Orient - 3 months ago 30

C++ Question

Is it possible to determine how many variable names should I to specify in square brackets using structured bindings syntax to match the number of data members of a plain right hand side

`struct`

I want to make a part of generic library, which uses structured bindings to decompose arbitrary classes into its constituents. At the moment there is no variadic version of structured bindings (and, I think, cannot be for current syntax proposed), but my first thought is to make a set of overloadings of some function

`decompose()`

`struct`

`decompose()`

`struct`

`constexpr if`

`sizeof...`

`auto [a, b, c]`

`decltype`

Surely I want to have builtin operator (with syntax like

`sizeof[] S`

`sizeof[](S)`

`class S`

`template< typename type, typename = void >`

struct sizeof_struct

{

};

template< typename type >

struct sizeof_struct< type, std::void_t< decltype([] { auto && [p1] = std::declval< type >(); void(p1); }) > >

: std::integral_constant< std::size_t, 1 >

{

};

template< typename type >

struct sizeof_struct< type, std::void_t< decltype([] { auto && [p1, p2] = std::declval< type >(); void(p1); void(p2); }) > >

: std::integral_constant< std::size_t, 2 >

{

};

... etc up to some reasonable arity

Maybe

`constexpr`

Will it be possible with coming

Answer

```
struct two_elements {
int x;
double y;
};
struct five_elements {
std::string one;
std::unique_ptr<int> two;
int * three;
char four;
std::array<two_elements, 10> five;
};
struct anything {
template<class T> operator T()const;
};
namespace details {
template<class T, class Is, class=void>
struct can_construct_with_N:std::false_type {};
template<class T, std::size_t...Is>
struct can_construct_with_N<T, std::index_sequence<Is...>, std::void_t< decltype(T{(void(Is),anything{})...}) >>:
std::true_type
{};
}
template<class T, std::size_t N>
using can_construct_with_N=details::can_construct_with_N<T, std::make_index_sequence<N>>;
namespace details {
template<std::size_t Min, std::size_t Range, template<std::size_t N>class target>
struct maximize:
std::conditional_t<
maximize<Min, Range/2, target>{} == (Min+Range/2)-1,
maximize<Min+Range/2, (Range+1)/2, target>,
maximize<Min, Range/2, target>
>
{};
template<std::size_t Min, template<std::size_t N>class target>
struct maximize<Min, 1, target>:
std::conditional_t<
target<Min>{},
std::integral_constant<std::size_t,Min>,
std::integral_constant<std::size_t,Min-1>
>
{};
template<std::size_t Min, template<std::size_t N>class target>
struct maximize<Min, 0, target>:
std::integral_constant<std::size_t,Min-1>
{};
template<class T>
struct construct_searcher {
template<std::size_t N>
using result = ::can_construct_with_N<T, N>;
};
}
template<class T, std::size_t Cap=20>
using construct_airity = details::maximize< 0, Cap, details::construct_searcher<T>::template result >;
```

This does a binary search for the longest construction airity of `T`

from 0 to 20. 20 is a constant, you can increase it as you will, at compile-time and memory cost.

If the data in your struct cannot be constructed from an rvalue of its own type, it won't work in C++14, but I believe guanteed elision occurs in C++17 here (!)

Turning this into structured bindings requires more than a bit of a pile of manual code. But once you have, you should be able to ask questions like "what is the 3rd type of this `struct`

" and the like.

If a `struct`

can be decomposed into structured bindings without the `tuple_size`

stuff being done, the airity of it determines how many variables it needs.

Unfortunetally `std::tuple_size`

is not SFINAE friendly even in C++17. But, types that use the `tuple_size`

part also need to ADL-enable `std::get`

.

Create a namespace with a `failure_tag get<std::size_t>(Ts const&...)`

that `using std::get`

. Use that to detect if they have overridden `get<0>`

on the type (`!std::is_same< get_type<T,0>, failure_tag >{}`

), and if so go down the `tuple_element`

path to determine airity. Stuff the resulting elements into a `std::tuple`

of `decltype(get<Is>(x))`

and return it.

If that fails, use the above `construct_airity`

, and use that to figure out how to use structured bindings on the type. I'd probably then send that off into a `std::tie`

, for uniformity.

We now have `tuple_it`

which takes anything structured-binding-like and converts it to a tuple of references or values.
Now both paths have converged, and your generic code is easier!