Don’t you need to move the surviving young generation objects after the ones they’re surrounded by die? Otherwise the array is going to end up rather sparse, with a lot of unused array entries.

Without either a free list or compaction, I don’t really see how you’re collecting garbage at all.

Yes, I do need to compact the young generation during GC. Eg. Let's say I have the young generation starting at index 1000 and I do GC with 1100 items, with 10 items surviving. I'll have to compact the remaining 10 items to the 1000..1010 span of the vector, after which I can also decide to promote the bottom two young generation indexes to the old generation, making the next young generation start index 1002.