c++ - 将 constexpr 结构转换为运行时结构

Question

我正在尝试使用模板类(此处为 Foo )，其基本类型如下:

• hana::tuple<hana::pair<hana::type<int>, Runtime>>与 Runtime一个显然不能是 constepxr 的类.

但是类型可以用多种方式构造，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类型转换为第二个元组类型。不知hana里面有没有东西那可以帮助我。或者更好的是，关于这种“转换”的一些技巧。

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;
}

如您所见，我尝试了一些 c++1z convertMap与 hana::transform和 lambdas , 但第二个元组不能是 constexpr , 所以我不能把它传递给 hana::make_type希望得到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

您只关心类型 - 因此您可以在非 constexpr 实现函数中隐藏类型的计算，然后使用 decltype(...){ 到 “force 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)){};
}

live wandbox example

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另请注意，在函数签名中使用 auto 是 gcc 扩展 - 您应该使用模板参数来代替标准兼容。