c++ - 用于默认模板参数的参数包

Question

我想做这样的事情:

template<int... myints>
struct A
{}

template<int... myints, class traits = A<myints...>>
struct B
{}

我知道参数包应该只出现在最后，但我记得有一些异常(exception)。您认为是否有办法使此代码(或类似代码)起作用？

谢谢

Answer 1

一种方式:

• 将类型包装成元组。

• 使用特化将类型列表转换为类型

#include <tuple>
#include <utility>

template<int...myints> struct A {};

namespace detail {
    template<class Sequence>
    struct make_A_from;

    template<class T, T...ts>
    struct make_A_from<std::integer_sequence<T, ts...>> {
        using type = A<ts...>;
    };
}
template<class Sequence> using make_A_from = typename detail::make_A_from<Sequence>::type;

template<class Sequence, class TraitsTuple = make_A_from<Sequence> >
struct B;

template<typename Int, Int...myints, class Traits> 
struct B<std::integer_sequence<Int, myints...>, Traits>
{
    using ints_tuple = std::tuple<std::integral_constant<Int, myints>...>;
    using traits_type = Traits;
};

namespace detail {
    template<typename Int, Int...is>
    struct make_B
    {
        using type = B<std::integer_sequence<Int, is...>>;
    };
}

template<int...is> using make_B = B<std::integer_sequence<int, is...>>;

int main()
{
    make_B<1,3,4,5> b;
    B<std::integer_sequence<int, 5,6,7,8>> b2;
    B<std::integer_sequence<int, 5,6,7,8>, class CustomTrait> b3;
}