[swift-users] Class vs Structures

Taylor Swift kelvin13ma at gmail.com
Fri Jun 30 23:53:13 CDT 2017

An addendum: if something represents a specific “thing” that can’t be
duplicated without consequence, often that means it should be stored in one
specific place in your code, not made into a class.

On Fri, Jun 30, 2017 at 10:10 PM, Brent Royal-Gordon via swift-users <
swift-users at swift.org> wrote:

> On Jun 29, 2017, at 10:40 AM, Vitor Navarro via swift-users <
> swift-users at swift.org> wrote:
> Do you guys have any guideline regarding usage here?
> Here's my suggestion: Use a class when the object represents a specific
> *thing* that can't be duplicated without consequence; use a struct (or
> enum) when the instance represents some abstract data with no concrete
> existence. Some examples using framework types:
> * A piece of text is just some data; duplicating it doesn't do anything
> except maybe use more memory. So `String` should be a struct.
> * A label is a particular thing that exists at a particular place on
> screen; if you duplicated it, you'd end up with two labels on the screen,
> or with an off-screen copy of a label that wouldn't have any effect when
> you mutated it. So `UILabel` should be a class.
> * A URL is just some data; if you construct two URLs with the same
> contents, they are completely interchangeable. So `URL` should be a struct.
> * A connection to a web server to retrieve the contents of a URL is a
> particular thing; if you duplicated it, you would either establish another
> connection, or the two instances would interfere with each other (e.g.
> canceling one would cancel the other). So `URLSessionTask` and
> `NSURLConnection` are classes.
> Sometimes the same problem, approached in slightly different ways, would
> allow you to use either one. For instance, a database record is a
> particular *thing* and should probably be a class, but copy the values of
> the fields (perhaps omitting the ID) out of it and suddenly you have a
> plausible struct. As a *general* rule, it's usually better to use structs
> where possible because it's easier to reason about their behavior—mutations
> in one function don't suddenly pop up in a totally unrelated function—but
> sometimes a particular type works very easily as a class and very awkwardly
> as a struct. Ultimately, it's a judgement call.
> The other point I will make is this: "Protocol-oriented programming" is
> new and exciting and often very useful, but it's a tool, not a religion. If
> subclassing works well for your use case, then subclass away.
> --
> Brent Royal-Gordon
> Architechies
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