Use nontrivial C++ library in Swift package

Using Swift
1

I'd like to use the C++ library GitHub - google/robotstxt: The repository contains Google's robots.txt parser and matcher as a C++ library (compliant to C++11). in a Swift package, but I'm having trouble putting it all together.

I've reached a point where Xcode recognizes and can import robotstxt, but am stuck with the following build error:

error: 'googlebot.RobotsMatcher' cannot be constructed because it has no accessible initializers

At a very high level, my approach so far has been the following:

  1. Create a Swift executable package.
  2. Fetch and build the external library. (One caveat here is that the main class for this library has deleted its copy constructor, making it a noncopyable type. In this case we need to map it to a Swift reference type before building).
  3. Move the library headers into Sources/robotstxt/include and the built libs into Sources/robotstxt/lib.
  4. Create a modulemap for robotstxt.
  5. Set up Package.swift with the library target and set the headers and linker path to the ones mentioned above.

After all this, I can import robotstxt in main.swift and can even see code completion for its various symbols. However, when I run swift build, I get the error error: 'googlebot.RobotsMatcher' cannot be constructed because it has no accessible initializers.

Package.swift

// swift-tools-version: 5.10
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "spm-with-cpp",
    targets: [
        .target(
            name: "robotstxt",
            linkerSettings: [
                .unsafeFlags(["-L./Sources/robotstxt/lib"]),
                .linkedLibrary("robots"),
            ]
        ),
        .executableTarget(
            name: "spm-with-cpp", 
            dependencies: ["robotstxt"],
            swiftSettings: [.interoperabilityMode(.Cxx)]
        ),
    ]
)

Sources/robotstxt/include/module.modulemap

module robotstxt {
    header "robots.h"
    export *
}

Sources/spm-with-swift/main.swift

// The Swift Programming Language
// https://docs.swift.org/swift-book

import robotstxt

let _ = googlebot.RobotsMatcher()
// Error: 'googlebot.RobotsMatcher' cannot be constructed because it has no accessible initializers

Appendix - the specific steps i took

# 1. Initialize the package
swift package init --type executable
mkdir Sources/spm-with-cpp
mv Sources/main.swift Sources/spm-with-cpp
mkdir External && cd External

# 2. Fetch the library
git clone https://github.com/google/robotstxt
cd robotstxt
# Edit  robots.h to include `<swift/bridging>` and add `SWIFT_UNSAFE_REFERENCE` to the end of the RobotsMatcher class declaration.

# 3. Build the library
mkdir c-build && c-build
cmake .. # with cmake version 3.30.0
make

# 4. Move the headers and build artifacts to the target
cd ../../..
mkdir -p Sources/robotstxt/include/absl
mkdir Sources/robotstxt/lib
cp External/robotstxt/robots.h Sources/robotstxt/include
cp -R External/robotstxt/c-build/libs/abseil-cpp-src/absl/* Sources/robotstxt/include/absl
find Sources/robotstxt/include/absl -name '*.cc' | xargs rm
find Sources/robotstxt/include/absl -name '*.c' | xargs rm
cp External/robotstxt/c-build/librobots.a Sources/robotstxt/lib
cp External/robotstxt/c-build/librobots.dylib Sources/robotstxt/lib

Appendix - actual usage of the C++ library

Helpfully, the google library also ships with a CLI executable. Seeing how they use the RobotsMatcher class, it looks very straightforward.

googlebot::RobotsMatcher matcher;
std::string url = argv[3];
bool allowed = matcher.AllowedByRobots(robots_content, &user_agents, url);
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2

It turns out the problem was that the class googlebot.RobotsMatcher needed to be mapped to a Swift reference type.

The class declaration deletes its copy constructor and disallows assignment:

// Disallow copying and assignment.
RobotsMatcher(const RobotsMatcher&) = delete;
RobotsMatcher& operator=(const RobotsMatcher&) = delete;

Mixing Swift and C++ says:

C++ structures and classes with a deleted copy constructor are represented as non-copyable Swift types (~Copyable). If a C++ type has a valid copy constructor, it is still possible to make it non-copyable in Swift by annotating it with a SWIFT_NONCOPYABLE macro.

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