[swift-evolution] Pitch: Copy constructors for structs

[Joe Groff]

Currently, in most places where this is desired, we treat the reference type's interface as part of the value's interface and use copy-on-write to manage the referenced storage, as with arrays, dictionaries, and sets. Eager copying by a C++-like mechanism is interesting, and we designed the runtime with an eye toward future C++ interop, but this still has pretty massive impacts on the rest of the language and implementation that need deeper consideration than "no impact on existing code". When exactly are copy constructors run? When, if ever, is the compiler allowed to elide copies? Is move optimization possible? Do you have destructors, and if so, when are they run? How and when do we know copying a shared value can be done in a thread-safe way? In addition to looking at the C++ model, I'd also recommend thinking about simply allowing custom retain/release of otherwise bitwise-identical copies, or something like D's "post-blit constructor", the design of which avoids some of the pitfalls of C++.

-Joe

> On Dec 23, 2015, at 1:03 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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
> 
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