将constexpr结构转换为运行时结构

Question

我正在尝试使用模板类(此处Foo),其基本类型如下:

• hana::tuple<hana::pair<hana::type<int>, Runtime>>与Runtime一个不可思议的课程constepxr.

但是类型可以用几种方式构造,这就是我使用的原因:

• hana::tuple<hana::pair<hana::type<int>, hana::type<Runtime>>> 在编译时完成工作.

所以问题基本上是如何从第一个元组转换为第二个类型.我想知道是否有什么hana可以帮助我.或者甚至更好,关于那种"转换"的一些提示.

namespace hana = boost::hana;
using namespace hana::literals;

struct Runtime { std::vector<int> data; };

template < typename T >
struct Foo {
  T data;
};

constexpr decltype(auto)  convertMap(auto storageMap) {

return hana::make_type(hana::transform(
    storageMap,
    [] (auto pair) {
      return hana::make_pair(
        hana::first(pair),
        typename decltype(hana::typeid_(hana::second(pair)))::type {});
    }));
}

int main() {

  constexpr auto map = hana::make_tuple(
      hana::make_pair(hana::type_c<int>, hana::type_c<Runtime>)
      );

  constexpr auto result = convertMap(map);
  static_assert(result ==
     hana::type_c<hana::tuple<hana::pair<hana::type<int>, Runtime>>>);

  Foo<typename decltype(result)::type> test;
}

正如你所看到的,我尝试了一些convertMap带有hana::transform和的c ++ 1z lambdas,但第二个元组不能constexpr,所以我无法通过它来hana::make_type希望得到一个hana::type_c.

test.cpp: In function ‘int main()’:
test.cpp:70:41: error: ‘constexpr decltype(auto) convertMap(auto:27) [with auto:27 = boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, boost::hana::type_impl<Runtime>::_> >]’ called in a constant expression
   constexpr auto result = convertMap(map);
                                         ^
test.cpp:53:27: note: ‘constexpr decltype(auto) convertMap(auto:27) [with auto:27 = boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, boost::hana::type_impl<Runtime>::_> >]’ is not usable as a constexpr function because:
 constexpr decltype(auto)  convertMap(auto storageMap) {
                           ^~~~~~~~~~
test.cpp:53:27: error: temporary of non-literal type ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ in a constant expression
In file included from /usr/include/boost/hana/detail/struct_macros.hpp:29:0,
                 from /usr/include/boost/hana/adapt_adt.hpp:15,
                 from lib/hana/include/boost/hana.hpp:59,
                 from test.cpp:1:
/usr/include/boost/hana/tuple.hpp:68:12: note: ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ is not literal because:
     struct tuple
            ^~~~~
/usr/include/boost/hana/tuple.hpp:68:12: note:   ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ has a non-trivial destructor

Answer 1

所有你关心的是一个类型-所以你可以隐藏在非类型的计算constexpr实现的功能,事后与调用它 decltype(...){}以"力constexpr":

template <typename T>
decltype(auto) convertMapImpl(T storageMap)
{
    return hana::make_type(hana::transform(storageMap, [](auto pair) {
        return hana::make_pair(hana::first(pair),
            typename decltype(hana::typeid_(hana::second(pair)))::type{});
    }));
}

template <typename T>
constexpr decltype(auto) convertMap(T storageMap)
{
    return decltype(convertMapImpl(storageMap)){};
}

直播wandbox示例

另请注意,auto在函数签名中使用是gcc扩展名 - 您应该使用模板参数来代替标准.