always.qbk
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[#always]
[section always]
[h1 Synopsis]
template <class P, class T>
struct always;
This is a [link parser_combinator parser combinator].
[table Arguments
[[Name] [Type]]
[[`P`] [[link parser parser]]]
[[`T`] [[link metaprogramming_value template metaprogramming value]]]
]
[h1 Description]
It accepts an input if and only if `P` accepts it, but the result of parsing
will be `T` instead of the result `P` returned.
[h1 Header]
#include <boost/metaparse/always.hpp>
[h1 Expression semantics]
For any `p` parser and `t` class the following are equivalent:
always<p, t>
last_of<p, return_<t>>
[h1 Example]
#include <boost/metaparse/always.hpp>
#include <boost/metaparse/lit_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<lit_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]