Using Structs (includes byte Array) with SWIFT - Struct to NSData and NSData to Struct

Question

In continuation of my original post: Using Structs (Bytes) with SWIFT - Struct to NSData and NSData to Struct

I'm now finding that if I have a struct with bytes mixes with a byte array it doesn't pack correctly.

Example Code:

struct exampleStruct {
    var ModelNumber: Byte
    var MajorVersion: Byte
    var MinorVersion: Byte
    var Revision: Byte
    var Message: [Byte]
}

var myStruct = exampleStruct (
    ModelNumber: 1,
    MajorVersion: 2,
    MinorVersion: 3,
    Revision: 4,
    Message: [0x48, 0x45, 0x4C, 0x4C, 0x4F] // HELLO
)

println(myStruct.Message) returns correct array with values of [72,69,76,76,79]

However, when I convert this struct to NSData using:

// Struct to NSData.
var data = NSData(
    bytes: & myStruct,
    length: sizeof(exampleStruct)
)

I get unexpected results of: "data: <01020304 00000000 108c91fd a87f0000>". I was expecting "data: <01020304 48454c4c 4f>"

It seems like this is because the [Byte] Array length is not set. Can it be set in SWIFT? When I try the following:

struct exampleStruct {
   var ModelNumber: Byte
   var MajorVersion: Byte
   var MinorVersion: Byte
   var Revision: Byte
   var Message: Byte[5] // << Set array length 5
}

I get a warning that states: "Fixed-length arrays are not yet supported".

Anyway to work around this limitation?

[Byte] is a Swift struct Array and not a C array.

Martin R
– Martin R

2014-12-04 21:11:09 +00:00
Commented Dec 4, 2014 at 21:11 — Martin R
– Martin R, Commented Dec 4, 2014 at 21:11

Martin R · Accepted Answer · 2014-12-04 21:19:09Z

2

var Message: [Byte] declares a variable of the type struct Array:

struct Array<T> : MutableCollectionType, Sliceable {

    /// The type of element stored by this `Array`
    typealias Element = T

    /// Always zero, which is the index of the first element when non-empty.
    var startIndex: Int { get }

    /// A "past-the-end" element index; the successor of the last valid
    /// subscript argument.
    var endIndex: Int { get }
    subscript (index: Int) -> T

    // ... and much more ...
}

so this is not just a "C array" of bytes. The actual storage is opaque and only accessible through methods and properties.

You can define a tuple of fixed size:

struct exampleStruct {
    var ModelNumber: Byte
    var MajorVersion: Byte
    var MinorVersion: Byte
    var Revision: Byte
    var Message: (Byte, Byte, Byte, Byte, Byte)
}

var myStruct = exampleStruct (
    ModelNumber: 1,
    MajorVersion: 2,
    MinorVersion: 3,
    Revision: 4,
    Message: (0x48, 0x45, 0x4C, 0x4C, 0x4F) // HELLO
)
var data = NSData(
    bytes: &myStruct,
    length: sizeof(exampleStruct)
)

println(data) // <01020304 48454c4c 4f>

However, I don't think that Swift makes any guarantees about the binary representation of its structures, so this may break in the future.

answered Dec 4, 2014 at 21:19

Martin R

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2 Comments

Mavro Over a year ago

This is great - Thanks! For some reason I keep thinking SWIFT is more similar to C than it actually is.

Martin R Over a year ago

@Mavro: You are welcome. – If you need a well-defined representation (e.g. for data exchange) then you are probably more on the safe side by defining a C struct and importing that into Swift (or define your own marshaling functions).

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