This globally good, two remarks however:

> you don’t need to write destructors

I think this is not accurate. Destructors are not deallocators, they are supposed to set the object field in an invalid state. Now truth is that both are often called together, e.g `delete`.

> Typically arena lifetime is the whole program, so you don’t need to worry about freeing it

A technic I use is to increment a counter on `arena.alloc` and decrement it on `arena.dealloc`, and then free the memory (if it's on the heap) accordingly.