Question

It says here that

The unbounded array is similar to a
std::vector in that in can grow in
size beyond any fixed bound. However
unbounded_array is aimed at optimal
performance. Therefore unbounded_array
does not model a Sequence like
std::vector does.

What does this mean?

Answer 1

It appears to lack insert and erase methods. As these may be “slow,” ie their performance depends on size() in the vector implementation, they were omitted to prevent the programmer from shooting himself in the foot.

insert and erase are required by the standard for a container to be called a Sequence, so unlike vector, unbounded_array is not a sequence.

No efficiency is gained by failing to be a sequence, per se.

However, it is more efficient in its memory allocation scheme, by avoiding a concept of vector::capacity and always having the allocated block exactly the size of the content. This makes the unbounded_array object smaller and makes the block on the heap exactly as big as it needs to be.