digitalmars.D.learn - return scope vs. scope return
- 0xEAB (9/14) Nov 05 2022 Apparently there a difference between:
- Paul Backus (23/37) Nov 05 2022 Basically, the `return` attribute is a modifier, and has no
Apparently there a difference between: - ```d Message withBody(Body body_) return scope { /* … */ } ``` - ```d Message withBody(Body body_) scope return { /* … */ } `````` Deprecation: returning `this._body` escapes a reference to parameter `this` perhaps change the `return scope` into `scope return` ```What is it? And why (…does the order of attributes matter)?
Nov 05 2022
On Saturday, 5 November 2022 at 16:13:18 UTC, 0xEAB wrote:Apparently there a difference between: - ```d Message withBody(Body body_) return scope { /* … */ } ``` - ```d Message withBody(Body body_) scope return { /* … */ } ```Basically, the `return` attribute is a modifier, and has no meaning on its own. Either it modifies `ref` to create `return ref`, or it modifies `scope` to create `return scope`. (In the case of a struct method, like in your example, the `ref` is implicit.) In the past, if you used all 3 of `ref`, `scope`, and `return` on a single parameter, it was ambiguous whether the `return` was modifying `ref` or `scope`. This lead to a lot of confusion and frustration (see [this thread][1] for the gory details). To resolve the ambiguity, Walter decided to make the *order* of the keywords significant: if you write `return scope`, in exactly that order, then `return` modifies `scope`; otherwise, it modifies `ref`. I believe that the plan is to eventually require `return ref` to be written in exactly that order as well (see Walter's reply in [this thread][2]), but that's a breaking change, and will require a deprecation period. So in the meantime, we have kind of an awkward half-solution, where keyword order is significant for `return scope` but not for `return ref`. [1]: https://forum.dlang.org/post/nbbtdbgifaurxoknyeuu forum.dlang.org [2]: https://forum.dlang.org/post/snnd0r$132p$1 digitalmars.com``` Deprecation: returning `this._body` escapes a reference to parameter `this` perhaps change the `return scope` into `scope return` ```What is it? And why (…does the order of attributes matter)?
Nov 05 2022