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- [#middle_of]
- [section middle_of]
- [h1 Synopsis]
- template <class P1, class P2, class P3>
- struct middle_of;
- This is a [link parser_combinator parser combinator].
- [table Arguments
- [[Name] [Type]]
- [[`P1`] [[link parser parser]]]
- [[`P2`] [[link parser parser]]]
- [[`P3`] [[link parser parser]]]
- ]
- [h1 Description]
- `middle_of` applies `P1`, `P2` and `P3` in sequence. It accepts an input when
- all of these three parsers accept it. The result of parsing is the result of
- `P2`.
- [h1 Header]
- #include <boost/metaparse/middle_of.hpp>
- [h1 Expression semantics]
- For any `p1`, `p2` and `p3` parsers
- middle_of<p1, p2, p3>
- is equivalent to
- nth_of<1, p1, p2, p3>
- [h1 Example]
- #include <boost/metaparse/middle_of.hpp>
- #include <boost/metaparse/int_.hpp>
- #include <boost/metaparse/lit_c.hpp>
- #include <boost/metaparse/token.hpp>
- #include <boost/metaparse/start.hpp>
- #include <boost/metaparse/string.hpp>
- #include <boost/metaparse/is_error.hpp>
- #include <boost/metaparse/get_result.hpp>
-
- using namespace boost::metaparse;
-
- using int_token = token<int_>;
- using left_paren_token = token<lit_c<'('>>;
- using right_paren_token = token<lit_c<')'>>;
-
- using int_in_parens = middle_of<left_paren_token, int_token, right_paren_token>;
-
- static_assert(
- get_result<
- int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start>
- >::type::value == 13,
- "it should return the result of the middle parser"
- );
-
- static_assert(
- is_error<
- int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start>
- >::type::value,
- "it should reject the input when there are no parens"
- );
-
- static_assert(
- is_error<
- int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start>
- >::type::value,
- "it should reject the input when there is no closing paren"
- );
- [endsect]
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