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- [#look_ahead]
- [section look_ahead]
- [h1 Synopsis]
- template <class P>
- struct look_ahead;
- This is a [link parser_combinator parser combinator].
- [table Arguments
- [[Name] [Type]]
- [[`P`] [[link parser parser]]]
- ]
- [h1 Description]
- Parses the input using parser `P`. When `P` returns an error, `look_ahead`
- returns the error. When `P` returns a result, `look_ahead` returns the result
- without consuming anything from the input string.
- [h1 Header]
- #include <boost/metaparse/look_ahead.hpp>
- [h1 Expression semantics]
- For any `p` parser, `s` compile-time string and `pos` source position
- look_ahead<p>::apply<s, pos>
- is equivalent to
- return_<get_result<p::apply<s, pos>>::type>::apply<s, pos>
- when `p::apply<s, pos>` succeeds. It is
- p::apply<s, pos>
- otherwise.
- [h1 Example]
- #include <boost/metaparse/look_ahead.hpp>
- #include <boost/metaparse/int_.hpp>
- #include <boost/metaparse/start.hpp>
- #include <boost/metaparse/string.hpp>
- #include <boost/metaparse/is_error.hpp>
- #include <boost/metaparse/get_result.hpp>
- #include <boost/metaparse/get_remaining.hpp>
-
- #include <type_traits>
-
- using namespace boost::metaparse;
-
- static_assert(
- get_result<
- look_ahead<int_>::apply<BOOST_METAPARSE_STRING("13"), start>
- >::type::value == 13,
- "it should return the same result as the wrapped parser"
- );
-
- static_assert(
- std::is_same<
- BOOST_METAPARSE_STRING("13"),
- get_remaining<
- look_ahead<int_>::apply<BOOST_METAPARSE_STRING("13"), start>
- >::type
- >::type::value,
- "it should not consume any input"
- );
-
- static_assert(
- is_error<
- look_ahead<int_>::apply<BOOST_METAPARSE_STRING("six"), start>
- >::type::value,
- "it should fail when the wrapped parser fails"
- );
- [endsect]
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