Synopsis

template <char C, class T>
struct always_c;

This is a parser combinator.

Description

It accepts inputs beginning with the C character. It consumes that character and the result of parsing is T. Other inputs as rejected.

Header

#include <boost/metaparse/always_c.hpp>

Expression semantics

For any c character and t class the following are equivalent:

always_c<c, t>

always<lit_c<c>, t>

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"
);