I want to share a little conversation from Swift-Users. KS Sreeram wrote:

I’m trying to initialize a byte-array efficiently with minimal copying with the following steps:

1. Create an empty byte array.
2. Reserve sufficient capacity in the array.
3. Use mutable pointers into the array to fill in the data.
4. The actual size that was filled is known only after it is filled in.
5. I would like to set the size of the array to the actual size.

I couldn’t find any methods for doing the last step. Is there a way to do this?

Dave Abrahams’ response:

Create a Sequence that represents the elements you’re going to append, e.g.:

 var a = [1, 2]
 a.reserve(256)
 a += sequence(first: 3, next: {$0 < 1000 ? ($0 + 3) * 2 : nil})

There are lots of ways to make Sequences, but the overloaded sequence() functions are probably the easiest.

Sadly:

@jckarter @ericasadun Unfortunately, Dave’s solution doesn’t work for say serialization code. Code not suitable for control inversion.

— KS Sreeram (@kssreeram) August 10, 2016