Interoperating with C pointer to an Int array in Swift

Question

I've got a legacy codebase with this function:

-(void)foo: (int *)buffer {
  myArray[0] = buffer[0];
  myArray[1] = buffer[1];
  myArray[2] = buffer[2];
  myArray[3] = buffer[3];
}

In Swift, this method is written as the following:

func foo(buffer: UnsafeMutablePointer<Int32>) {
}

The int pointer is now considered an UnsafeMutablePointer<UInt32>. I'm having trouble accessing the subscripts of buffer in Swift; i.e. I cannot call buffer[0] like this:

func foo(buffer: UnsafeMutablePointer<Int32>) {
  myArray[0] = buffer[0] // won't compile, buffer treated as Int32
}

Out of desperation, even knowing that UnsafeMutablePointer<Int32> is ultimately resolved as a Int32 type in Swift, I've tried the following:

guard let buffer = buffer.memory as? [Int] else { fatalError() }

guard let buffer = buffer as? [Int] else { fatalError() }

guard let buffer = buffer.memory as? [Int32] else { fatalError() }

guard let buffer = buffer as? [Int32] else { fatalError() }

Could someone point me to the right direction on understanding how I could access the subscript members like how the Objective-C code did?

Related: Converting an UnsafePointer with length to a Swift Array type. — Martin R
– Martin R, Commented Jun 23, 2016 at 18:21

Code Different · Accepted Answer · 2016-06-23 17:52:32Z

3

You are overthinking it:

class MyClass {
    var myArray = [Int32](count: 4, repeatedValue: 0)

    func foo(buffer: UnsafeMutablePointer<Int32>) {
        for i in 0..<4 {
            myArray[i] = buffer[i]
        }
    }
}

You can change myArray to [Int32] to make it match buffer. Or you can cast buffer to Int:

myArray[i] = Int(buffer[i])

answered Jun 23, 2016 at 17:52

Code Different

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5 Comments

Kelvin Lau Over a year ago

Thanks, I did overthink it; For future reference to others, UnsafeMutablePointer<Int32> DOES allow for subscripting.

Kelvin Lau Over a year ago

As a side note, I'm curious why guard let buffer = buffer.memory as? [Int32] else { fatalError() } would fail. Any thoughts on that?

Code Different Over a year ago

memory points to the first element of your array, not the array itself hence not castable

Kelvin Lau Over a year ago

How about guard let buffer = buffer as? [Int32] else { ... }?

Alexander Over a year ago

Didn't realize UnsafeMutablePointer was subscriptable, that's good to know. Not a fan of this approach in general, as there's a cleaner, built-in way to do this

Alexander · Accepted Answer · 2016-06-23 18:04:22Z

3

UnsafePointer is meant to model a single location in memory. For dealing with a buffer, use UnsafeBufferPointer.

UnsafeBufferPointer can be subscripted like an Array, and has bound checking (since it requires you give it a count).

You don't even have to do that manual conversion like in your Objective C code, you can initialize Array directly from UnsafeBufferPointer, very easily:

func foo(buffer: UnsafeMutablePointer<Int32>) {
    myArray = Array(UnsafeBufferPointer(start: pointer, count: 4))
}

This solution is even generic: UnsafeBufferPointer can infer its type from the UnsafePointer you give it, and pass that info along to Array, to make it automagically create a [Int32]

Note: Remember to deallocate your UnsafePointer!

edited Jun 23, 2016 at 18:04

answered Jun 23, 2016 at 17:48

Alexander

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3 Comments

Kelvin Lau Over a year ago

Thanks. Really cool approach. I confirmed that this also works. May I ask if you know of a method to safely find the count of the UnsafeMutablePointer<Int32>?

Alexander Over a year ago

Nope, this is too "low level" for that. The UnsafeMutablePointer just knows an address and a type. It has no way of knowing the length, which is why you have to supply it. If you're dealing with a C string, you can iterate to find the first NUL character. Other than that, there's no way to know

Alexander Over a year ago

You can probably obtain the length from whatever source gave you the pointer.

tommybananas · Accepted Answer · 2016-06-23 17:47:58Z

1

Maybe you instead use UnsafeBufferPointer?

http://swiftdoc.org/v2.2/type/UnsafeBufferPointer/

answered Jun 23, 2016 at 17:47

tommybananas

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