Initialize std::array with implied length

Question

In C you can do int a[] = {1,2,3,4,5}, but C++11 std::array<int> a = {1,2,3,4,5} will give a "too few template parameters" compile error. Any way around this?

score 5 · Accepted Answer · answered Feb 27 '14 at 15:56

5

The best you can have is a make_array, something like:

template<typename T, typename...Ts>
constexpr std::array<T, 1 + sizeof...(Ts)> make_array(T&& head, Ts&&...tail)
{
     return {{ std::forward<T>(head), std::forward<Ts>(tail)... }};
}

answered Feb 27 '14 at 15:56

Jarod42

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Why is it `1 + sizeof...(Ts)`? What is the extra space for? – David G Feb 27 '14 at 16:37
@0x499602D2: `1` is for the first parameter `T head` (we deduce the type from the `head`). – Jarod42 Feb 27 '14 at 16:45

score -3 · Answer 2 · edited Feb 27 '14 at 15:56

-3

Implementation of std::array:

template<typename T, std::size_t N>
struct array {
    T array_impl[N];
};

So this Should work:

std::array<std::int, 5> a = {{ 1, 2, 3, 4, 5 }};

which is essentially just like (as the compiler agreed to drop off the inner curly brackets.

std::array<std::int, 5> a = { 1, 2, 3, 4, 5 };

See

edited Feb 27 '14 at 15:56

juanchopanza

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answered Feb 27 '14 at 15:52

Dory Zidon

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So how can you do that without saying that the length of the array is 5? – HelloWorld123456789 Feb 27 '14 at 15:54
you do specify the array Rikayan, look at the second template parameter that's passed to the template.. – Dory Zidon Feb 27 '14 at 15:55
6

@DoryZidon This doesn't answer what he is asking. – Zac Howland Feb 27 '14 at 15:56

Initialize std::array with implied length

2 Answers2