使用hana :: transform在C++ 14中转换元组内部的类型

Question

我正在尝试使用Boost hana::transform来改变一个类型hana::tuple.举个例子,说我有

constexpr auto some_tuple = hana::tuple_t<int, char *, bool>;

而我想制作

constexpr auto transformed_tuple = hana::tuple_t<std::vector<int>,
                                                 std::vector<char *>, 
                                                 std::vector<bool>>;

尝试1

解决方案对我来说似乎很容易:使用hana::transform并使应用函数返回hana::type_c<std::vector<decltype(T)::type>>.但是,我无法做到这一点:

constexpr auto transformed_tuple = hana::transform(some_tuple, [](auto T) {
    using inner_type = typename decltype(T)::type;
    return hana::type_c<std::vector<inner_type>>;
});

这有一个问题,lambda表达式不是constexpr- 我想留在C++ 14,即lambdas不能constexpr.

尝试2

我的下一个想法:如果我把它包裹hana::transform成一个decltype,然后使用hana::type_c它怎么办？这样,lambda永远不需要被评估(只需要推导它的返回类型),并且constexprness应该无关紧要:

constexpr auto transformed_tuple = 
    hana::type_c<decltype(hana::transform(some_tuple, [](auto T) {
        using inner_type = typename decltype(T)::type;
        return hana::type_c<std::vector<inner_type>>;
    }))>;

但是,现在我遇到的问题是lambda表达式可能不会出现在"未评估的上下文"中.

我的方法完全错了吗？我应该使用别的东西hana::transform吗？

谢谢你的帮助.

编辑:

示例代码:

#include <boost/hana.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;

#include <vector>

constexpr auto some_tuple = hana::tuple_t<int, char *, bool>;

/** What I want:
 *
 *   constexpr auto transformed_tuple 
 *       = hana::tuple_t<std::vector<int>,
 *           std::vector<char *>, 
 *           std::vector<bool>>;
**/

#if ATTEMPT1
    constexpr auto transformed_tuple = hana::transform(some_tuple, [](auto T) {
        using inner_type = typename decltype(T)::type;
        return hana::type_c<std::vector<inner_type>>;
    });

#elif ATTEMPT2
    constexpr auto transformed_tuple = hana::type_c<decltype(hana::transform(some_tuple, [](auto T) {
        using inner_type = typename decltype(T)::type;
        return hana::type_c<std::vector<inner_type>>;
    }))>;

#endif

Answer 1

Boost.Hana hana::template_将类型应用于返回类型的模板.

#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/transform.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
#include <vector>

namespace hana = boost::hana;


int main() {
    constexpr auto some_tuple = hana::tuple_t<int, char *, bool>;
    constexpr auto expected_tuple = hana::tuple_t<std::vector<int>,
                                                  std::vector<char*>,
                                                  std::vector<bool>>;

    constexpr auto transformed_tuple = hana::transform(some_tuple, hana::template_<std::vector>);

    BOOST_HANA_CONSTANT_CHECK(transformed_tuple == expected_tuple);
}