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- [#always_c]
- [section always_c]
- [h1 Synopsis]
- template <char C, class T>
- struct always_c;
- This is a [link parser_combinator parser combinator].
- [table Arguments
- [[Name] [Type]]
- [[`C`] [character]]
- [[`T`] [[link metaprogramming_value template metaprogramming value]]]
- ]
- [h1 Description]
- It accepts inputs beginning with the `C` character. It consumes that character
- and the result of parsing is `T`. Other inputs as rejected.
- [h1 Header]
- #include <boost/metaparse/always_c.hpp>
- [h1 Expression semantics]
- For any `c` character and `t` class the following are equivalent:
- always_c<c, t>
-
- always<lit_c<c>, t>
- [h1 Example]
- #include <boost/metaparse/always_c.hpp>
- #include <boost/metaparse/start.hpp>
- #include <boost/metaparse/string.hpp>
- #include <boost/metaparse/is_error.hpp>
- #include <boost/metaparse/get_result.hpp>
-
- #include <type_traits>
-
- using namespace boost::metaparse;
-
- using always13 = always_c<'x', std::integral_constant<int, 13>>;
-
- static_assert(
- !is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
- "always13 should accept x"
- );
-
- static_assert(
- std::is_same<
- get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type,
- std::integral_constant<int, 13>
- >::type::value,
- "the result of parsing should be the integral_constant type"
- );
-
- static_assert(
- is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value,
- "always13 should reject characters other than x"
- );
- [endsect]
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