Cannot convert empty array of arrays to JSON with Swift 2

Question

What is wrong with this piece of code (which was inspired by this example)? It currently prints JSON string "(<5b5d>, 4)" instead of the expected "[]".

var tags: [[String]] = []
// tags to be added later ...
do {
    let data = try NSJSONSerialization.dataWithJSONObject(tags, options: [])
    let json = String(data: data, encoding: NSUTF8StringEncoding)
    print("\(json)")
}
catch {
    fatalError("\(error)")
}

Martin R · Accepted Answer · 2015-07-31 08:02:40Z

4

Short answer: The creation of the JSON data is correct. The problem is in the conversion of the data to a string, what you want is the NSString method:

let json = NSString(data: data, encoding: NSUTF8StringEncoding) as! String

which produces the expected [].

Slightly longer answer: Your code

let json = String(data: data, encoding: NSUTF8StringEncoding)

calls the String init method

/// Initialize `self` with the textual representation of `instance`.
/// ...
init<T>(_ instance: T)

and the result is the textual representation of the tuple (data: data, encoding: NSUTF8StringEncoding):

(<5b5d>, 4)

Actually you can call String() with arbitrary arguments

let s = String(foo: 1, bar: "baz")
print(s) // (1, "baz")

in Swift 2. This does not compile in Swift 1.2, so I am not sure if this is intended or not. I have posted a question in the Apple Developer Forums about that:

String init accepts arbitrary arguments.

edited Jul 31, 2015 at 8:02

answered Jul 31, 2015 at 7:10

Martin R

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5 Comments

Drux Over a year ago

What a "bridge"! :) Thx.

gnasher729 Over a year ago

Absolutely brilliant. Anyway, you only convert JSON data to a string for debugging.

gnasher729 Over a year ago

And I haven't found any guarantee anywhere that the JSON data would be UTF-8 encoded.

Martin R Over a year ago

@gnasher729: It's not a guarantee, but tools.ietf.org/html/rfc7159#section-8.1 states: "The default encoding is UTF-8, ...". – On the other hand, OP's original code now works as expected, because String has an init?(data: NSData, encoding: NSStringEncoding) method now.

Martin R Over a year ago

@gnasher729: Also the documentation for dataWithJSONObject explicitly states: "The resulting data is encoded in UTF-8."

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