Synopsis

template <class T, class NotErrorCase>
struct unless_error;

This is a lazy template metafunction.

Description

Checks if T is a parsing error or not. When it is, the result is T. When it is not, the result is NotErrorCase.

Header

#include <boost/metaparse/unless_error.hpp>

Expression semantics

For any t and c classes the following are equivalent:

unless_error<t, c>

boost::mpl::if_<is_error<t::type>::type, t, c>

Example

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

#include <type_traits>

using namespace boost::metaparse;

BOOST_METAPARSE_DEFINE_ERROR(sample_error, "Sample error message");

using accept1 =
  accept<std::integral_constant<int, 11>, BOOST_METAPARSE_STRING("foo"), start>;

using accept2 =
  accept<std::integral_constant<int, 13>, BOOST_METAPARSE_STRING("bar"), start>;

using reject1 = reject<sample_error, start>;

struct returns_accept1 { using type = accept1; };
struct returns_accept2 { using type = accept2; };

static_assert(
  std::is_same<accept2, unless_error<accept1, accept2>::type>::type::value,
  "it returns the second argument when the first argument is an accept"
);

static_assert(
  std::is_same<reject1, unless_error<reject1, accept2>::type>::type::value,
  "it returns the first argument when that is a reject"
);

static_assert(
  std::is_same<
    accept2,
    unless_error<returns_accept1, returns_accept2>::type
  >::type::value,
  "it supports lazy evaluation"
);