How to return a String from swift to cpp caller

Question

I am working on a C++ library with a Swift backend (the C++ code mainly serves as a bridge to Rust).

I have the following:

char* get_string(void* pBackend) {
    Backend* backend = (Backend*) pBackend;
    return backend.get_string ???; // or does this need more processing?
}

In turn Rust will call get_string from the cpp library, but never mutate it.

The question is how to properly handle this from the Swift side, I have lost count of all the numerous pointer swift and cpp types.

in Swift:

struct Backend {
  var myString: String? = nil
  func getString() -> ??? { // which type to return?
    if let s = myString {
      return ??? // how to get it?
    } else {
      "no String"
    }
  }
}

I know there may be some lifetime concerns, but as soon as Rust call get_string, it copies the contents.

(The backend has more internal functions that will actually set myString.)

You can just cast to convert a Swift String to std::string. See swift.org/documentation/cxx-interop/… — Sweeper
– Sweeper, Commented Sep 2 at 10:06

Sweeper · Accepted Answer · 2025-09-02 11:15:21Z

7

It is possible to just return a Swift String on the Swift side. Swift Strings can be easily converted to std::strings. See also the documentation.

// Make these public so that they can be accessed from C++
public struct Backend {
    var myString: String? = nil
    
    public func getString() -> String {
        return myString ?? "no String"
    }
}

On the C++ side, you can do:

char* get_string(void* pBackend) {
    Backend* backend = (Backend*) pBackend;
    swift::String stringFromSwift = backend->getString();
    std::string cppString = (std::string)stringFromSwift;
    return strdup(cppString.c_str());
}

Since the string needs to outlive get_string, it needs to be copied to the heap. The caller of get_string is responsible for freeing this.

answered Sep 2 at 11:15

Sweeper

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3 Comments

Plegeus Sep 2 at 12:05

Just a shame the returned char* needs to be freed given that the actual swift value will persist in Backend after the call. I do prefer this answer over @ali_nazzal's answer since it works on Backend directly making the swift code cleaner. I am also reluctant to use @_cdecl because Apple is not clear about what the future of this is going to be. Thank you.

Duncan C Sep 2 at 18:45

I don't use C++ very much lately. Does the C++ string c_str() function return a char* serialization of a C++ string object that supports unicode?

Sweeper Sep 3 at 13:57

@DuncanC I don't use C++ regularly either. The documentation for c_str() is here. As for Unicode support, this answer explains it quite well IMO.

Ali Nazzal · Accepted Answer · 2025-09-02 10:13:24Z

2

Return a C string with stable storage and a matching free. Don’t return a pointer into Swift’s String.

import Darwin

final class Backend { var myString: String? }

@_cdecl("backend_get_string")
public func backend_get_string(_ p: UnsafeMutableRawPointer?) -> UnsafeMutablePointer<CChar>? {
    guard let p else { return nil }
    let b = Unmanaged<Backend>.fromOpaque(p).takeUnretainedValue()
    let s = b.myString ?? "no String"
    return strdup(s)                 // caller must free
}

@_cdecl("backend_string_free")
public func backend_string_free(_ p: UnsafeMutablePointer<CChar>?) { if let p { free(p) } }

answered Sep 2 at 10:13

Ali Nazzal

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