123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475 |
- [#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]
|