to_array from LFTS with updates

Introduction

This paper proposes to adopt to_array from Library Fundamentals TS along with an rvalue-reference overload based on the feedback from the users.

auto lv = to_array("foo");               // array<char, 4>
auto rv = to_array<int>({ 'a', 'b' })    // array<int, 2>

Discussion

In C++17 we introduced deduction guide on std::array:

auto a = array{ 1, 2, 3 };

But it is impossible to create an array<char, 4> from array{ "foo" } because otherwise, one won’t be able to explain why array{ "foo", "bar" } creates array<char const*, 2>. So one overload set, despite whether it’s std::array deduction guide or make_array from LFTSv2, is not enough to serve the purpose of “creating a std::array from elements” and “creating a std::array by copying from a built-in array” at the same time. That is why^[1] we introduced to_array in LFTSv2:

auto a1 = to_array("foo");          // array<char, 4>
auto a2 = array{ "foo" };           // array<char const*, 1>

For the same reason, to_array should be adopted into the IS.

However, the original version of to_array only takes an lvalue reference-to-array, and users reported^[2] some issues regarding this design. The first issue is that it becomes very hackish to deduce only the array bound from a braced-init-list:

auto x = to_array<int const>({ 2, 4 });     // array<int, 2>

The language treats braced-init-list as a prvalue of an array type when deducing against reference-to-array. The code above adds const to coin a reference that can bind to such a prvalue, and leaves the const to be stripped later. Anyhow, the code isn’t doing what it says.

The second issue is that it does not support move-only elements, such as unique_ptr, no matter how you hack.

So we should add the rvalue-reference overload to fill this hole regarding type system.

Impact on the Standard

The following table illustrates the uses and the corresponding overload resolution after adopting the proposed addition. Given

int a[3] = {};
struct A { int a; double b; };

The last entry is a valuable case to study. Compound literals are lvalues in the C standards, but C does not have rvalue objects of compound types anyway. Both Clang and GCC implement C99 compound literals as extensions in C++ and treat them as prvalues. Let’s take a closer look at this feature,

(int[]){3, 4}

It is an array literal formed by a C-style cast notation (like (char)) for an array type followed by an initializer list. In the same way, we can interpret to_array as

to_array<int>({3, 4})

a std::array literal formed by a C++ style cast notation (like static_cast<char>(...)) for the array element type surrounding an initializer list.

An interesting consequence of this proposal is that we no longer need make_array:

In the future, we may have array<long>{ 1, 2, 3 } to replace this specific use of to_array, but that will not defeat the primary purpose of to_array – a facility to create std::array by copying or moving from a built-in array.

Supplying the bound with to_array<T, N>({ ... }) is more strict compared to array<T, N>{ ... }, because brace elision in aggregate initialization can sometimes give unexpected outcomes:

using E = complex<double>;

// Nicol's quiz time
auto a = array<E, 10>{{1, 2}, {3, 4}};
auto b = array<E, 10>{{{1, 2}, {3, 4}}};
auto c = array<E, 10>{{1, 2}};

Implementation

Godbolted: https://godbolt.org/g/GqbygN

Wording

This wording is relative to N4762.

New section 21.3.7.6 [array.creation] (between [array.zero] and [array.tuple], which was 21.3.7.6):

21.3.7.6 Array creation functions [array.creation]

template <class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);
template <class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

Returns: For all $i$, $0 \le i <$ N, an array<remove_cv_t<T>, N> initialized with { a[$i$]... } for the first form, or { std::move(a[$i$])... } for the second form.

Add those signatures to 21.3.2 [array.syn]:

namespace std {
  // 21.3.7, class template array
  template<class T, size_t N> struct array;
  
  …
  template<class T, size_t N>
    void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));

  // 21.3.7.6, Array creation functions
  template <class T, size_t N>
    constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);
  template <class T, size_t N>
    constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);
  …
}

Acknowledgements

Thank Peter Sommerlad for the inputs on this paper.

References