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2628 :: "Error: expression has no address" issue with an array type

• def_pri_pub (orginal) [2016-11-19T03:04:35+01:00] view original

  Right now I'm trying to do a very tiny OpenGL in nim. I'm at the stage where I need to bind my buffer data. The compiler is giving me this error:

  
  Error: expression has no address
  Run

  Here is the C code I'm trying to port:

  float points = [
    0.0, 0.5, 0.0,
    0.5, -0.5, 0.0,
    -0.5, -0.5, 0.0
  ];
  
  // ...
  
  GLuint vbo = 0;
  glGenBuffers(1, &vbo);
  glBindBuffer(GL_ARRAY_BUFFER, vbo);
  glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), points, GL_STATIC_DRAW);
  Run

  And here is my nim code right now:

  let
    points = [
      0.0, 0.5, 0.0,
      0.5, -0.5, 0.0,
      -0.5, -0.5, 0.0
    ]
    numVertexBuffers = GlSizei(1)
    pointsBufferSize = GlSizeiptr(sizeof(points))
  
  # ...
  
  var
    vbo: GlUint
  
  glGenBuffers(numVertexBuffers, vbo.addr)
  glBindBuffer(GL_ARRAY_BUFFER, vbo)
  glBufferData(GL_ARRAY_BUFFER, pointsBufferSize, points.addr, GL_STATIC_DRAW)  # It errors here on `points.addr`
  
  Run

  Here is the definition for the glBufferData() proc: https://github.com/nimious/gles/blob/master/src/gles2/gl2.nim#L494 I have no idea what's wrong here.

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• def_pri_pub (orginal) [2016-11-19T04:16:12+01:00] view original

  Alright, I figured out my own solution. I had to change that let points to a var points.

  Can I get some insight into why I can't call addr on a let'd variable?

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• Arrrrrrrrr (orginal) [2016-11-19T09:38:39+01:00] view original

  You can do it with unsafeAddr. When you declare variables with the keyword let, you are telling the compiller that you want to protect it against any change. And having its address can potentially alter the variable.

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• def_pri_pub (orginal) [2016-11-19T18:37:48+01:00] view original

  Oh, did not know that. Thanks! I didn't find any mention of that in the Nim manual, it might be something to add here: http://nim-lang.org/docs/manual.html#statements-and-expressions-the-addr-operator

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