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5539 :: Differences between simple assignment, shallowCopy and deepCopy

• drkameleon (orginal) [2019-11-16T13:09:02+01:00] view original

  I've been doing several experiments on the matter and read a lot, but to make sure II got it right, I would be grateful if I had your input.

  So...

  What is the difference between:

  • a = b
  • shallowCopy(a,b)
  • deepCopy(a,b)

  when:

  • a/b are objects
  • a/b are ref objects
  • a/b are strings or sequences
  • a/b is everything else (int - for example)

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• Stefan_Salewski (orginal) [2019-11-16T13:46:39+01:00] view original

  I think for most of us it works exactly as we suppose.

  Some years ago there was no exact documentation available, so indeed you request writing documentation. But I think explaining this is not that easy, as it should be understandable to newcomers, but at the same time not boring for experts. Maybe you can try yourself, and araq or mratsim can correct it. But maybe we need two version, the short for the manual, and the longer one for a beginner book.

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• treeform (orginal) [2019-11-16T17:02:22+01:00] view original

  At first I was like, what a simple question... then I started to explain it I was like whaaaaa

  You can think of it as shallowCopy with some extra.

  • a/b are objects - shallowCopy of the struct including packed objects (which might appear like deepCopy).
  • a/b are ref objects - shallowCopy only the ref is copied.
  • a/b are strings or sequences - shallowCopy only the size and the array is copied (which might appear like deepCopy but first layer of the array is not deep, might appear like deepCopy for non refs)
  • a/b is everything else (int - for example) - shallowCopy

  Or you can think of it as deepCopy till you get a nim ref or a nim pointer, then just the pointer is copied.

  • a/b are objects - deepCopy till you get a nim ref or a nim pointer
  • a/b are ref objects - shallowCopy only the ref is copied.
  • a/b are strings or sequences - deepCopy till you get a nim ref or a nim pointer (note internal C pointer to array data does not count as a "nim pointer")
  • a/b is everything else (int - for example) - deepCopy that's kind of like shallowCopy as it does not matter for leaf objects.

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• mratsim (orginal) [2019-11-17T00:06:16+01:00] view original

  There are 2 semantics in Nim and low-level programming languages like C, C++, Rust.

  1. Value semantics.
  2. Reference semantics.

  Value semantics means that the copy owns its memory and lives separately from the copied.

  Reference semantics means that the copy and the copied refer to the same underlying memory location.

  Now:

  • raw objects have value semantics: on assignment you get a full copy. On deepCopy you get a full copy and on shallowCopy you also get a full copy. They are also called trivial types or POD for Plain Old Data.

  Now you might want to now why shallowCopy actually deep copies, well that's because they are values they don't refer to any memory location so you can't just copy the reference.

  • ref objects have reference semantics. When you do an assignment you actually copy the reference to a memory location. A shallow copy does the same, a deepCopy instead will recursively create a new memory location and copy the memory referred to to that location.
  • sequence and strings have value semantics. They copy on assignments. This avoids many errors where you copy an objects, modify it and use the old one and don't realize that it has been modified. Now internally they are implemented with a reference to a resizable and movable memory location. And copying that is quite costly so Nim exposes the internal details via shallowCopy when avoiding such a copy is desirable.
  • int, bool, enum are like objects, trivial types/POD.

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• drkameleon (orginal) [2019-11-18T08:00:08+01:00] view original

  Thanks a lot. Very thorough explanation!

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• Araq (orginal) [2019-11-18T09:18:31+01:00] view original

  In additon to what @mratsim said both shallowCopy and deepCopy will likely be deprecated. shallowCopy(x, y) should have been x = move(y) and deepCopy should be something like send (send data to a different thread).

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• cdunn2001 (orginal) [2019-11-19T00:50:43+01:00] view original

  deepCopy has value independent of memory management. I don't use it much, if it's not a problem I'd vote for it to remain.

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