This project demonstrates using Swift/C++ interop, creating two executables and two libraries. One executable is written in C++, calling into a Swift library, which calls into a C++ library. The other executable is written in Swift, calling into a C++ library, which calls into a Swift library, bouncing control flow between the two languages to compute fibonacci numbers.

• Swift 5.9 or later
• Clang 11 or Apple Clang shipped in Xcode 12 or newer
• CMake 3.26 or later
• Ninja

 $ mkdir build
 $ cd build
 $ cmake -G 'Ninja' ../
 $ ninja

Start in the build directory you created above. There are two executables under src/. src/fibonacci_cpp is a program written in C++ that calls into a Swift library, which calls back into C++. src/fibonacci_swift is a program written in Swift that calls into a C++ library, which calls back into Swift. Both compute the n'th fibonacci number, where n is 5.

Each step of computing the numbers prints whether it's running in Swift or C++, and the value x at that step.

 $ ./src/fibonacci_cpp
x [swift]: 5
x [cpp]: 4
x [swift]: 3
x [cpp]: 2
x [swift]: 1
x [swift]: 0
x [cpp]: 1
x [swift]: 2
x [cpp]: 1
x [cpp]: 0
x [cpp]: 3
x [swift]: 2
x [cpp]: 1
x [cpp]: 0
x [swift]: 1
8

$ ./src/fibonacci_swift
x [cpp]: 5
x [swift]: 4
x [cpp]: 3
x [swift]: 2
x [cpp]: 1
x [cpp]: 0
x [swift]: 1
x [cpp]: 2
x [swift]: 1
x [swift]: 0
x [swift]: 3
x [cpp]: 2
x [swift]: 1
x [swift]: 0
x [cpp]: 1
8