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What is the difference between a clang (C++, Objective-C, C) module and a Swift module?

· 3 min read
Ben Butterworth

They are different. At the end of the build process though, they both need to be linked to your application/ library's other .o and .dylib files for it to run.

Swift modules​

  • From Swift Serialization.md docs:

    The fundamental unit of distribution for Swift code is a module. A module contains declarations as an interface for clients to write code against.

  • Swift acccess control docs:

    A module is a single unit of code distribution: a framework or application that’s built and shipped as a single unit and that can be imported by another module with Swift’s import keyword.

  • Configured by .target()'s in Package.swift

  • Cannot have submodules, so users cannot import Module.Submodule in Swift. Users can still import specific entities, import struct PackageModel.Manifest, but this is a lot more verbose than importing submodules.

  • Its interface exists as a .swiftmodule. What is a .swiftmodule?. The documentation says:

    Conceptually, the file containing the interface for a module serves much the same purpose as the collection of C header files for a particular library.

  • The compiler produces this .swiftmodule file a lot, like a generated objective-C header, but instead of text, its a binary repesentation. It includes the bodies of inlinable functions, much like static inline functions in objective-C or header implementations in C++. However, Swift modules does include the names and types of private declarations. This allows you to refer to them in the debugger, but it does mean you shouldn't name a private variable after your deepest darkest secret. from WWDC 2018: Behind the Scenes of the Xcode Build Process

    • So private declarations are exposed in your .swiftmodule (Swift module interface).

  • When importing pure Objective-C frameworks into Swift, the Swift compiler uses its built-in clang compiler to import an Objective-C header.

    The importer finds declarations in the headers exposed in Clangs .modulemap for that framework. (again, from WWDC2018)

  • When importing Objective-C + Swift frameworks into Swift, the Swift compiler uses the Umbrella header.

Clang modules

  • Configured by YourModuleName.modulemap file (previously module.map, but this is deprecated), formatted like this
  • Can have submodules, e.g. std module has std.io and std.complex.
  • A clang module exposes header files specified in the module map. Private details (in .m) are not exposed at all.
  • Is an improvement of the original #include or #import style imports to improve the build process (This is a big topic, read the Clang module docs).

I originally posted this question and answer on Stack Overflow here. I had to do some digging and reading for this one.