using Swift Library from c++ code - calllbacks ?

Question

Created Nov ’24

Replies 4

Boosts 0

Participants 2

Hello, Im developing an app entirely with C++, and I need to call various swift functions because it requires the Swift library. Ive seen several posts on forums about C++ callbacks and honestly I dont understand how its done exactly. I get the general idea but I am not able to understand it and make it work. I feel like people throw vague ideas and weird function names and everything gets confusing. Could anyone give me the smallest example that works please ?

Just to make sure you know what I mean, here is an example of what I want to do, but you dont have to generate the code exactly for this, I want an example that I can understand please, but the swift code has to depend on a swift library. I dont want to simply call a swift function that returns x*2 ...

{1} notif.swift file : coded in swift language include <UserNotifications/UserNotifications.h> function A that show notification in swift code.

{2} mainwindow.cpp file : coded in C++ language

import notif.swift ??

button connected to slot/function mybuttonclicked. MainWindow::mybuttonclicked(){ std::string my_result = call function A_from_swift_file(argument_1); } --The end ---

I wrote the notif.swift with '?' because I dont know how you include the swift file from your cpp code and I could not find that anywhere. Maybe it is obvious, but I would really appreciate getting some help on this, Thank you everyone

Boost

Answer 1

DTS Engineer OP

Apple

Nov ’24

Is this you over on Swift Forums? Or is that just a coincidence?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 2

buubuu66 OP

Nov ’24

yes

0

Answer 3

DTS Engineer OP

Apple

Nov ’24

I dont know how you include the swift file from your cpp code

OK. My C++ Fu is very weak, but I can help you get past that roadblock.

Here’s what I did today:

Using Xcode 16.1, I created a new project from the macOS > Command Line Tool template, choosing C++ as the language, and naming it Test768928.
I created a new file from the macOS > Swift template and named it MySwiftClass.swift.
This asked me whether I wanted to create a bridging header. I agreed to that. Xcode created Test768928-Bridging-Header.h in my project. That’s not directly related to this task, but I’ll come back to it below.

I populated the file with this code:

public class MySwiftClass {
    public init() {
        print("MySwiftClass lives!")
    }
}

I set the C++ and Objective-C Interoperability build setting to C++/Objective-C++.
I added this to main.cpp:
```
#include "Test768928-Swift.h"
```
Note that this header is managed by Xcode. It contains a C++ interface to all the Swift stuff that you can use from C++. The Test768928 component comes from the module name. For an executable target, like this one, that matches the product name.
And this code within the main function in that file:
```
auto i = Test768928::MySwiftClass::init();
```
Again, the Test768928 namespace comes from the module name.
I chose Product > Run. It printed:
```
Hello, World!
MySwiftClass lives!
```

You can also go the other way — use C++ from Swift — by including C++ headers in the bridging header that you created in step 3.

IMPORTANT There are two headers in play here:

The bridging header, Test768928-Bridging-Header.h, is managed by you. Edit it to include the headers for any C++ stuff that you want to use from Swift.
Test768928-Swift.h is managed by Xcode. Xcode automatically populates it in with a C++ interface to the Swift stuff that you can use from C++

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 4

buubuu66 OP

2w

Hello Quinn “The Eskimo!”, I tried to follow your steps, but could not understand the most crucial part of it all, which is the step 6. How do you create this header file exactly ? because the other bridging-header is actually for the other way around if I understand correctly. My goal is to use a swift function from C++, and even without Xcode ideally, but I can create the header via xcode before getting rid of xcode I suppose.

0