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>>
Runtime
一个无法constepxr
的类。
但是该类型可以通过多种方式构造,这就是我使用的原因:
hana::tuple<hana::pair<hana::type<int>, hana::type<Runtime>>>
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
。我想知道hana
是否有可以帮助我的东西。或者更好的是,关于这种"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;
}
如您所见,我尝试了一些带有hana::transform
和lambdas
的c ++ 1zconvertMap
,但是第二个元组无法constexpr
,所以我无法将其传递给希望获得hana::type_c
的hana::make_type
。
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
你关心的只是一个类型 - 因此你可以将类型的计算隐藏在一个非constexpr
实现函数中,然后用decltype(...){}
调用它来"强制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)){};
}
活魔杖盒示例
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