Mix Swift and C++

Written by Ramit Sharma

Description: Learn how you can use Swift in your C++ and Objective-C++ projects to make your code safer, faster, and easier to develop. We'll show you how to use C++ and Swift APIs to incrementally incorporate Swift into your app.

Basics of interoperability

Incremental adoption

  • Builds on Objective-C interoperability
    • Supports incremental Swift adoption

Using Swift in more places

  • Adopt Swift in large C++ codebases.
    • Use C++ libraries in Swift.
    • Remove Objective-C bridging.
    • While using C++ frameworks, Xcode imports C++ APIs automatically and don’t need a bridging header.
    • Enable C++ interoperability in the build settings.
  // Calling a C++ method from Swift
  func loadImage(_ image: UIImage) {
  // Load an image into the shared C++ class.

  // importing a C++ framework 
  import CxxImageKit

Bidirectional interoperability

  • Incremental Swift adoption.
    • Completely automatic integration by the Swift compiler without writting a Bridging layer.
    • Direct calls and no overheads when calling APIs in Swift and vice-versa.
    • C++ APIs availablity in Swift:
      • can import most C++ collections, both from the standard library and elsewhere
        • Collections: std::vector, std::map, User-defined
      • handlew function templates and class template specializations
      • supports managing memory using shared pointer and similar user-defined types.
        • Smart pointers: std::shared_ptr, user-defined
      • Members: methods, properties and intializers
      • Generics: structs and enums
      • Standard library: swift::Array, swift::String, and swift::Optional
      • Library evolution: resilient structs and classes
  • Xcode Compatiblity
    • Code completion
    • Jump-to-definition
    • Global rename
    • Debugger support

Natural Swift APIs

  • Swift compiler is able to automatically import most C++ APIs and represent them as safe Swift APIs.
    • Getters and Setters -> Methods
    • Value types -> Structs
    • Reference Types -> Pointers
    • Operators -> Operators
    • Containers -> Swift Collections
    • Constructors -> initializers
    • Swift compiler allows us to fine-tune how APIs are imported and expose APIs that feel even more natural by providing the compiler with more information about your APIs through the use of annotations.
    • structs represent value types and classes represent reference types.
    • C++ types will be imported as value types in Swift.

Imported value types

  • Short lifetimes and deep copies.
    • Copy constructor and destructor manage lifetime.


  • Imports any type with begin and end methods as Swift collection.
    • Swift Collections can be used in for-loops, map, filter

Swift Collections Safety

  • use these Swift Collection APIs rather than C++ iterator APIs which do not fit into Swift's safety model.
    • The Swift compiler will help guide you towards these safer APIs by marking unsafe C++ APIs as unavailable and suggesting a safer alternative.
    • C++ doesn't have a strong distinction between value types and reference types.
    • Swift also gives you the option to import some things as reference or class types by adding an annotation to your C++ code.

Foreign reference types

  • map CxxImageEngine to a Swift class using the SWIFT_SHARED_REFERENCE attribute.
    • Enforces reference sematics.
    • Removes UnsafePointer indirection.
    • Manages lifetime with custom retain and release operations.
  //importing swift/bridging
  #import <swift/bridging>

  //Applying SWIFT_SHARED_REFERENCE attribute to CxxImageEngine
  struct SWIFT_SHARED_REFERENCE(IKRetain, IKRelease) CxxImageEngine {
      // ...

Computed properties

SWIFT_COMPUTED_PROPERTY attribute maps getter and setter to property.

   /// \returns all images that have been loaded into the engine. Includes any modifications that were
   /// applied to the images.
   inline std::vector<Image *_Nonnull> getImages() const;

Future evolution

  • Evolve based on your feedback.
    • Opt-in to changes with interoperability versions.
    • You can join the workgroup and get involved on the forums. Just head to Mixing Swift and C++

Missing anything? Corrections? Contributions are welcome 😃


Written by

Ramit Sharma

Ramit Sharma

Indie iOS Dev. Swift, SwiftUI, Obj-C, UX and related.