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Сообщение [trick] Вывод типа в сообщении об ошибке в Visual Studio от 09.08.2014 18:09

Изменено 04.01.2018 22:29 Vain

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[trick] Вывод типа в сообщении об ошибке в Visual Studio

Не так давно столкнулся с ситуацией, когда буст генерит кучу непонятных ошибок и не понятно в чём проблема.
В моём случае это был variant + операции поиска и выборки из него.
Вот пример.

	Заголовки
	`#include <boost/variant.hpp> #include <boost/mpl/vector.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/find.hpp> #include <boost/type_traits.hpp> namespace mpl = boost::mpl;`

начальный пример

struct test_dummy
{
};

typedef boost::variant<int, long, test_dummy> variant_t0;
typedef mpl::vector<int, long, test_dummy> mpl_variant_t0;

template <typename V, typename T>
struct Find
{
  typedef typename mpl::at<V, mpl::int_<mpl::find<mpl_variant_t0, T>::type::pos::value> >::type type;
};

struct test : boost::static_visitor<>
{
  template <typename T>
  void operator()(const T & value) const
  {
    // на самом деле char здесь не виден как конкретный тип, это только для демонстрации проблемы
    typedef typename Find<mpl_variant_t0, char>::type found_t0;
    found_t0 i = 0; // как-то используем что имеем, здесь не важно как
  }
};

void main()
{
  variant_t0 variant_test1;
  // ... где-то в дебрях кода variant_test1 получает что-то или не получает, но мы не знаем что ...
  boost::apply_visitor(test(), variant_test1);
}

Под студией 2010 оно выдаст примерно следующее:

	ошибка компиляции (выводится 3 раза)
	error C2440: 'initializing' : cannot convert from 'int' to 'found_t0' No constructor could take the source type, or constructor overload resolution was ambiguous boost_1_55_0\boost/variant/variant.hpp(1048) : see reference to function template instantiation 'void test::operator ()<const T>(const T &) const' being compiled with [ T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(130) : see reference to function template instantiation 'void boost::detail::variant::invoke_visitor<Visitor>::internal_visit<const T>(T &,int)' being compiled with [ Visitor=const test, T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(173) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<Visitor,VoidPtrCV,T>(int,Visitor &,VoidPtrCV,T ,boost::mpl::true_)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(256) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke<Visitor,VoidPtrCV,T0,NoBackupFlag>(int,Visitor &,VoidPtrCV,T ,NoBackupFlag,int)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, T=T0 ] boost_1_55_0\boost/variant/variant.hpp(2367) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl<first_which,first_step,Visitor,VoidPtrCV,boost::variant<T0_,T1,T2>::has_fallback_type_>(const int,const int,Visitor &,VoidPtrCV,boost::mpl::false_,NoBackupFlag,Which ,step0 )' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T0_=int, T1=long, T2=test_dummy, NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, Which=first_which, step0=first_step ] boost_1_55_0\boost/variant/variant.hpp(2389) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor_impl<Visitor,const void>(int,int,Visitor &,VoidPtrCV)' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void * ] boost_1_55_0\boost/variant/variant.hpp(2411) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor<boost::detail::variant::invoke_visitor<Visitor>>(boost::detail::variant::invoke_visitor<Visitor> &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=const test ] boost_1_55_0\boost/variant/detail/apply_visitor_unary.hpp(76) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::apply_visitor<const Visitor>(Visitor &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=test ] test.cpp(117) : see reference to function template instantiation 'void boost::apply_visitor<test,const variant_t0>(const Visitor &,Visitable &)' being compiled with [ Visitor=test, Visitable=const variant_t0 ]

Ни намёка на то, что за тип такой found_t0.

В инете можно найти несколько способов показать тип. К примеру, способ с incomplete type error:

пример с incomplete type

#define INCOMPLETE_TYPE_LOOKUP_BY_ERROR(type_name)   static_cast< ::util::incomplete_type_lookup< type_name > >(0);

namespace util
{
  template <typename T>
  struct incomplete_type_lookup;
}

struct test : boost::static_visitor<>
{
  template <typename T>
  void operator()(const T & value) const
  {
    typedef typename mpl::at<mpl_variant_t0, mpl::int_<mpl::find<mpl_variant_t0, T>::type::pos::value> >::type found_t0;
    INCOMPLETE_TYPE_LOOKUP_BY_ERROR(found_t0);
    //found_t0 i = 0;
  }
};

Но это не всегда работает. Компилятор из Visual Studio 2010 показывает не внутренний тип, а внешний:

