如何访问C11中可能不存在的类型别名？

Question

我想这样写：

template <class T>
using MyTypeOrTupple = typename std::conditional<has_member_type_MyType<T>::value,
                                                 typename T::MyType,
                                                 std::tuple<> >::type;

我使用https://en.wikibooks.org/wiki/More_C++_惯用语/成员检测器

然而，GCC（4.8.4）仍然抱怨在T中未定义MyType时使用T:：MyType。有办法解决这个问题吗？

Answer 1

我认为这需要2级间接：

template<typename ...>
struct void_type
{
    using type = void;
};

template<typename ...T>
using void_t = typename void_type<T...>::type;

#define HAS_TYPE(NAME) \
template<typename, typename = void> \
struct has_type_##NAME: std::false_type \
{}; \
template<typename T> \
struct has_type_##NAME<T, void_t<typename T::NAME>>: std::true_type \
{}

HAS_TYPE(MyType);

template<typename T, bool = has_type_MyType<T>::value>
struct MyTypeOrTupple_impl;

template<typename T>
struct MyTypeOrTupple_impl<T, true>
{ using type = typename T::MyType; };

template<typename T>
struct MyTypeOrTupple_impl<T, false>
{ using type = std::tuple<>; };

template<typename T> using MyTypeOrTupple = typename MyTypeOrTupple_impl<T>::type;

演示

Answer 2

template <typename...>
struct voider { using type = void; };

template <typename... Ts>
using void_t = typename voider<Ts...>::type;

template <typename, typename = void_t<>>
struct MyTypeOrTupple { using type = std::tuple<>; };

template <typename T> 
struct MyTypeOrTupple<T, void_t<typename T::MyType>> { using type = typename T::MyType; };

template <typename T>
using MyTypeOrTupple_t = typename MyTypeOrTupple<T>::type;

演示

Answer 3

不要使用维基百科上的疯狂东西。

template <class T>
using MyType_t = typename T::MyType;

template<class T> 
using MyTypeOrTuple = detected_or_t<std::tuple<>, MyType_t, T>;

其中，detected_或_t是来自library fundamentals TS v2的std:：experimental:：detected_或_t，可按如下方式实现：

namespace detail {

    template<class...> struct voidify { using type = void; };
    template<class...Ts> using void_t = typename voidify<Ts...>::type;

    template <class Default, class AlwaysVoid,
              template<class...> class Op, class... Args>
    struct detector {
      using value_t = std::false_type;
      using type = Default;
    };

    template <class Default, template<class...> class Op, class... Args>
    struct detector<Default, void_t<Op<Args...>>, Op, Args...> {
      using value_t = std::true_type;
      using type = Op<Args...>;
    };

} // namespace detail

template <class Default, template<class...> class Op, class... Args>
using detected_or = detail::detector<Default, void, Op, Args...>;

template< class Default, template<class...> class Op, class... Args >
using detected_or_t = typename detected_or<Default, Op, Args...>::type;