It’s not so brilliant when you have an array of size 2^n and you append and then pop off the end in a loop and each operation takes time proportional to the length of the vector.

Usually, dynamic arrays aim for amortized constant time for append and pop. This trick loses that property.

Restoring it only needs one additional bit of storage, however (whether the buffer is currently 2⌊n/2⌋ or 4⌊n/2⌋ elements in size), which we can probably squeeze in somewhere.

More disappointing is that libc still forces us to use the free(p) API, without a length argument, meaning we are still paying to store a(n approximation of) the length somewhere in the allocation metadata.