Parameter type UnsafePointer<Void> in glDrawElements - ios

Question

I am trying to get my context (EAGLContext) to render in Swift; but for days I haven't been able to the function glDrawElements to work.
I have read a couple of similar questions here on Stackoverflow but to no avail.
My glDrawElements is as follows:
glDrawElements(GLenum(GL_TRIANGLES), GLsizei(Indices.count), GLenum(GL_UNSIGNED_BYTE), &offset)
I am having problem with the last parameter - offset, which expects a UnsafePointer<Void>.
I have tried the following:
let offset: CConstVoidPointer = COpaquePointer(UnsafePointer<Int>(0))
The above no longer works, because CConstVoidPointer doesn't seem to be available anymore in Swift 1.2.
And:
var offset = UnsafePointer<Int>(other: 0)
The above gives a warning that I should use bitPattern: instead.
Although I don't believe bitPattern: should be used here (because the parameter expects a Word), I decided to give it a try according to the suggestion provided such as the following:
var offset = UnsafePointer<Int>(bitPattern: 0)
glDrawElements(GLenum(GL_TRIANGLES), GLsizei(Indices.count), GLenum(GL_UNSIGNED_BYTE), &offset)
I would get a EXE_BAD_ACCESS (code=EXC_I386_GPFLT) error.
In vain, I also tried something as simple as the following using:
var offsetZero : Int = 0
and subsequently feeding it to the last parameter of glDrawElements like so:
glDrawElements(GLenum(GL_TRIANGLES), GLsizei(Indices.count), GLenum(GL_UNSIGNED_BYTE), &offsetZero)
I am getting the same EXE_BAD_ACCESS like in the case above.
How can I properly form a type suitable for the last parameter of glDrawElements that expects the type UnsafePointer<Void>?
#RobMayoff
Update (adding code for VBO set-up and variables declarations and definitions):
struct vertex
{
var position: (CFloat, CFloat, CFloat)
var color: (CFloat, CFloat, CFloat, CFloat)
}
var vertices =
[
vertex(position: (-1, -1, 0) , color: (1, 1, 0, 1)),
vertex(position: (1, -1, 0) , color: (1, 1, 1, 1)),
vertex(position: (-1, 0, 1) , color: (0, 1, 0, 1)),
vertex(position: (-1, 1, 1), color: (1, 0, 0, 1))
]
let indices: [UInt8] = [ 0, 2, 3, 3, 1, 0 ]
class mainGLView : UIView
{
var layer : CAEAGLLayer!
var context : EAGLContext!
var cBuffer : GLuint = GLuint()
var pos : GLuint = GLuint()
var color : GLuint = GLuint()
var iBuffer : GLuint = GLuint()
var vBuffer : GLuint = GLuint()
var vao : GLuint = GLuint()
override class func layerClass() -> AnyClass
{
return CAEAGLLayer.self
}
required init(coder aDecoder: NSCoder)
{
super.init(coder: aDecoder)
//setting up context, buffers, shaders, etc.
self.configureVBO()
self.setupRendering()
}
func configureVBO()
{
glGenVertexArraysOES(1, &vao);
glBindVertexArrayOES(vao);
glGenBuffers(GLsizei(1), &vBuffer)
glBindBuffer(GLenum(GL_ARRAY_BUFFER), vBuffer)
glBufferData(GLenum(GL_ARRAY_BUFFER), vertices.size(), vertices, GLenum(GL_STATIC_DRAW))
glEnableVertexAttribArray(pos)
var ptr = COpaquePointer(UnsafePointer<Int>(bitPattern: 0))
glVertexAttribPointer(GLuint(pos), GLint(3), GLenum(GL_FLOAT), GLboolean(GL_FALSE), GLsizei(sizeof(vertex)), &ptr)
glEnableVertexAttribArray(color)
let fColor = UnsafePointer<Int>(bitPattern: sizeof(Float) * 3)
glVertexAttribPointer(GLuint(color), 4, GLenum(GL_FLOAT), GLboolean(GL_FALSE), GLsizei(sizeof(vertex)), fColor)
glGenBuffers(1, &iBuffer)
glBindBuffer(GLenum(GL_ELEMENT_ARRAY_BUFFER), iBuffer)
glBufferData(GLenum(GL_ELEMENT_ARRAY_BUFFER), indices.size(), indices, GLenum(GL_STATIC_DRAW))
glBindBuffer(GLenum(GL_ARRAY_BUFFER), 0)
glBindVertexArrayOES(0)
}
func setupRendering()
{
glBindVertexArrayOES(vao);
glViewport(0, 0, GLint(self.frame.size.width), GLint(self.frame.size.height));
indices.withUnsafeBufferPointer
{
(pointer : UnsafeBufferPointer<UInt8>) -> Void in
glDrawElements(GLenum(GL_TRIANGLES), GLsizei(indices.count), GLenum(GL_UNSIGNED_BYTE), UnsafePointer<Void>(pointer.baseAddress))
Void()
}
self.context.presentRenderbuffer(Int(GL_RENDERBUFFER))
glBindVertexArrayOES(0)
}
}

Answer 1

Because of your choices for the count and type arguments, the indices argument (the last argument) needs to be a pointer to the first element of an array, of (at least) length Indices.count, of unsigned bytes. This pointer needs to be converted to UnsafePointer<Void> in Swift.
You didn't show the declaration of your Indices. Let's assume it's declared like this:
let indices: [UInt8] = [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]
Then you can call glDrawElements by jumping through these hoops:
indices.withUnsafeBufferPointer { (pointer: UnsafeBufferPointer<UInt8>) -> Void in
glDrawElements(GLenum(GL_TRIANGLES), GLsizei(indices.count),
GLenum(GL_UNSIGNED_BYTE),
UnsafePointer<Void>(pointer.baseAddress))
Void()
}