Yeah, but I'm not entirely clear how I'd for example get std::vector to use a mmap'ed range, that I could zero-copy grow.

Also in that case I might want to grow it by a fixed amount, once it grows past a certain threshold.

It's a shame one needs to write another vector implementation just for that.

Why wouldn't writing an allocator class be enough for mmap?

Probably not, because the allocators don't expose the "extend" or "realloc" concept, so all you'll see from within your allocator is a request for an entire new region that vector wants in order to copy the new stuff into.

To implement a growing allocation , you'll have to create a custom vector, pretty much.

How would you ensure nothing was used in the region after the original allocation?