error<want_empty::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
"empty accepts the empty input"
);
static_assert(
is_error<want_empty::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
"empty should reject non-empty input"
);
static_assert(
std::is_same<
get_result<want_empty::apply<BOOST_METAPARSE_STRING(""), start>>::type,
BOOST_METAPARSE_STRING("result")
>::type::value,
"the result of parsing should be the given value"
);
[endsect]