	почти такая же бесполезная ошибка, вид сбоку 3 раза
	error C2440: 'static_cast' : cannot convert from 'int' to 'util::incomplete_type_lookup<T>' with [ T=found_t0 ] Source or target has incomplete type boost_1_55_0\boost/variant/variant.hpp(1048) : see reference to function template instantiation 'void test::operator ()<const T>(const T &) const' being compiled with [ T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(130) : see reference to function template instantiation 'void boost::detail::variant::invoke_visitor<Visitor>::internal_visit<const T>(T &,int)' being compiled with [ Visitor=const test, T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(173) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<Visitor,VoidPtrCV,T>(int,Visitor &,VoidPtrCV,T ,boost::mpl::true_)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(256) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke<Visitor,VoidPtrCV,T0,NoBackupFlag>(int,Visitor &,VoidPtrCV,T ,NoBackupFlag,int)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, T=T0 ] boost_1_55_0\boost/variant/variant.hpp(2367) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl<first_which,first_step,Visitor,VoidPtrCV,boost::variant<T0_,T1,T2>::has_fallback_type_>(const int,const int,Visitor &,VoidPtrCV,boost::mpl::false_,NoBackupFlag,Which ,step0 )' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T0_=int, T1=long, T2=test_dummy, NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, Which=first_which, step0=first_step ] boost_1_55_0\boost/variant/variant.hpp(2389) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor_impl<Visitor,const void>(int,int,Visitor &,VoidPtrCV)' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void * ] boost_1_55_0\boost/variant/variant.hpp(2411) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor<boost::detail::variant::invoke_visitor<Visitor>>(boost::detail::variant::invoke_visitor<Visitor> &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=const test ] boost_1_55_0\boost/variant/detail/apply_visitor_unary.hpp(76) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::apply_visitor<const Visitor>(Visitor &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=test ] test.cpp(117) : see reference to function template instantiation 'void boost::apply_visitor<test,const variant_t0>(const Visitor &,Visitable &)' being compiled with [ Visitor=test, Visitable=const variant_t0 ]

Идея заключается в том, чтобы вызвать у неизвестного типа то, что у него быть не может, компилятор начнёт искать это что-то, раскрывая всё подряд, ну и раскроет и наш тип:

наборчик для вскрывания типов %)

// generates compilation error and shows real type name (and place of declaration in some cases) in an error message
#define UTIL_TYPE_LOOKUP_BY_ERROR(type_name)   (*(typename ::util::type_lookup< type_name >::type*)0).operator ,(*(::util::dummy*)0)

// the macro only for msvc compiler which has more useful error output if a scope class and a type can be separated from each other
#ifdef _MSC_VER

#define UTIL_TYPE_LOOKUP_BY_ERROR_CLASS(class_name, type_name)   (*(typename ::util::type_lookup< class_name >::type_name*)0).operator ,(*(::util::dummy*)0)

#else

#define UTIL_TYPE_LOOKUP_BY_ERROR_CLASS(class_name, type_name)    UTIL_TYPE_LOOKUP_BY_ERROR(class_name::type_name)

#endif

namespace util
{
  struct dummy;
  template <typename T>
  struct type_lookup
  {
    typedef T type;
  };
}

пример использования

struct test : boost::static_visitor<>
{
  template <typename T>
  void operator()(const T & value) const
  {
    typedef typename Find<mpl_variant_t0, char>::type found_t0;
    UTIL_TYPE_LOOKUP_BY_ERROR(found_t0);
    UTIL_TYPE_LOOKUP_BY_ERROR(mpl_variant_t0);
    UTIL_TYPE_LOOKUP_BY_ERROR(variant_t0);
    //found_t0 i = 0;
  }
};

	ошибки компиляции со вскрытыми типами
	error C2039: ',' : is not a member of 'boost::mpl::void_' boost_1_55_0\boost/mpl/void.hpp(29) : see declaration of 'boost::mpl::void_' boost_1_55_0\boost/variant/variant.hpp(1048) : see reference to function template instantiation 'void test::operator ()<const T>(const T &) const' being compiled with [ T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(130) : see reference to function template instantiation 'void boost::detail::variant::invoke_visitor<Visitor>::internal_visit<const T>(T &,int)' being compiled with [ Visitor=const test, T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(173) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke_impl<Visitor,VoidPtrCV,T>(int,Visitor &,VoidPtrCV,T ,boost::mpl::true_)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T=T0 ] boost_1_55_0\boost/variant/detail/visitation_impl.hpp(256) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl_invoke<Visitor,VoidPtrCV,T0,NoBackupFlag>(int,Visitor &,VoidPtrCV,T ,NoBackupFlag,int)' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, T=T0 ] boost_1_55_0\boost/variant/variant.hpp(2367) : see reference to function template instantiation 'void boost::detail::variant::visitation_impl<first_which,first_step,Visitor,VoidPtrCV,boost::variant<T0_,T1,T2>::has_fallback_type_>(const int,const int,Visitor &,VoidPtrCV,boost::mpl::false_,NoBackupFlag,Which ,step0 )' being compiled with [ Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void , T0_=int, T1=long, T2=test_dummy, NoBackupFlag=boost::variant<int,long,test_dummy>::has_fallback_type_, Which=first_which, step0=first_step ] boost_1_55_0\boost/variant/variant.hpp(2389) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor_impl<Visitor,const void>(int,int,Visitor &,VoidPtrCV)' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=boost::detail::variant::invoke_visitor<const test>, VoidPtrCV=const void * ] boost_1_55_0\boost/variant/variant.hpp(2411) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::internal_apply_visitor<boost::detail::variant::invoke_visitor<Visitor>>(boost::detail::variant::invoke_visitor<Visitor> &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=const test ] boost_1_55_0\boost/variant/detail/apply_visitor_unary.hpp(76) : see reference to function template instantiation 'void boost::variant<T0_,T1,T2>::apply_visitor<const Visitor>(Visitor &) const' being compiled with [ T0_=int, T1=long, T2=test_dummy, Visitor=test ] test.cpp(119) : see reference to function template instantiation 'void boost::apply_visitor<test,const variant_t0>(const Visitor &,Visitable &)' being compiled with [ Visitor=test, Visitable=const variant_t0 ] error C2039: ',' : is not a member of 'boost::mpl::vector<T0,T1,T2>' with [ T0=int, T1=long, T2=test_dummy ] error C2039: ',' : is not a member of 'boost::variant<T0_,T1,T2>' with [ T0_=int, T1=long, T2=test_dummy ]