Swift: convert const char ** output parameter to String

Question

I'm interacting with a C++ library (with the header in C) which uses const char ** as an output parameter.

After executing a method in that library, the value I need is written in that variable, for example:

CustomMethod(const char **output)

CustomMethod(&output)

// Using the `output` here

Normally, in Swift it's possible to pass just a standard Swift String as a parameter and it will be transparently transformed into the const char * (Interacting with C Pointers - Swift Blog).

For example, I already use the following construct a lot with the same library:

// C
BasicMethod(const char *input)

// Swift
let string = "test"
BasicMethod(string)

However, when it comes to working with const char **, I couldn't just pass a pointer to the Swift String, as I'd expected:

// C
CustomMethod(const char **output)

// Swift
var output: String?
CustomMethod(&output)

Getting an error:

Cannot convert value of type 'UnsafeMutablePointer<String?>' to expected argument type 'UnsafeMutablePointer<UnsafePointer?>' (aka 'UnsafeMutablePointer<Optional<UnsafePointer>>')

The only way I could make it work is by manipulating the pointers directly:

// C
CustomMethod(const char **output)

// Swift
var output: UnsafePointer<CChar>?
CustomMethod(&output)
let stringValue = String(cString: json)

Is there any way to use the automatic Swift string to const char ** conversion, or does it only work with const char *?

To answer the question from the last paragraph - no, it's not possible to get automatic conversion, you'll have to write some Swift pointer code. — Cristik
– Cristik, Commented Jun 21, 2021 at 14:41

Cristik · Accepted Answer · 2021-06-21 15:04:35Z

3

The bridged C function expects a mutable pointer to a CChar pointer, so you'll need to provide one, there's no automatic bridging here.

var characters: UnsafePointer<CChar>?
withUnsafeMutablePointer(to: &characters) {
    CustomMethod($0)
}

if let characters = characters {
    let receivedString = String(cString: characters)
    print(receivedString)
}

Same code, but in a more FP manner:

var characters: UnsafePointer<CChar>?
withUnsafeMutablePointer(to: &characters, CustomMethod)

var receivedString = characters.map(String.init)
print(receivedString)

edited Jun 21, 2021 at 15:04

answered Jun 21, 2021 at 14:58

Cristik

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1 Comment

Richard Topchii Over a year ago

Thanks for your clarification! It seems that I've missed withUnsafeMutablePointer method, which is important according to the developer documentation: developer.apple.com/documentation/swift/…

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