76 lines
1.9 KiB
Plaintext
76 lines
1.9 KiB
Plaintext
[#one_of]
|
|
[section one_of]
|
|
|
|
[h1 Synopsis]
|
|
|
|
template <class... Ps>
|
|
struct one_of;
|
|
|
|
This is a [link parser_combinator parser combinator].
|
|
|
|
[table Arguments
|
|
[[Name] [Type]]
|
|
[[`Ps`] [[link parser parser]s]]
|
|
]
|
|
|
|
[h1 Description]
|
|
|
|
It accepts an input when any of the `Ps...` parsers accept it. The result of
|
|
parsing is the result of applying the first parser that accepts the input. The
|
|
parsers are tried in order, therefore in case of ambiguous grammars the result
|
|
of parsing depends on the order of the `Ps...` parsers.
|
|
|
|
On compilers, which are not C++11-compliant, the maximum number of accepted
|
|
parsers is defined by the `BOOST_METAPARSE_LIMIT_ONE_OF_SIZE` macro. Its default
|
|
value is `20`.
|
|
|
|
[h1 Header]
|
|
|
|
#include <boost/metaparse/one_of.hpp>
|
|
|
|
[h1 Expression semantics]
|
|
|
|
For any `p1`, ..., `pn` parsers, `s` compile-time string and `pos` source
|
|
position
|
|
|
|
one_of<p1, ..., pn>::apply<s, pos>
|
|
|
|
is equivalent to
|
|
|
|
pk::apply<s, pos>
|
|
|
|
when there is a `k`, that `pi::apply<s, pos>::type` returns an error for every
|
|
`i` in the range `[1..k)` and `pk::apply<s, pos>::type` doesn't return an error.
|
|
|
|
The parser combinator returns an error when there is no such `k`.
|
|
|
|
[h1 Example]
|
|
|
|
#include <boost/metaparse/one_of.hpp>
|
|
#include <boost/metaparse/lit_c.hpp>
|
|
#include <boost/metaparse/start.hpp>
|
|
#include <boost/metaparse/string.hpp>
|
|
#include <boost/metaparse/get_result.hpp>
|
|
#include <boost/metaparse/is_error.hpp>
|
|
|
|
using namespace boost::metaparse;
|
|
|
|
using whitespace =
|
|
one_of<lit_c<' '>, lit_c<'\n'>, lit_c<'\r'>, lit_c<'\t'>, lit_c<'\v'>>;
|
|
|
|
static_assert(
|
|
get_result<
|
|
whitespace::apply<BOOST_METAPARSE_STRING(" "), start>
|
|
>::type::value == ' ',
|
|
"the result of parsing should be the first character of the input"
|
|
);
|
|
|
|
static_assert(
|
|
is_error<whitespace::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
|
|
"it should return an error when the input does not begin with a whitespace"
|
|
);
|
|
|
|
|
|
[endsect]
|
|
|