Friday, August 12, 2022

Swift Pitch: “borrow” and “take” Parameter Ownership Modifiers

Joe Groff:

We propose new borrow and take parameter modifiers to allow developers to explicitly choose the ownership convention that a function uses to receive immutable parameters. This allows for fine-tuning of performance by reducing the number of ARC calls or copies needed to call a function, and provides a necessary prerequisite feature for move-only types to specify whether a function consumes a move-only value or not.


Looking to the future, as part of our ongoing project to add ownership to Swift, we will eventually have move-only values and types. Since move-only types do not have the ability to be copied, the distinction between the two conventions becomes an important part of the API contract: functions that borrow move-only values make temporary use of the value and leave it valid for further use, like reading from a file handle, whereas functions that take a move-only value consume it and prevent its further use, like closing a file handle. Relying on implicit selection of the parameter convention will not suffice for these types.


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