As far as I can tell, you're essentially creating a new stack for every dynamically-sized variable (because otherwise nothing would be able to grow by bump allocation without moving). But this means you're creating a separate virtual memory allocation for each of these objects, which (I presume?) then rounds the size up to the OS's minimum page size (4KiB or similar).

Doesn't that waste a lot of space? Or is there something I'm missing?

Also it would seem to force every allocation to go through the OS kernel to allocate unless there's some trick to avoid that.

These AI generated docs would make more sense if you instruct it to not include numbers unless they form a complete example.

You can also just do that in C++. I do that as part of my messaging usually, so that the objects are trivially relocatable and reinterpret-castable. It only works so long as you combine a set of canonical types, but that can be verified or converted using C++26 reflection.

I have both what I call inline sequences, and out-of-line ones (store an offset to the object itself rather than pointers).

I made all my sizes and offset 32-bit, but I suppose they could have been 16-bit.

I wish it was more clear about the memory layout of nested variable sized data structures. As far as I can work out the max capacity is decided at construction time which makes the whole thing a lot less intriguing.