Just a quick example of how to make a Swift Package that includes a library target in C++ and a Swift target that calls into that library C++ code

A developer on Mastodon posted a query:

I want to use some C++ code from a Swift Package, so I:

1. Added a target with the C++ code
2. Added an umbrella header
3. Added this new target as a dependency to the client target
4. Import the new module in the client target

But now when I try to call a C++ function, which I can autocomplete to, the linker throws “Undefined symbol”.

I’d never done this before so I thought I’d take a look. The definitive documentation I could find seemed to be “Setting up Mixed-Language Swift and C++ Projects” on Swift.org.

The poster on Mastodon wanted to use a couple of C++ files from https://github.com/aappleby/smhasher/tree/master, specifically MurmurHash3.h / MurmurHash3.cpp. Not really sure what this repo is all about, but I grabbed those files and played around some.

Got it linking and executing without crashing :) and you can see a test repo here in this repo. (ignore the hacky Swift code trying to make some test parameters to call the C++ MurmurHash3_… function with; not sure what this function is really expecting but I was just trying to solve the linking issue the poster was running into.)

The key to getting this to work is the addition of:

swiftSettings: [.interoperabilityMode(.Cxx)])

on the target that is going to use the C++ containing target.

(Repo was tested with Xcode 16.2, macOS 14.7.5).