iOS Interview Guide: How Swift compiler optimize performance with immutable collections?

Level: Intermediate, Priority: High

Q. How does the Swift compiler optimize performance when using immutable collections?

Efficient use of immutable collections ensures minimal memory overhead and faster execution, particularly in scenarios involving frequent data manipulation. Swift compiler employs several optimization techniques when working with immutable collections to improve performance.

Here are some of the key optimization techniques:

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Copy-on-Write (CoW)

Collections like Arrays and Dictionaries use copy-on-write for immutable instances. When you assign an immutable collection to a new variable or pass it to a function, instead of creating a full copy, the new variable or function parameter points to the original data storage. Only when one of the variables attempts to modify the collection is a copy made, preventing unnecessary copies. An example:

let originalArray = [1, 2, 3]

// no copying occurs here
var copiedArray = originalArray

// actual copy happens at this point
copiedArray.append(4)

In the above example, no copy has been made when you assign originalArray instance to copiedArray. It gets copied when you modify the copiedArray instance.