Performance notes — copy-on-write
Understand Swift String performance: copy-on-write (COW), substring sharing, and how to manage memory efficiently.
Performance notes — copy-on-write is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 3. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 3 lessons in total.
Intro
Goal: Learn how Strings use copy-on-write (COW) to balance performance and safety. Substrings also share memory until needed.
Shared storage
Assigning a String does not immediately copy characters. Both share the same storage.
var a = "Hello World"
var b = a // no real copy yet
print(a, b)Mutation triggers copy
A copy occurs only when one of the values mutates. This is copy-on-write.
b.append("!!!") // triggers a real copy
print("a:", a)
print("b:", b)Substrings
Substrings share the original string's memory. Converting to String makes a full copy.
let text = "Hello Swift"
let part = text.prefix(5) // Substring
print(part)Substring → String
Explicitly create a String from a Substring if you want it to own storage.
let s: String = String(part)
print("Converted:", s)Tips
Tips:
- Assignments are cheap until modified.
- Be cautious with long-lived Substrings—they keep the full original string in memory.
- Convert to String if you need independent storage.
COW question
Quick check: What does copy-on-write do?
Recap
Recap: Swift Strings use copy-on-write to be efficient. Assignments and substrings share memory; a real copy happens only on mutation or explicit conversion.
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Frequently Asked Questions
Is the “Performance notes — copy-on-write” lesson free?
Yes — any 3 lessons of the Swift Academy learning path, including “Performance notes — copy-on-write”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 3 lessons in total.
What will I learn in “Performance notes — copy-on-write”?
Understand Swift String performance: copy-on-write (COW), substring sharing, and how to manage memory efficiently. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Swift Academy?
No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 3.
How long does the “Performance notes — copy-on-write” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Swift Academy lesson?
Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Strings & Unicode — graphemes, indices, substrings, APIs
- Common APIs — prefix/suffix/contains, split/join
- Performance notes — copy-on-write