[swift-evolution] Pitch: Copy constructors for structs

[Charles Srstka]

Introduction:

This is a request for a copy constructor mechanism for structs in Swift.

Motivation:

Suppose you have a class stored inside a struct, like so:

class C {
	func copy() -> C { … }
}

struct S {
	var i: Int
	var c: C
}

and you create a couple of the structs, like so:

let c = C()
let foo = S(i: 1, c: c)
var bar = foo
bar.i = 2

Since the ‘bar’ variable was mutated, it now contains a copy of the original ‘foo’ struct. However, both structs still carry the same pointer to ‘c'. There may be cases where you would want a copy of the struct to make a copy of any reference types stored within; however, that does not seem to be possible currently.

Proposed Solution:

Adding a copy constructor to S that would be called when a copy of the struct is about to be made. This constructor would simply create a new instance, initialize it, and return it. The copy constructor would look like this:

struct S {
	var i: Int
	var c: C

	copy {
		return S(i: self.i, c: self.c.copy())
	}
}

Structs that do not implement the copy constructor would get the same behavior as they do currently.

Impact on Existing Code:

There should be no impact on existing code that does not implement the copy constructor.

Charles