I doubt there are any vector implementations that don't use exponential growth. Without exponential growth, push_back doesn't have constant time, and adding n elements doesn't take O(n) time. Since push_back is required to take amortized constant time, an implementation without exponential growth (or some kind of magic) wouldn't be standard conforming.
Whether the exponential growth is in the form of doubling or not is just a constant factor. There's nothing magical about using powers of two. Depending on the accounting used by the allocator you're using, there may be synergies, or there might not be - some allocators require a few bytes more than the allocation request, and this requirement might inflate the underlying memory request more than necessary.
Whether the exponential growth is in the form of doubling or not is just a constant factor. There's nothing magical about using powers of two. Depending on the accounting used by the allocator you're using, there may be synergies, or there might not be - some allocators require a few bytes more than the allocation request, and this requirement might inflate the underlying memory request more than necessary.