Synopsis

template <class C>
struct is_error;

This is a lazy template metafunction that supports currying.

Description

Determines if C is a parsing error or not. Returns a boxed boolean value.

Header

#include <boost/metaparse/is_error.hpp>

Expression semantics

For any e parsing error is_error<c>::type is a wrapped compile-time true value, for any other c class is_error<c>::type is a wrapped compile-time false value.

Example

#include <boost/metaparse/is_error.hpp>
#include <boost/metaparse/accept.hpp>
#include <boost/metaparse/reject.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/define_error.hpp>

#include <type_traits>

using namespace boost::metaparse;

BOOST_METAPARSE_DEFINE_ERROR(sample_error, "Sample error message");

struct returns_reject
{
  typedef reject<sample_error, start> type;
};

static_assert(
  !is_error<
    accept<
      std::integral_constant<int, 13>,
      BOOST_METAPARSE_STRING("foo"),
      start
    >
  >::type::value,
  "an accept should not be an error"
);

static_assert(
  is_error<reject<sample_error, start>>::type::value,
  "an reject should be an error"
);

static_assert(
  is_error<>::type::apply<reject<sample_error, start>>::type::value,
  "it should support currying"
);

static_assert(
  is_error<returns_reject>::type::value,
  "it should support lazy evaluation"
);