Decltype of struct data members, using structured binding

Question

I have a template taking a type that is structurally-bindable to two aliases (could be a tuple but also a struct). I need the type of those two variables the aliases point to.

template <typename T>
T obj;
// type of a/b in auto&& [a, b] = obj ?

I need to know the type before I actually use the structured binding:

template <typename S>
void fn(ranges::any_view<S> range_of_tuple_or_struct_or_pair) {

    last_from = std::numeric_limits<?????>::max();

    // ????? should be the type of from (or to, they should be the same types) of: 
    //     auto&& [from, to] = <range element>

    for (auto&& [from, to] : range_of_tuple_or_struct_or_pair) {
         if (from != last_from) {
             ...;
             last_from = from;
         }
    }
}

score 4 · Accepted Answer · answered Sep 17 '19 at 09:02

I think you can do something like this:

#include <type_traits>

template <typename T, typename U>
struct Identity {
  using type_first = T;
  using type_second = U;
};

template <typename T>
constexpr auto GetTypes(const T& obj) noexcept {
  const auto& [x, y] = obj;
  return Identity<std::remove_cv_t<decltype(x)>, 
                  std::remove_cv_t<decltype(y)>>{};
}

template <typename T>
void foo(T obj) {
  // T is structurally-bindable with two fields
  constexpr auto Types = GetTypes(obj);
  using t1 = typename decltype(Types)::type_first;
  using t2 = typename decltype(Types)::type_second;

  // t1 is the first type
  // t2 is the second type
}

Complete Example Here

Note that with constexpr there is no overhead, even without enabling optimizations.

Thanks. If I don't have the object at compile time, I should construct it and do GetTypes(T())? — Adam, Sep 17 '19 at 09:17
@Adam Yes, In order to bind the values you need an instance of the object — BiagioF, Sep 17 '19 at 09:42

P. Dmitry · Answer 2 · 2019-09-17T09:05:14.583

2

Probably you can add one extra structured binding before last_from declaration and take from type from it. I.e.

const auto& [first_from, first_to] = *range_of_tuple_or_struct_or_pair.begin();
last_from = std::numeric_limits<std::decay_t<decltype(first_from)>>::max();

Not quite cool, but maybe will work

edited Sep 17 '19 at 09:05

answered Sep 17 '19 at 08:57

P. Dmitry

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Decltype of struct data members, using structured binding

2 Answers2