As you might have noticed in some previous examples,
reassigning a Table, tuple or object will create a copy of
it and its containing values.
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When working with references instead of with values is needed,
The ref keyword can be used.
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This declares a variable as a reference to an int.
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var ref1: ref int
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A declared, but unassigned variable of reference type will hold nil.
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echo "ref1 is nil? ", isNil ref1
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To create a reference, use the new proc.
The actual value of the reference will be zero-valued.
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var ref2 = int.new()
echo "ref2 is nil? ", isNil ref2
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When required, a reference can be de-referenced with [] operator.
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echo ref2[]
ref2[] = 5
echo ref2[]
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Trying to de-reference a nil reference will cause a crash.
Use isNil proc to check if reference points to a valid value.
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if not isNil ref1:
echo ref1[]
else:
echo "ref1 is nil!"
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An object type can be defined as reference-based by declaring it as ref object.
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type CatRef = ref object
name: string
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let c1 = CatRef(name: "Mittens")
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| When accessing object fields, de-referencing is not required (it is done implicitly). |
echo c1[].name, " == ", c1.name
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| Since the type is reference-based, reassignment will not make a copy of the object (contrary to value-based types). |
let c2 = c1
c2.name = "Kitty"
echo c1[], c2[]
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Here, we define a new object type Person, and a new object type PersonRef,
which is just a reference to the Person object.
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type Person = object
name: string
type PersonRef = ref Person
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let per_ref = PersonRef(name: "Bob")
echo per_ref[]
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PersonRef type can now be de-referenced to Person type.
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var per1: Person = per_ref[]
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| Assigning a de-referenced object will make a copy of it (de-referenced object modification does not modify reference's object). |
var per2: Person = per_ref[]
per_ref.name = "Alice"
per1.name = "Mary"
per2.name = "Jane"
echo per_ref[], per1, per2
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$ nim c -r references.nim ref1 is nil? true ref2 is nil? false 0 5 ref1 is nil! Mittens == Mittens (name: "Kitty")(name: "Kitty") (name: "Bob") (name: "Alice")(name: "Mary")(name: "Jane") |
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Previously: Objects