How should I construct a smaller std::array as a slice of a larger one?

Question

I want to represent elements i through i+k-1 of an std::array as another std::array, for constexpr i and k - preferably with no copying and in a constexpr function.

Can this be done? If so, what's the right way to approach this?

Answer 1

Copying is inevitable with std::array. You can avoid copying if you can use span<T, N> from the Microsoft Guideline Support Library.

If you have to stick with std::array then you can introduce this helper function:

template<std::size_t OFFSET, std::size_t K, class T, std::size_t N>
std::array<T, K>
subarray(const std::array<T, N>& a)
{
    static_assert(OFFSET+K<=N, "your error message here");

    std::array<T, K> s;
    std::copy(a.begin()+OFFSET, a.begin()+OFFSET+K, s.begin());
    return s;
};

Use it like this:

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

Answer 2

What you are asking is for a slice of an array or may be even a vector. Sadly, We do not have anything in the standard library (until C++17), but as Leon has mentioned in his answer, you can use span for that.

GSL implementation: https://github.com/Microsoft/GSL/blob/master/include/span.h

std::array<int, 4> arr = {1, 2, 3, 4};
span<int> s{&arr[0], 2};

My implementation of slice(test) : https://github.com/arun11299/Slices-in-Cpp\

std::vector<int> vec{1,2,3,4,5,6,7,8,9,10};

// all elements
auto vslice = make_slice(vec, 0, vec.size());
assert(len(vslice) == 10);

vslice = vslice(beg, 8);

assert(len(vslice) == 8);