Awesome Open Source
Awesome Open Source

Struct vs Class Performance

This is a POC for my Medium article Stop Using Structs!! which compares performance characteristics of structs and classes, with a view to show where structs are misused and contain other reference types.


The article and POC was published for Swift version 4.0 and this POC is valid for Swift 4.0 and -very likely- previous versions. But with Swift 4.1 and 4.2 Apple changed calling convention and ownership model and these changes directly effect language's performance characteristics.

Therefore the results of this POC may vary and is not perfectly valid for Swift versions after 4.0.


I tested each configuration with 100 iterations, results are averages in milliseconds, smaller is better :)

Optimization Level Class Struct Ratio
-Onone (unoptimized) 47926 89411 ~1.86x
-O (single file optimization) 5981 16503 ~2.76x
-O -whole-module-optimization 372 24809 ~67x


  • Xcode 9.0
  • Swift 4.0


xcrun -sdk macosx swiftc -O -whole-module-optimization StructvsClassPerformance.swift


Licensed under the terms of the MIT license. See the LICENSE file.

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