I’m building an iOS library that contains both Swift and C++ code.

Using the new C++ <> Swift interop from Swift 5.10, I can now skip the Objective-C bridge – which was annoying to write, and also slowed everything down in performance critical code.

I managed to set up the CocoaPod and my example project accordingly – SWIFT_OBJC_INTEROP_MODE is set to objcxx, and I’m using C++ 20.

Now I created CxxInstance.hpp:

#include "MyPod-Swift.h"

enum class SomeEnum { FIRST, SECOND }; 

class CxxInstance {
public:
  static void runIt() {
    auto swiftInstance = MyPod::SwiftInstance::init();
    int i = swiftInstance.getInt(); // <-- works ✅
    SomeEnum e = swiftInstance.getEnum(); // <-- build error! ❌
  }
};

And SwiftInstance.swift:

public class SwiftInstance {
  public init() { }

  public func getInt() -> Int {
    return 13
  }

  public func getEnum() -> SomeEnum {
    return .FIRST
  }
}

MyPod-Swift.h was generated successfully and I can call MyPod::SwiftInstance::init().

But while getInt() -> Int works fine and can be called from C++, getEnum() -> SomeEnum fails to build because C++ cannot resolve that method.

In Swift it exists and I can use the enum SomeEnum just fine – it’s just the C++ part that fails to find the getEnum() method.

According to their Mixing Swift and C++ guide, calling Swift functions from C++, where the Swift function uses C++ types should be a supported feature, but I think they created separate “Modules” – one for C++ and one for Swift.

In my case I have a single .podspec which includes all files – Swift and C++ – as source_files:

Pod::Spec.new do |s|
  s.name         = "NitroImage"
  s.version      = package["version"]
  s.summary      = package["description"]
  s.homepage     = package["homepage"]
  s.license      = package["license"]
  s.authors      = package["author"]

  s.platforms    = { :ios => min_ios_version_supported }

  s.source_files = [
    # C++ part
    "cpp/**/*.{hpp,cpp}",
    # Implementation (Swift)
    "ios/**/*.{swift}",
  ]

  s.pod_target_xcconfig = {
    # Use C++ 20
    "CLANG_CXX_LANGUAGE_STANDARD" => "c++20",
    # Enables C++ <-> Swift interop (by default it's only C)
    "SWIFT_OBJC_INTEROP_MODE" => "objcxx",
  }
end

And CocoaPods always generates a default module.modulemap:

module MyPod {
  umbrella header "MyPod-umbrella.h"

  export *
  module * { export * }
}

I think I am using dynamically linked use_frameworks!, if that matters.

Any ideas of how I can properly have cyclic dependencies on C++ <> Swift code within the same Pod?