[swift-users] Clarification on the role of `CustomStringConvertible`

Brent Royal-Gordon brent at architechies.com
Wed Jun 29 17:42:12 CDT 2016

> On Jun 29, 2016, at 1:33 PM, Daniel Dunbar via swift-users <swift-users at swift.org> wrote:
> I would like some clarification regarding the expected role of `CustomStringConvertible` for types which have a natural, lossless, string representation.
> I don't have good terminology to use here, but is the expectation that the "textual string representation" is a readable description of the object? This seems to correspond to the synthesized (?) default definition for struct types, as well as some of the uses for Foundation types without natural string representations (i.e. `Foundation.Notification` reports something like "name = foo, object = ..., userInfo = ...").
> Or, is the expectation that it provide a string *representation* of the object, for objects where that makes sense. This corresponds to the use in `Foundation.UUID`, or the example of `Point.description` from the stdlib's `CustomStringConvertible` documentation.

CustomStringConvertible is intended to be a non-localized but human-readable representation of the instance. It is permitted to be imprecise or lossy.

Swift takes the position that it should always be possible to convert an instance into some kind of string. For pure Swift types, no default implementation is provided, but String's from-anything initializer itself attempts a number of increasingly desperate fallbacks, like the debugDescription and reflection. Objective-C is a little different; there, NSObject has a `description` property which subclasses can override, and a separate `debugDescription` property which calls through to `description` by default.

What I would suggest is:

* If there's an end-user-readable representation that captures the gist of the instance, use CustomStringConvertible.

* If you want to customize the way it's displayed but the audience is programers, use CustomDebugStringConvertible.

* Always use `String(_:)` (which is going to become `String(describing:)`) or string interpolation, rather than calling .description or .debugDescription directly. That will automatically choose the most human-readable representation available. (If you prefer the debug description, use `String(reflecting:)`.)

> Another way of phrasing this question is: for types which have a reversible string representation, should I expect `CustomStringConvertible.description` to give me a string I could use to reconstruct an instance _without_ knowing its type?

In some cases, like Ints or (potentially) UUIDs, the CustomStringConvertible representation actually is lossless and unambiguous and can be converted back into an identical instance. SE-0089 introduces a LosslessStringConvertible protocol which refines CustomStringConvertible to add an `init?(_ description: String)` initializer. There's no guarantee it'll make Swift 3, but people are working on implementing it.


However, LosslessStringConvertible should only be used when the *human-readable* representation happens to be machine-readable as well. Human readability is the first priority with `description`.

Brent Royal-Gordon

More information about the swift-users mailing list