Swift-C++ Interoperability: Bridging Two Worlds

The Swift type system emphasises ownership and optionals, while C++ exposes raw pointers and templates. Our approach started with Clang’s ASTImporter – we added a translation layer that maps C++ class templates into Swift generic constraints and preserves noexcept semantics as Swift error-handling attributes.

Value types were particularly tricky. We leaned on Swift’s move-only experiments to mirror the lifetime of C++ std::unique_ptr and introduced synthesized deinitializers that forward to the C++ destructor when a Swift wrapper goes out of scope.

Interoperability only works if symbol names line up perfectly. We extended the Swift name mangler to understand Itanium C++ ABI components and wrote exhaustive tests that round-tripped demangling between the two compilers. This prevented Swift from ever generating a symbol that Clang could not locate.

Binary size mattered as well. We implemented lazy member loading so importing <vector> no longer pulled dozens of unused template instantiations into Swift build artifacts.

Interop is only as good as the developer experience. We updated SourceKit to surface C++ doc comments inside Xcode auto-complete and wrote SIL-level diagnostics when a developer accidentally used an owning Swift type where a borrowed C++ reference was expected.

The end result: Apple platform teams adopted Swift wrappers around std::chrono and folly types without giving up performance or hitting ABI traps.