CODEWITHSUNDEEP
c++arrays
Daniel Langr
Daniel Langr

Reputation: 23527

Current status of std::make_array

What is the current status of std::make_array function proposed here? I cannot find any information about its potential acceptance. According to cppreference.com, it's in the std::experimental namespace. It's not mentioned at all on C++ compiler support nor on Wikipedia-C++17, Wikipedia-C++20, and C++17 Standard draft.

Upvotes: 18

Views: 9013

Answers (5)

Elliott
Elliott

Reputation: 2623

From C++20 we can use std::to_array, like so:

auto arr = std::to_array<CustomDataType>({a, b, c});

So long as a, b, c are each convertible to CustomDataType.

Upvotes: 1

einpoklum
einpoklum

Reputation: 131960

As @DeiDei writes, C++17 includes template argument deduction for classes, so you can now write:

std::pair p (foo, bar);
std::array arr = { 1, 2, 3, 4, 5 };

and so on. But there are some (somewhat subtle) remaining use cases where make_pair or make_array can be useful, and you can read about them in: Usefulness of std::make_pair and std::make_tuple in C++1z

@Ruslan correctly notes in a comment that the above is mostly useful when the type of the elements is "obvious" to the compiler, or made explicit for each element. If you want to explicitly construct elements of some type, the above looks like;

std::array arr = { MyType{1,2}, MyType{3,4}, MyType{5,6}, MyType{7,8} };

which is too wordy; and that's one of the cases where you fall back on:

std::make_array<MyType>{ {1,2}, {3,4}, {5,6}, {7,8} };

Upvotes: 17

alfC
alfC

Reputation: 16272

There is an experimental make_array now.

https://en.cppreference.com/w/cpp/experimental/make_array

Defined in header <experimental/array>

template <class D = void, class... Types> constexpr std::array<VT /* see below */, sizeof...(Types)> make_array(Types&&... t);

(library fundamentals TS v2)

Upvotes: 2

PiotrNycz
PiotrNycz

Reputation: 24412

This answer provided the status of the proposal - however - it is pretty easy to implement in C++17 - at least this part:

[Example:

int i = 1; int& ri = i;
auto a1 = make_array(i, ri);         // a1 is of type array<int, 2>
auto a2 = make_array(i, ri, 42L);    // a2 is of type array<long, 3>
auto a3 = make_array<long>(i, ri);   // a3 is of type array<long, 2>
auto a4 = make_array<long>();        // a4 is of type array<long, 0>
auto a5 = make_array();              // ill-formed    auto a6 = make_array<double>(1, 2);  // ill-formed: might narrow –end example]

See:

template <typename Dest=void, typename ...Arg>
constexpr auto make_array(Arg&& ...arg) {
   if constexpr (std::is_same<void,Dest>::value)
      return std::array<std::common_type_t<std::decay_t<Arg>...>, sizeof...(Arg)>{{ std::forward<Arg>(arg)... }};
   else
      return std::array<Dest, sizeof...(Arg)>{{ std::forward<Arg>(arg)... }};
}

The proof:

int main() {
    int i = 1; int& ri = i;
    auto a1 = make_array(i, ri);         // a1 is of type array<int, 2>
    std::cout << print<decltype(a1)>().get() << std::endl; 
    auto a2 = make_array(i, ri, 42L);    // a2 is of type array<long, 3>
    std::cout << print<decltype(a2)>().get() << std::endl;
    auto a3 = make_array<long>(i, ri);   // a3 is of type array<long, 2>
    std::cout << print<decltype(a3)>().get() << std::endl;
    auto a4 = make_array<long>();        // a4 is of type array<long, 0>
    std::cout << print<decltype(a4)>().get() << std::endl;
    // auto a5 = make_array();              // ill-formed
    // auto a6 = make_array<double>(1, 2);  // ill-formed: might narrow
}

Output:

std::__1::array<int, 2ul>
std::__1::array<long, 3ul>
std::__1::array<long, 2ul>
std::__1::array<long, 0ul>

The last line make_array<double>(1, 2) produces "narrowing cast" errors - as required in proposal. It can be "improved" by adding static_cast in implementations.

On latest clang - demo.

Upvotes: 6

T.C.
T.C.

Reputation: 137425

LEWG voted to forward the merge paper for C++20 back in 2016 (this was after the C++17 feature freeze). Its LWG review is on hold at the author's request pending the resolution of LWG issue 2814.

Upvotes: 9

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