I would like to do something like
std::array<int, 5> array1 = {{ ... }};
const std::array<int, 3>& array2 = array1[1:4]; // [x:y] doesn't exist
That is, get an array that is a sort of view of another array, without having to copy it.
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5In the C++ standard library, they get around this problem by passing pairs of iterators rather than a reference to an array. Random access iterators can easily be offset for a sub-range.Mark Ransom– Mark Ransom2016-03-16 20:42:08 +00:00Commented Mar 16, 2016 at 20:42
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Except that then you are not telling the compiler the distance between the first and the last element. Without knowing you have three elements, you can't unroll the loop.Ant6n– Ant6n2016-03-16 21:26:27 +00:00Commented Mar 16, 2016 at 21:26
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4 Answers
No, you can't do that. All standard library containers are unique owners of their data, and std::array is no exception. In fact, std::array is constrained to be implemented in such a way so that the elements are stored inside an actual array member of the class, which would make aliasing impossible.
There is a proposal for an array_view class that would represent a non-owning view into a contiguous block of data. You can read about it here. I don't know the status of this proposal (the current C++ standardization process confuses me).
You can if you use valarray instead of array http://en.cppreference.com/w/cpp/numeric/valarray/slice
Edit: from C++20 you can refer to a continuous sub-array with std::span
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Yes you can, std::span in C++20 provides a view over a contiguous sequence of objects.
std::array<int, 5> array1 = {20, 10, 35, 30, 5};
std::span array2(array1.begin(), array1.begin() + 3); // {20, 10, 35}; subview of array1
The underlying data is not copied but rather referenced, therefore care should be taken to ensure the original array still exists when using the span.
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std::array<int, 3>& array2 = reinterpret_cast<std::array<int, 3>&>(array1);
7 Comments
Weak to Enuma Elish
Doesn't this violate strict aliasing?
R Sahu
@JamesRoot, it definitely does.
user4581301
Violates it most violently. I can think of several off colour analogies I won't be using in a public forum.
M.M
This would rely on
std::array wrapping a C-style array (in which case I am not convinced there is an aliasing violation) but the standard doesn't require that
alfC
@user4581301, I have been using this technique for a while and interestingly I never managed to make
clang or gcc to produce a strict aliasing warning when using std::array. Perhaps there is something special about the internal representation of std::array<T,N> as T[N] that makes it work. I am talking about this code for example std::array<int,3> aa = {{1,2,3}}; std::array<int,2>& ee = reinterpret_cast<std::array<int,2>&>(aa[1]); assert(ee[0] == 2); |