CODEWITHSUNDEEP
c++templates
motam79
motam79

Reputation: 3824

In C++ template arguments, is it possible to specify the type and number of occurrence of that type?

I have a variadic template class in C++ (similar to std::tuple). When instantiating the class, I need to type a lot of types, e.g.

MyClass<int, int, double, double, double> my_obj;

This approach works when the number of types is small. However, say if I have 10 ints followed by 20 doubles, typing it would be cumbersome and error-prone.

Is there is mechanism in C++, to specify types followed by number of occurrence in the template <> argument? Something like this:

MyClass<some_magic(int,2), some_magic(double, 3)> my_obj;

Upvotes: 0

Views: 111

Answers (1)

Jans
Jans

Reputation: 11250

This will make it:

namespace detail{
    template <class T, auto> using always_t = T;

    template <class T, std::size_t... Idx>
    auto repeat_impl(std::index_sequence<Idx...>) -> std::tuple<always_t<T, Idx>...>;

    template <class T, class...>
    struct pack
    {
        using type = T;
    };
    template <class... T, class... R, class... Tuple>
    struct pack<std::tuple<T...>, std::tuple<R...>, Tuple...>
        : pack<std::tuple<T..., R...>, Tuple...>
    { };

    template <class> struct tuple_to_class;
    template <class... T> struct tuple_to_class<std::tuple<T...>>
    {
        using type = MyClass<T...>;
    };
}

template <class T, std::size_t N>
using repeat_t = decltype(detail::repeat_impl<T>(std::make_index_sequence<N>{}));

template <class... Repeats>
using ToMyClass = typename detail::tuple_to_class<typename detail::pack<Repeats...>::type>::type;

using C = MyClass<int, int, double, double, double>;

static_assert(std::is_same_v<C,
              ToMyClass<repeat_t<int, 2>, repeat_t<double, 3>>
              >);

Live demo

Upvotes: 3

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