[swift-users] Memory management without ARC?

[Jens Alfke]

> On Jan 30, 2016, at 7:17 AM, Matthias Zenger via swift-users <swift-users at swift.org> wrote:
> 
> So, my question is: What are Swift programmers supposed to do if they are dealing with inherently cyclic data structures that cannot be broken up via weak references? It almost seems like there's some memory management functionality missing in Swift that makes this possible.

You need to write code that walks through the data structures, recognizes which parts are no longer live, and clears the strong references to them. This is one of the age-old fundamental problems with reference counting — I remember dealing with it in Smalltalk-80, whose GC was based on refcounts.

I think you’re going to have to end up writing a mark/sweep garbage collector for your Scheme interpreter. A basic implementation is really simple.

Just off the top of my head, I think you’d use only unowned references between your Scheme objects. Then to keep the objects alive, you have a master array/set/whatever that holds strong references to all of them. After doing the recursive marking of all your objects, you traverse the master set and remove all the objects whose mark flags aren’t set.

—Jens
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