Implement swift API for C++ multi-type structure

Using Swift
1

Consider a C++ method that retrieve struct of native typed arguments like enum class, sub-structs, std::string, int, etc...

I'd like to create a swift API that return the same struct but in swift variables

for example :

class ErrorMessage {
 public:
  int status;
  std::string message;
};

class serverResponse {
 public:
  ErrorMessage error;
  std::string str_value;
  std::uint16_t int_val;
  std::time_t last_seen;
  EnumVal status;  
};

serverResponse getServerResponse();

So I'd like to convert it to the swift equivalent struct with native members

open class serverResponseSwift : NSObject {

    open class var error: ErrorMessage { get }

    open var str_value: String { get }

    open var int_val: UInt16 { get }

    open var status: EnumVal { get }
 };

I know that direct conversion is not yet possible so I need to use objective-C++ code as a mediator. So I've used a bridging header to include the converting method in objective-C++ which will look like this :

@interface Converter
- (serverResponseSwift) getServerStatusSwift;
@end

and the equivalent .mm file will implement the conversion function, but can I use the swift Class in objective-c in order to fill it up according to the CPP serverResponse ?

@implementation Converter
- (serverResponseSwift) getServerStatusSwift {
   serverResponse x = getServerResponse();  

    /// How do I create serverResponseSwift out of serverResponse
}

Thanks !

2

By the looks of it you are familiar with neither swift nor obj-c, so there'll be lots for you to cover. This should get you started:

// ----------
// ServerResponseObjc.h file
#import <Foundation/Foundation.h>

@class ServerResponseSwift;

@interface ServerResponseObjc: NSObject
- (ServerResponseSwift*)serverResponseSwift;
@end

// ----------
// ServerResponseObjc.mm file

#import "ServerResponseObjc.h"
#import <UIKit/UIKit.h> // or similar
#import "ModuleName-Swift.h" // "product module name" here with "-Swift.h" suffix
#import "ServerResponseCpp.h" // your c++ header

@implementation ServerResponseObjc
- (ServerResponseSwift*)serverResponseSwift {
    serverResponse* r = new serverResponse();
    // TODO: check for no error
    NSString* errorString = [[NSString alloc] initWithUTF8String:r->error.message.c_str()];
    NSDictionary* userInfo = @{NSLocalizedDescriptionKey: errorString};
    NSError* error = [[NSError alloc] initWithDomain:@"Error" code:r->error.status userInfo:userInfo];
    NSString* str = [[NSString alloc] initWithUTF8String:r->str_value.c_str()];
    NSDate* lastSeen = [NSDate dateWithTimeIntervalSince1970:r->last_seen];
    int status = 0; // TODO: status
    return [[ServerResponseSwift alloc] initWithError:error str_value:str int_val:r->int_val last_seen:lastSeen status:status];
}
@end

// ----------
// ServerResponseSwift.swift file

import Foundation

class ServerResponseSwift: NSObject {
    @objc init(error: NSError, str_value: String, int_val: UInt16, last_seen: Date, status: Int) {
        self.error = error
        self.str_value = str_value
        self.int_val = int_val
        self.last_seen = last_seen
        self.status = status
        super.init()
    }
    var error: NSError
    var str_value: String
    var int_val: UInt16
    var last_seen: Date
    var status: Int // TODO
};

A few notes:

  • you can model your ErrorMessage with NSError
  • you can use NS_ENUM with your EnumVal (that you didn't show)
  • you have to return a pointer to "ServerResponseSwift" from Obj-C and properly allocate it in there
  • instead of using init with parameters (as shown above) you can have an empty init + set individual fields.
  • you need to put @objc before members you want to be accessible from Obj-C
  • search for "Product Module Name" in Xcode settings. You can either change it in there or change it in the source file, the two must be the same.
3

Hi and thanks for the comprehensive solution.

From why I read... In swift it's better dealing with errors using exceptions, and not by passing the error reference variable. However, if I throw an exception in the objective-c or the cpp part (for example, in method - (ServerResponseSwift*)serverResponseSwift { , than it wouldn't be supported in the swift part..

So I thought about adding method to fetch the data from swift (besides @objc init) and use it every time I want to fetch some data... But perhaps there's a better way (I know that in init we cannot use exceptions) ?

   getData() {
        self.error = error
        // here I'll throw an exception if the error is set
        ...    
    }
4

Swift Error values are not exceptions, and it's important to drop whatever conceptions of error handling you have with them right away when using Swift or Obj-C. Using an error parameter in Obj-C will be automatically translated into throwing function in Swift (makes with throws) as long as it's properly structured.

5

@Jon_Shier . Hi and thanks for the comment. regarding your statement

Using an error parameter in Obj-C will be automatically translated into throwing function in Swift (makes with throws) as long as it's properly structured.

Perhaps you can give me minimal example so I can understand how it works ? I assume that if the signature of the objc method include (NSError *) as last parameter, it's parsed by the swift context and converted into exception if error != nil. right ?

7

@tera, I followed you guidelines, and it works great ! thanks.

Perhaps you can just add some usage example of how to call this API from swift code.
For example what i did, but i don't know how to handle the error - I'd like to get an exception if the object from type serverResponseSwift has valid error field

            let obj = ServerResponseObjc().serverResponseSwift()
            let dateFormatter = DateFormatter()
            self.textField = "address = " + obj!.address_
            self.textField += "\nport = " + "\(obj!.port_)"
8

As @Jon_Shier rightly mentioned swift error handling is not called exceptions: even if it uses similar try and catch keywords the mechanism is totally different. This shows a few examples: Handling Cocoa Errors in Swift.

Your obj-c serverResponseSwift needs to return BOOL and have NSError** as the last parameter, and in swift you'll call it with try try? or try! as in the examples.

Using and Creating Error Objects.

9

The thing is that I would rather return the swift object and not BOOL so that i can use it later on the code.
The NSError** part is hidden in swift so i don't mind adding it.

is there an option that I can write the objective-c signature with NSError** but returning the ServerResponseSwift object and still get the exception

In objc code :

@interface ServerResponseObjc
- (ServerResponseSwift*)serverResponseSwiftWithError:(NSError **)error;
@end

and use in swift like this :

do {
    let obj = ServerResponseObjc().serverResponseSwift()
} catch let error as NSError {
    print(error)
}

is it supported ?

11

Try this, it shall also work:

- (ServerResponseSwift*)serverStatusSwift:(NSError**)error;

and consistently return non nil when error == nil and return nil when error <> nil.

on swift side you'd call it like:

do {
    let result = try serverStatusSwift()
    // use result here
} catch {
    print(error)
}

or one of these:

let result = try? serverStatusSwift()
let result = try! serverStatusSwift()
12

Thanks, that exactly what I needed !