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Comment by inferiorhuman | Hacker News Reader

Agreed – but why would you want to box an array instead of simply using a Vec?

You can save memory by having fewer fields. This can matter when you have lots of small arrays.

Vec<u8> has {usize length, usize capacity, void* data}. Box<[u8]> has {usize length, void* data}. Box<[u8;N]> has {void* data}.

For a typical use case that seems like a rather extreme optimization, no? If you have a lot of objects with many small arrays and you're keeping them in a Vec, they'll be on the heap. If you're dealing with a bunch of small parts of a big blob of binary data, you'd use slices and not create new arrays. If you're on an embedded system you're not likely to have an allocator anyways.

(without trying to be too argumentative) right? Or?

Edit since I've been throttled:

  For example it can make a difference between passing values per register or per
  stack in some situations. … But then for some fields where C++ is currently very
  prevalent it might matter all the time.

That's an interesting one I hadn't thought about (and I didn't realize that the register keyword was deprecated in C++17). In a rather broad sense I hope Rust catches on in the kinda niche stuff where C++ is often popular. For example I've only done a little bit of dabbling with Rust in an embedded context but overall I thought it brought a lot to the table.

In a system at $WORK I recently optimized a structure from String to Box<str> (similar optimization to remove the 8 byte capacity field) and saved ~16Gb of memory. Granted, the program uses 100-200Gb of RAM at peak, but it still was a nice win for basically no work. It's also a semantic win, since it encodes "this string can't be mutated" into the type.

yes but also no,

In some situations "optimizing smart pointers" to just be a single pointer size (Box<[T; N]>) instead of two pointer sizes (Box<[T]>) or instead of three pointer sizes (Vec<T>) can make a difference.

For example it can make a difference between passing values per register or per stack in some situations.

Or it could make the difference of how many instances of the boxed slice fit efficiently into the stack part of a smallvec. Which if it's the difference between the most common number fitting or not fitting can make a noticeable difference.

Through for a lot of fields of programming you likely won't opt. to do such optimizations as there are more important things to do/better things to spend time optimizing at. But then for some fields where C++ is currently very prevalent it might matter all the time.

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