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Getting started with C++ Interoperability

docs/CppInteroperability/GettingStartedWithC++Interop.md

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** ‼️ The official C++ interoperability documentation is live at Swift.org and provides an up-to-date guide for mixing Swift and C++ ‼️ **

Getting started with C++ Interoperability

This document is designed to get you started with bidirectional API-level interoperability between Swift and C++.

Table of Contents

Creating a Module to contain your C++ source code

  • Create a new target in Xcode via File | New | Target, select Library.
  • Within the directory of the newly created target, create a new C++ implementation and header file
  • For this example we will call the files CxxTest, so we should have a CxxTest.cpp and CxxTest.hpp.
  • Next create an empty file and call it module.modulemap, in this file create the module for your source code, and define your C++ header (requires cplusplus isn't required but it's convention for C++ modules, especially if they use C++ features).
// In module.modulemap
module CxxTest {
    header "CxxTest.hpp"
    requires cplusplus
}

Adding C++ to an Xcode project

Add the C++ module to the include path and enable C++ interop:

  • Navigate to your project directory

  • In Project navigate to Build Settings -> Swift Compiler

  • Under Custom Flags -> Other Swift Flags add -cxx-interoperability-mode=default

  • Under Search Paths -> Import Paths add your search path to the C++ module (i.e, ./ProjectName/CxxTest).

  • This should now allow your to import your C++ Module into any .swift file.

//In ContentView.swift
import SwiftUI
import CxxTest

struct ContentView: View {
    var body: some View {
        Text("CxxTest function result: \(cxxFunction(7))")
            .padding()
    }
}

// In CxxTest.hpp

#ifndef CxxTest_hpp
#define CxxTest_hpp

int cxxFunction(int n);

#endif

// In CxxTest.cpp

#include "CxxTest.hpp"

int cxxFunction(int n) {
    return n;
}

Creating a Swift Package

After creating your Swift package project, follow the steps Creating a Module to contain your C++ source code in your Source directory

  • In your Package Manifest, you need to configure the Swift target's dependencies and compiler flags
  • In this example the name of the package is CxxInterop
  • Swift code will be in Sources/CxxInterop called main.swift
  • C++ source code follows the example shown in Creating a Module to contain your C++ source code
  • Under targets, add the name of your C++ module and the directory containing the Swift code as a target.
  • In the target defining your Swift target, add adependencies to the C++ Module, the path, source, and swiftSettings with unsafeFlags with the source to the C++ Module, and enable -cxx-interoperability-mode=default
//In Package Manifest

import PackageDescription

let package = Package(
    name: "CxxInterop",
    platforms: [.macOS(.v12)],
    products: [
        .library(
            name: "CxxTest",
            targets: ["CxxTest"]),
        .library(
            name: "CxxInterop",
            targets: ["CxxInterop"]),
    ],
    targets: [
        .target(
            name: "CxxTest",
            dependencies: []
        ),
        .executableTarget(
            name: "CxxInterop",
            dependencies: ["CxxTest"],
            path: "./Sources/CxxInterop",
            sources: [ "main.swift" ],
            swiftSettings: [.unsafeFlags([
                "-I", "Sources/CxxTest",
                "-cxx-interoperability-mode=default",
            ])]
        ),
    ]
)
  • We are now able to import our C++ Module into our swift code, and import the package into existing projects
//In main.swift

import CxxTest

public struct CxxInterop {

    public func callCxxFunction(n: Int32) -> Int32 {
        return cxxFunction(n: n)
    }
}

print(CxxInterop().callCxxFunction(n: 7))
//outputs: 7

Building with CMake

After creating your project follow the steps Creating a Module to contain your C++ source code

  • Create a CMakeLists.txt file and configure for your project
  • Inadd_library invoke cxx-support with the path to the C++ implementation file
  • Add the target_include_directories with cxx-support and path to the C++ Module ${CMAKE_SOURCE_DIR}/Sources/CxxTest
  • Add the add_executable to the specific files/directory you would like to generate source, withSHELL:-cxx-interoperability-mode=default.
  • In the example below we will be following the file structure used in Creating a Swift Package
// In CMakeLists.txt

cmake_minimum_required(VERSION 3.18)

project(CxxInterop LANGUAGES CXX Swift)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS OFF)

add_library(cxx-support ./Sources/CxxTest/CxxTest.cpp)
target_compile_options(cxx-support PRIVATE
  -fno-exceptions
  -fignore-exceptions)
target_include_directories(cxx-support PUBLIC
  ${CMAKE_SOURCE_DIR}/Sources/CxxTest)

add_executable(CxxInterop ./Sources/CxxInterop/main.swift)
target_compile_options(CxxInterop PRIVATE
  "SHELL:-cxx-interoperability-mode=default")
target_link_libraries(CxxInterop PRIVATE cxx-support)


//In main.swift

import CxxTest

public struct CxxInterop {
    public static func main() {
        let result = cxxFunction(7)
        print(result)
    }
}

CxxInterop.main()