[Idea] Return a reference to the element which were just pushed onto a Vec

TimDiekmann · 2018-12-20T04:21:49.118Z

I often push an element to a Vec and then call last().unwrap() to get a reference to it. Is there a reason why push should not return the reference to the pushed element?

mjbshaw · 2018-12-20T04:26:32.034Z

This seems like something you could use a trait to solve. Besides, what kind of reference would push return? & or &mut?

TimDiekmann · 2018-12-20T04:29:27.090Z

As push needs a &mut self anyway, it should return &mut.

canndrew · 2018-12-20T07:00:40.312Z

TimDiekmann:

Is there a reason why push should not return the reference to the pushed element?

We can’t change the type of such a commonly used pre-existing method for compatibility reasons. There should definitely be a new method for this though.

Dylan-DPC · 2018-12-20T07:42:48.965Z

This feels a bit odd. Since you are pushing an item to the vec, you already have the item with you. So having push return a item feels redundant

djc · 2018-12-20T08:03:15.660Z

The item will often be owned by the container you’re pushing into, so now you no longer have it. I, too, have definitely encountered the case where I wanted to push something into a container and get back a reference.

Maybe we need something akin to HashMap::entry() for Vec?

Dylan-DPC · 2018-12-20T09:06:13.745Z

Ah true. Yeah the entry API would make sense and keep things consistent with other collections.

scottmcm · 2018-12-20T09:39:09.042Z

I’ve definitely wanted this for HashMaps – at least with Vec it’s cheap to get a reference to what I just inserted.

jschievink · 2018-12-20T12:21:22.358Z

For maps we already have the entry API, which should solve this problem (HashMap::entry)

cuviper · 2018-12-20T21:09:23.109Z

I don’t know that we need a full entry API, since this doesn’t need to look up existing items – although that could be a thing too. Here I would suggest:

fn push_last(&mut self, value: T) -> &mut T

Maybe insert deserves a variant too, but I don’t have a name suggestion.

TimDiekmann · 2018-12-20T21:16:34.335Z

I think insert is the corresponded entry.

One advantage of a full entry API would be, that you could also return the index of the last element, no need to another call to len.

scottmcm · 2018-12-23T01:15:22.301Z

Not for the key, though, which is also moved.

system · 2019-03-25T08:31:20.211Z

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