CODEWITHSUNDEEP
carraysswiftunsafe-pointers
Ephemera
Ephemera

Reputation: 8970

Converting an UnsafePointer with length to a Swift Array type

I'm looking for the simplest ways to achieve reasonable C interoperability in Swift, and my current block is converting an UnsafePointer<Int8> (which was a const char *), into an [Int8] array.

Currently, I have a naïve algorithm that can take an UnsafePointer and a number of bytes and converts it to an array, element by element:

func convert(length: Int, data: UnsafePointer<Int8>) {

    let buffer = UnsafeBufferPointer(start: data, count: length);
    var arr: [Int8] = [Int8]()
    for (var i = 0; i < length; i++) {
        arr.append(buffer[i])
    }
}

The loop itself can be sped up by using arr.reserveCapacity(length), however that does not remove the issue of the loop itself.

I'm aware of this SO question which covers how to convert UnsafePointer<Int8>to String, however String is a different beast entirely to [T]. Is there a convenient Swift way of copying length bytes from an UnsafePointer<T> into a [T]? I'd prefer pure Swift methods, without passing through NSData or similar. If the above algorithm is really the only way to do it, I'm happy to stick with that.

Upvotes: 33

Views: 21954

Answers (3)

Moose
Moose

Reputation: 2737

You can also use the Array init function:

init(unsafeUninitializedCapacity: Int, initializingWith initializer: (inout UnsafeMutableBufferPointer<Element>, inout Int) throws -> Void) rethrows

First convert the unsafeRawPointer to unsafePointer if needed Then convert the pointer to a buffer pointer, and finally the buffer pointer to an array.

Example, assuming you have an unsafeRawPointer (dataPtr) and its size (dataSize)

let numberOfItems = dataSize / MemoryLayout<MyClass>.stride

let myArray = dataPtr.withMemoryRebound(to: MyClass.self,
                                        capacity: numberOfItems) { typedPtr in
    // Convert pointer to buffer pointer to access buffer via indices
    let bufferPointer = UnsafeBufferPointer(start: typedPtr, count: numberOfItems)
    // Construct array
    return [MyClass](unsafeUninitializedCapacity: numberOfItems) { arrayBuffer, count in
        count = numberOfItems
        for i in 0..<count {
            arrayBuffer[i] = bufferPointer[i]
        }
    }
}

Upvotes: 0

Martin R
Martin R

Reputation: 539765

You can simply initialize a Swift Array from an UnsafeBufferPointer:

func convert(length: Int, data: UnsafePointer<Int8>) -> [Int8] {

    let buffer = UnsafeBufferPointer(start: data, count: length);
    return Array(buffer)
}

This creates an array of the needed size and copies the data.

Or as a generic function:

func convert<T>(count: Int, data: UnsafePointer<T>) -> [T] {

    let buffer = UnsafeBufferPointer(start: data, count: count);
    return Array(buffer) 
}

where length is the number of items that the pointer points to.

If you have a UInt8 pointer but want to create an [T] array from the pointed-to data, then this is a possible solution:

// Swift 2:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {

    let buffer = UnsafeBufferPointer<T>(start: UnsafePointer(data), count: length/strideof(T));
    return Array(buffer) 
}

// Swift 3:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {
    let numItems = length/MemoryLayout<T>.stride
    let buffer = data.withMemoryRebound(to: T.self, capacity: numItems) {
        UnsafeBufferPointer(start: $0, count: numItems)
    }
    return Array(buffer) 
}

where length now is the number of bytes. Example:

let arr  = convert(12, data: ptr, Float.self)

would create an array of 3 Floats from the 12 bytes pointed to by ptr.

Upvotes: 52

tBug
tBug

Reputation: 827

extension NSData {

    public func convertToBytes() -> [UInt8] {
        let count = self.length / sizeof(UInt8)
        var bytesArray = [UInt8](count: count, repeatedValue: 0)
        self.getBytes(&bytesArray, length:count * sizeof(UInt8))
        return bytesArray
    }
}

You can convert row data to byts (Uint8)

Copy Extension and use it..

Upvotes: 2

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