How to create std::vector of char/std::byte where first byte is aligned to 16 byes, but there is no padding?

Question

There is an existing question that requires C++03 and has no answer, so I will open a new one.

Problem I have is that I want to have std::vector of std::byte, but so that .data()(first element of data array) is 16 byte aligned.

alignas on wrapped char does not help because I do not want to have alignment gaps in the array. In other words I want to keey alignment of 1 for elements, but I want alignment of 16 for the array.

Ideally I would like to avoid using a custom allocator. If there is any TBB or boost vector that does what I want that would be also great.

How about wrapping the `std::vector` and a `alignas(16) std::byte` into a class? — Fureeish, Jan 25 '19 at 12:16
On platforms where maximum native alignment is 16 or greater, you need to do nothing special if you use the default allocator. — eerorika, Jan 25 '19 at 12:24
@user463035818 OK, but I mean there may be a C++11 solution that nobody posted as an answer to that Q — NoSenseEtAl, Jan 25 '19 at 12:24
Answer: https://stackoverflow.com/questions/12942548/making-stdvector-allocate-aligned-memory — Nuzhny, Jan 25 '19 at 12:25
ah ok you meant "requires c++03" in the sense of "the other question requires c++03 but no later standard", sorry misunderstood — 463035818_is_not_an_ai, Jan 25 '19 at 12:28

JVApen · Accepted Answer · 2019-01-25T12:34:34.987

4

Aligning the data in a vector ain't provided by default. Not even for aligned classes.

The best way of doing alignment is with the aligned_allocator of boost.

Unfortunately, it doesn't prevent padding, it even overallocates to adapt the pointer on the alignment. From C++17, it can used aligned new (see std::aligned_val_t overloads). However, all implementations I've seen actually use the same trick.

An alternative is allocating a whole page at once, and do your own memory management with a custom allocator. You can do it, though it might take a lot of time to do correctly.

edited Jan 25 '19 at 12:34

answered Jan 25 '19 at 12:27

JVApen

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nitpick: you should provide link to aligned new since it is quite tricky to google for it, I assume you mean overloads taking std::align_val_t ? – NoSenseEtAl Jan 25 '19 at 12:30
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Yes, indeed, I'll add it – JVApen Jan 25 '19 at 12:31

How to create std::vector of char/std::byte where first byte is aligned to 16 byes, but there is no padding?

1 Answers1