[swift-users] Can I use a tuple to force a certain memory layout?

[Johannes Weiß]

Hi,

> On 20 Jul 2017, at 5:41 pm, Taylor Swift <kelvin13ma at gmail.com> wrote:
> 
> Does addressof count as legally observing it?
> 
>         var buffers:(GL.UInt, GL.UInt) = (0, 0)
>         glGenBuffers(n: 2, buffers: &buffers.0)
> 
> Also, I assume Swift performs a swizzle if the tuple is defined in a separate module from where the pointer to it is constructed?

yes, that's legal assuming the called function doesn't store the pointer and read/write it later.


> 
> On Thu, Jul 20, 2017 at 4:59 AM, Johannes Weiß <johannesweiss at apple.com> wrote:
> When you can (legally) observe it, tuples in Swift have guaranteed standard C-style layout.
> 
> John McCall confirms this here: https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20170424/004481.html
> 
> > On 20 Jul 2017, at 4:33 am, Taylor Swift via swift-users <swift-users at swift.org> wrote:
> >
> > Many APIs like OpenGL take arrays where the atomic unit is multiple elements long. For example, a buffer of coordinates laid out like
> >
> > :[Float] = [ x1, y1, z1, x2, y2, z2, ... , xn, yn, zn ]
> >
> > I want to be able to define in Swift (i.e., without creating and importing a Objective C module) a struct that preserves the layout, so that I can do withMemoryRebound(to:capacity:_) or something similar and treat the buffer as
> >
> > struct Point
> > {
> >     let x:Float,
> >         y:Float,
> >         z:Float
> > }
> >
> > :[Point] = [ point1, point2, ... , pointn ]
> >
> > The memory layout of the struct isn’t guaranteed, but will the layout be guaranteed to be in declaration order if I use a tuple inside the struct instead?
> >
> > struct Point
> > {
> >     let _point:(x:Float, y:Float, z:Float)
> >
> >     var x:Float
> >     {
> >         return self._point.x
> >     }
> >
> >     var y:Float
> >     {
> >         return self._point.y
> >     }
> >
> >     var z:Float
> >     {
> >         return self._point.z
> >     }
> > }
> >
> > This is an ugly workaround, but I can’t really think of any alternatives that don’t involve “import something from Objective C”. I am aware that the implementation of structs currently lays them out in declaration order, but I’m looking for something that’s actually defined in the language.
> >
> >
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> 
>