[swift-users] appending packed bytes to [UInt8]

[Kelvin Ma]

I thought tuples, like structs do *not* have fixed layout. And they get
padded too.

On Wed, Jan 10, 2018 at 5:01 PM, Nevin Brackett-Rozinsky <
nevin.brackettrozinsky at gmail.com> wrote:

> Conversely, since I’m pretty sure the memory layout of a *tuple* is
> guaranteed to be as it appears, I would probably start with something like
> this:
>
> typealias Vec3rgba = (x: Float, y: Float, z: Float, r: Int8, g: Int8, b:
> Int8, a: Int8)
>
> Nevin
>
>
> On Wed, Jan 10, 2018 at 4:27 PM, Jens Persson via swift-users <
> swift-users at swift.org> wrote:
>
>> I'm not sure what you mean by "I need the buffer to be a vector /.../"
>> but perhaps this may be of some help:
>>
>> struct S {
>>     var x: Float
>>     var y: Float
>>     var z: Float
>>     var r: UInt8
>>     var g: UInt8
>>     var b: UInt8
>>     var a: UInt8
>> }
>> var buf = S(x: 0.1, y: 1.2, z: 2.3, r: 11, g: 22, b: 33, a: 44)
>> print(MemoryLayout<S>.stride) // 16
>> withUnsafeMutableBytes(of: &buf) { ptr in
>>     print("x:", ptr.load(fromByteOffset: 0, as: Float.self)) // 0.1
>>     print("y:", ptr.load(fromByteOffset: 4, as: Float.self)) // 1.2
>>     print("z:", ptr.load(fromByteOffset: 8, as: Float.self)) // 2.3
>>     print("r:", ptr.load(fromByteOffset: 12, as: UInt8.self)) // 11
>>     print("g:", ptr.load(fromByteOffset: 13, as: UInt8.self)) // 22
>>     print("b:", ptr.load(fromByteOffset: 14, as: UInt8.self)) // 33
>>     print("a:", ptr.load(fromByteOffset: 15, as: UInt8.self)) // 44
>> }
>>
>> NOTE however that the memory layout of Swift-structs is not guaranteed to
>> remain like this and is thus not future-proof, although I think that
>> if/when things changes, there will be some way to tell the compiler that
>> you want the memory to be this "expected" "C-like" layout.
>>
>> /Jens
>>
>>
>> On Wed, Jan 10, 2018 at 10:03 PM, Kelvin Ma via swift-users <
>> swift-users at swift.org> wrote:
>>
>>> I want to create a buffer with the layout
>>>
>>> 0           4           8           12      13      14      15      16
>>> [   x:Float   |   y:Float   |   z:Float   | r:UInt8 | g:UInt8 | b:UInt8
>>> | _:UInt8 ]
>>>
>>> Normally, I’d use UnsafeRawBufferPointer for this, but I need the
>>> buffer to be a vector (i.e. with append(_:)), and for it to return a
>>> Swift-managed Array<UInt8>. How should I do this? The project does not
>>> use Foundation.
>>>
>>> _______________________________________________
>>> swift-users mailing list
>>> swift-users at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-users
>>>
>>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20180110/0a0f587c/attachment.html>