c - Converting an UnsafePointer with length to a Swift Array type

Question

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 na?ve 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.

Answer 1

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.