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5046 :: FFI: help converting macro into template

• mantielero (orginal) [2019-07-24T19:09:21+02:00] view original

  I am trying to convert C macro into a template. But I am struggling to understand the C language code.

  The macro is (defined here in case you need some context):

  #define  r(vr) comp->r[vr]
  Run

  where comp is a struct of type ModelInstance.

  I barely remember C, but it looks like comp->r[vr] refers to component vr of the r array within comp struct. Am I right? But the ModelInstance definition doesn't seem to suggest it is an array.

  Any help interpreting this please?

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• treeform (orginal) [2019-07-24T22:58:17+02:00] view original

  Yes that should be right:

  template r(vr: untyped): untyped =  comp.r[vr]
  Run

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• hugogranstrom (orginal) [2019-07-25T00:22:00+02:00] view original

  But the ModelInstance definition doesn't seem to suggest it is an array.

  fmi2Real *r;
  Run

  In C, arrays are a pointer to the first element in the array, thus r is both a pointer to a fmi2Real and an array of fmi2Reals.

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• hugogranstrom (orginal) [2019-07-25T09:53:09+02:00] view original

  Are comp a ModelInstance or is it a pointer to a ModelInstance? In C the "->" operator takes a pointer to a struct and the field we want and then dereferences it. The equivalent Nim code in that case is:

  template r(vr: untyped): untyped = comp[].r[vr]
  Run

  And if you want to write it like in the C code this template does it:

  template `->`(s, f: untyped): untyped = s[].f
  Run

  Then the template becomes:

  template r(vr: untyped): untyped = comp->r[vr]
  Run

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