如何将`std :: array`用于形式为`template< typename>班级? [英] How can I use `std::array` for a template parameter of the form `template<typename> class`?

问题描述

请考虑以下tree类

template<typename T, template<typename> class Tuple>
class tree
{
private:
    T m_value;
    Tuple<tree> m_children;
};

template<typename T, std::size_t N>
using static_tree = tree<T, std::array<T, N>>;

这是没有明确定义的. std::array<T, N>不是Tuple的合适模板参数.我认为static_tree的意图很明确.我们可以做类似的事情

which is not well-defined. std::array<T, N> is not a suitable template parameter for Tuple. I assume the intend of static_tree is clear. We could do something like

template<std::size_t N>
struct helper
{
    template<typename T>
    using type = std::array<T, N>;
};

template<typename T, std::size_t N>
using static_tree = tree<T, helper<N>::template type>;

如果没有helper类，还有其他选择吗?

Is there any other option without the helper class?

推荐答案

我建议这是规则，而不是让helper函数成为std::array的例外.代替采用模板template参数，而采用元函数类参数.当所有内容都是类型时(避免模板模板和非类型参数)，模板元编程会容易得多:

Rather than having the helper function be an exception for std::array, I'd propose that be the rule. Instead of taking a template template parameter, take a metafunction class parameter. Template metaprogramming is a lot easier when everything everywhere is a type (avoiding template templates and non-type arguments):

template<typename T, typename TupleMfn>
class tree
{
private:
    using Tuple = TupleMfn::template apply<tree>;

    T m_value;
    Tuple m_children;
};

具有:

template <size_t N>
struct array_builder {
    template <class T>
    using apply = std::array<T, N>;
};

template <typename T, size_t N>
using static_tree = tree<T, array_builder<N>>;

这也将使它更易于与其他类型的容器一起使用，因为我们可以为模板模板做一个包装，从而为我们提供一个元功能:

This will make it easier to use with other kinds of containers as well, since we can make a wrapper for template templates that gives us back a metafunction:

template <template <typename...> class X>
struct as_metafunction {
    template <class... Args>
    using apply = X<Args...>;
}; 

template <typename T>
using vector_tree = tree<T, as_metafunction<std::vector>>;

---

如果您特别讨厌，可以提供std::array的元编程友好版本:

---

If you're feeling especially feisty, you can provide a metaprogramming-friendly version of std::array:

template <class T, class N>
struct meta_array : std::array<T, N::value> // obviously I am terrible at naming things
{ };

template <size_t N>
using size_t_ = std::integral_constant<size_t, N>;

然后为tree提供占位符args:

And then provide placeholder args for tree to apply:

template <class T, size_t N>
using static_tree = tree<T, meta_array<_, size_t_<N>>>;

template <class T>
using vector_tree = tree<T, std::vector<_>>;

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