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[std-proposals] Making tuple an aggregate

From: Phil Endecott <std_proposals_list_at_[hidden]>
Date: Tue, 07 Jun 2022 17:54:57 +0100
Dear Experts,

Today I have been frustrated by std::tuple not being an aggregate.
I have some non-moveable types and it is difficult to use them
with tuples. That's in contrast to std::pair, which I can use
with piecewise_construct, and with std::array and plain structs,
which I can use thanks to copy elision:

struct NonMoveable
  NonMoveable(NonMoveable&&) = delete;
  NonMoveable(const NonMoveable&) = delete;

  NonMoveable(int a, int b);

struct S
  NonMoveable a;
  NonMoveable b;

void f()
  std::pair<NonMoveable,NonMoveable> p{ std::piecewise_construct,
                                        std::make_tuple(3,4) };

  std::array<NonMoveable,2> a{{ {1,2}, {3,4} }};

  S s{ {1,2}, {3,4} };

  std::tuple<NonMoveable,NonMoveable> t .....????.....

I ended up with the following, which I call "structuple" because it
is a hybrid struct and tuple:

template <typename... Ts>
struct structuple

template <typename T0>
struct structuple<T0>
  T0 f0;

template <typename T0, typename T1>
struct structuple<T0,T1>
  T0 f0;
  T1 f1;

template <typename T0, typename T1, typename T2>
struct structuple<T0,T1,T2>
  T0 f0;
  T1 f1;
  T2 f2;

So, what can we do to make std::tuple more like a struct?

One obvious "fix" would be to add std::piecewise_construct support
to std::tuple.

But I was wondering if there is another approach that would allow
tuple to be more like a struct, if we added something to the core
language. Pseudo-code:

template <typename... TYPES>
struct tuple {
  TYPES fields;...

I note that we can use pack expansion with ",". Why not also with ";" ?

Any thoughts?


Received on 2022-06-07 16:54:59