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8551 :: How to pass a "plain type" to a proc that expects a "ref type" in Nim ?

• Serge (orginal) [2021-10-27T12:39:47+02:00] view original

  In C or C++ or in Go it would just be a matter of using a reference operator : f(&x), x being a "plain type".

  How does it work in Nim?

  Ex: in the example hereunder, how do you pass ob1 to say() (obviously, it is not say(&ob1) or say(addr(ob1)))

  
  type
      rob = ref ob
      ob = object
          key: int
  
  proc say(r : rob) =
      echo r.key
  
  var ob1 = ob(key:42)
  Run

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• haxscramper (orginal) [2021-10-27T12:47:05+02:00] view original

  ref object and just object are different in nim - first one is a heap-allocated object while second one is stack allocated. You would have to copy the data using var rval: rob; new(rval); rval[] = ob

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• Serge (orginal) [2021-10-27T13:03:21+02:00] view original

  Thanks @haxscramper.

  So basically when planning to code an application in Nim, one has to decide beforehand whether to represent a given object as a "value type" or as a "reference type" and stick consistently with that choice throughout the app (otherwise there is the cost of making copies).

  An additional consequence is that this choice is basically dictated for you when you use a third-party library (which returns a given object as a "ref type" or as a "plain type").

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• shirleyquirk (orginal) [2021-10-27T13:44:45+02:00] view original

  In C or C++ or in Go f(&x)

  This is probably closer to a var T parameter in Nim, rather than a ref T, but the point is you certainly can't tell just by looking at it!

  Nim slices the different uses and usecases of pointers/references/addresses differently--more thoughtfully.

  as well as ref and ptr there's var, lent, {.byaddr.}, openArray proc... they work best when you can forget that a pointer is involved somewhere.

  instead of "The C++ rvlaue lifetime disaster" , the conflation between value category and lifetime, we have affine type annotations: sink et al.

  one has to decide beforehand whether to represent a given object as... ...choice is basically dictated for you when you use a third-party library..

  well, yes. ¯\_(ツ)_/¯ engineering & designing means making decisions.

  but type Foo = object; type FooRef = ref Foo; or type BarObj = object; type Bar = ref BarObj is an idiom

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• auxym (orginal) [2021-10-27T23:50:32+02:00] view original

  Yes, but keep in mind that Nim may pass references internally in some cases, even if your type is a value type. For example "var" arguments in prices are passed as reference (internally, as an implementation detail) so that it can be mutated. Non-var arguments may also be passed as a reference if that data is large enough, as a performance optimization.

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