except.qbk
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[#except]
[section except]
[h1 Synopsis]
template <class P, class Result, class ErrorMsg>
struct except;
This is a [link parser_combinator parser combinator].
[table Arguments
[[Name] [Type]]
[[`P`] [[link parser parser]]]
[[`Result`] [[link metaprogramming_value template metaprogramming value]]]
[[`ErrorMsg`] [[link parsing_error_message parsing error message]]]
]
[h1 Description]
`except` accepts the input when `P` rejects it and the result of parsing is the
`Result` argument. When `P` accepts the input, `except` rejects it and the
reason is `ErrorMsg`.
[h1 Header]
#include <boost/metaparse/except.hpp>
[h1 Expression semantics]
For any `p` parser, `c` class, `msg` parsing error message, `s` compile-time
string and `pos` source position the following are equivalent
get_result<except<p, c, msg>, s, pos>::type
c
get_remaining<except<p, c, msg>, s, pos>::type
s
get_position<except<p, c, msg>, s, pos>::type
pos
when `p` rejects the input. The result of the parser is an error with the error
message `msg` otherwise.
[h1 Example]
#include <boost/metaparse/except.hpp>
#include <boost/metaparse/int_.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/get_result.hpp>
#include <boost/metaparse/get_message.hpp>
#include <boost/metaparse/define_error.hpp>
#include <type_traits>
using namespace boost::metaparse;
BOOST_METAPARSE_DEFINE_ERROR(
number_is_not_allowed,
"numbers are not allowed here"
);
using except_int =
except<int_, std::integral_constant<int, 1>, number_is_not_allowed>;
static_assert(
get_result<
except_int::apply<BOOST_METAPARSE_STRING("foo"), start>
>::type::value == 1,
"it should accept the input when it is not an integer"
);
static_assert(
std::is_same<
number_is_not_allowed,
get_message<except_int::apply<BOOST_METAPARSE_STRING("13"), start>>::type
>::type::value,
"it should reject the input when it is an integer"
);
[endsect]