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5062 :: Tables - can't get the address of value

• r3c (orginal) [2019-08-01T21:40:49+02:00] view original

  I use table in type like:

  type
    RenderableObject* = object
      vertices_tbl*: Table[int, seq[float32]]
      indices_tbl*: Table[int, seq[uint32]]
  
  # init
  var FACE : RenderableObject
  FACE.vertices_tbl = initTable[int, seq[float32]]()
  FACE.indices_tbl  = initTable[int, seq[uint32]]()
  
  #init the seq
  proc CreateFace(f: int, pos: int) =
    if not hasKey(FACE.vertices_tbl, pos):
      FACE.vertices_tbl[pos] = @[]
      FACE.indices_tbl[pos] = @[]
  Run

  but in OpenGL function where i specify the container

  proc CreateBuffers*(obj: RenderableObject) {.inline.} =
  # # #
    glBufferData(GL_ARRAY_BUFFER, obj.vertices_tbl[f].len*sizeof(GLfloat), obj.vertices_tbl[f][0].unsafeAddr, GL_STATIC_DRAW)
  # # #
  Run

  It says it cant get the address.

  Is it because its not yet initialized? if I pass it with var like obj: var RenderableObject it works, but that kinda defeats the purpose and its very slow

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• demotomohiro (orginal) [2019-08-01T23:11:32+02:00] view original

  From: https://nim-lang.org/docs/manual.html#statements-and-expressions-the-addr-operator

  The addr operator returns the address of an l-value.

  From: https://nim-lang.org/docs/manual.html#procedures-var-return-type

  A proc, converter or iterator may return a var type which means that the returned value is an l-value and can be modified by the caller:

  In other words, when a return type is not a var type, it is not an l-value.

  From: https://nim-lang.org/docs/tables.html#%5B%5D%2CTable%5BA%2CB%5D%2CA https://nim-lang.org/docs/tables.html#%5B%5D%2CTable%5BA%2CB%5D%2CA_2

  proc `[]`A, B: B

  proc `[]`A, B: var B

  So in your CreateBuffers proc, proc `[]`[A, B](t: Table[A, B]; key: A): B is called and it doesn't return var type. You can get an address only from an l-value. That is why you cannt use unsafeAddr in your code.

  You can not access seq[0] when the lenght of the sequence is 0.

  I think your code shoulde be like:

  proc CreateBuffers*(obj: var RenderableObject) {.inline.} =
  # # #
    glBufferData(GL_ARRAY_BUFFER, obj.vertices_tbl[f].len*sizeof(GLfloat), if obj.vertices_tbl[f].len == 0: nil else: obj.vertices_tbl[f][0].unsafeAddr, GL_STATIC_DRAW)
  Run

  if I pass it with var like obj: var RenderableObject it works, but that kinda defeats the purpose of using tables and its very slow

  I don't understand neither why you are using tables in your code nor why var defeats the purpose of using tables only from your code.

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• r3c (orginal) [2019-08-01T23:43:28+02:00] view original

  When im creating the faces, i know their exact number, but to speedup the rendering, I want to merge them by their common texture. I dont know the number of the merged faces, so I have to make one loop trough all the faces and find the number of unique texture/lightmap combinations.

  Then from this number i have the size of the render data (number of merged faces) Then I do another loop to fill the data.

  With tables I dont have to do that, I can do that in the first loop

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