Understanding `append(_:)` implementation of _FixedArray16

Using Swift
1

Hello.
My question pertains to _FixedArray16 in the stdlib and is really simple: Why increment the count first, only to subtract from it right after, in order to store newValue?

extension _FixedArray16 {
  internal mutating func append(_ newElement: T) {
    _internalInvariant(count < capacity)
    _count += 1
    self[count-1] = newElement
  }
}

While writing this down I realised that they are not the same count:

var _count: Int8

// ...

internal var count: Int {
  @inline(__always) get { return Int(truncatingIfNeeded: _count) }
  @inline(__always) set { _count = Int8(newValue) }
}

But still I can't understand why not do this instead:

self[count] = newElement
_count += 1

Is there a performance reason behind this choice assuming they don't lead to the same machine code? If there is, can someone explain it to me?

Thank you.

2

The type has an invariant that a valid subscript index is greater or equal to zero, and less than _count. Your formulation would break that invariant; I'm sure it was written this way simply to make it more resistant to possible mistakes.

The subscript is defined at line 56 of FixedArray.swift

3 Likes
3

Oh right, that makes sense. Thanks!