std::span on containers of contiguous data

Question

std::span works great on std::vectors, std::arrays or c pointers. However, often the contiguous memory has a different type associated to it such as for example a vector of arrays. I'm trying to use a std::span on vectors of arrays, arrays of arrays, or more generally containers of contiguous data.

For example, if I have a function:

void func(std::span<double> vec){

}

and a vector of array:

std::vector<std::array<double,3>> vec;

I would like to be able to call it like that:

func(vec);

I would like to avoid the std::span<std::array<T,N>> since this is all one block of memory, and I would like to accept std::vector<T>, std::array<T>, and std::vector<std::array<T>> as well. It is possible to do something like that with raw pointer, but then the type is lost and it decays, so I am trying to avoid that.

I could do func(std::span(&vec[0][0], vec.size()*3)), but I am wondering if there is something better.

Personally I would just take the container as is like template <typename Container2D> func(Container2D cont) and if you want to you can use concepts to enforce the interface you want. — NathanOliver
– NathanOliver, Commented Nov 3, 2024 at 1:07
@NathanOliver: Though in that case you probably want to receive by reference or const reference; std::span is cheap to pass, actual containers would involve copies if not received by reference. — ShadowRanger
– ShadowRanger, Commented Nov 3, 2024 at 14:34

user17732522 · Accepted Answer · 2024-11-03 02:03:01Z

I could do func(std::span(&vec[0][0], vec.size()*3)), but I am wondering if there is something better.

That is going to end in undefined behavior. It is not possible to access beyond a single std::array element of the vector, even if they are laid-out contiguously.

There are rules in the standard currently that would permit a compiler to optimize under the assumption that it is impossible to go out of bounds of the individual std::array object from a pointer to one of the std::array's elements, even though I am not aware of any compiler currently doing so.

There have been proposals to change the standard to permit what you want, but I am not aware of any progress in that direction.

This is not specific to std::span either. With a raw pointer you have the same problem. Pointer arithmetic is only ever defined in a single array, not across array objects.

Instead of a vector of arrays of doubles, just use a vector of doubles wrapped in a class with member functions and operator overloads that handle the correct 2d-indexing for you. Then passing arbitrary sections as std::span is trivial and has defined behavior.

Thank you for your answer. I tried on clang and it works, which does not mean it is correct, as you said.

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