返回C++17可变模板的可变聚合（结构）和语法“构造扣除指南”

Question

template<typename T1,typename T2,typename T3>
struct many {
  T1 a;
  T2 b;
  T3 c;
};

// guide:
template<class T1, class T2, class T3>
many(T1, T2, T3) -> many<T1, T2, T3>;

auto f(){ return many{string(),5.7, unmovable()}; }; 

int main(){
   auto [x,y,z] = f();
}

是否可以创建many的可变版本，该版本可以使用任意数量的参数？

更新：在many的模板版本中，是否允许以下指南语法？

template<typename Args...>    
many(Args...) -> many<Args...>;

Answer 1

在C++17中，聚合初始化将能够初始化公共基类。所以您可以使用继承+包扩展来构建这样的类。要使其与结构化绑定一起工作，您必须公开元组接口:specializestd::tuple_size和std::tuple_element，并为您的类提供get函数：

//Headers used by "many" class implementation
#include <utility>
#include <tuple>

namespace rw {
    namespace detail {

    template <size_t index, typename T>
    struct many_holder
    { T value; };

    template <typename idx_seq, typename... Types>
    struct many_impl;

    template <size_t... Indices, typename... Types>
    struct many_impl<std::index_sequence<Indices...>, Types...>: many_holder<Indices, Types>...
    {};

    }

template <typename... Types>
struct many: detail::many_impl<typename std::make_index_sequence<sizeof...(Types)>, Types...>
{};

template<size_t N, typename... Types> 
auto get(const rw::many<Types...>& data) -> const std::tuple_element_t<N, rw::many<Types...>>&
{
    const rw::detail::many_holder<N, std::tuple_element_t<N, rw::many<Types...>>>& holder = data;
    return holder.value;
}

}

namespace std {
    template <typename... Types>
    struct tuple_size<rw::many<Types...>> : std::integral_constant<std::size_t, sizeof...(Types)> 
    {};

    template< std::size_t N, class... Types >
    struct tuple_element<N, rw::many<Types...> >
    { using type = typename tuple_element<N, std::tuple<Types...>>::type; };
}

//Headers used for testing
#include <iostream>
#include <string>

int main()
{
    rw::many<int, std::string, int> x = {42, "Hello", 11};
    std::cout << std::tuple_size<decltype(x)>() << '\n' << rw::get<1>(x);
}

演示（现在只适用于CLANG3.9):http://melpon.org/wandbox/permlink/9nbqkcbouurfvypt

备注：

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