I'm implementing a COW big int type but am running into problems with
non-mutating functions (e.g. the + operator). Simplified example code below
shows AFAIK the default way to implement COW, but the non-mutating method
doesn't identify the reference as unique (even with -O), resulting in a
needless copy.
I've tried everything I could think of, but only inout parameters seem to
work. How does the standard library do this, for e.g. String + String?
struct Foo {
var storage = Storage()
class Storage { var x = 0 }
init(_ x: Int) { storage.x = x }
mutating func negate() {
if !isKnownUniquelyReferenced(&storage) {
print("Copy")
}
storage.x = -storage.x
}
func negated() -> Foo {
var result = self // This counts as a second reference
result.negate()
return result
}
}
func test() {
var a = Foo(5)
a.negate()
print(a.storage.x)
let b = Foo(5)
let c = b.negated()
print(c.storage.x)
}
test()
*Output*
-5
Copy
-5
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