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### 1041. Add associative/unordered container functions that allow to extract elements

Section: 24.2.7 [associative.reqmts] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses UK 239 [CD1]

It is not possible to take a move-only key out of an unordered container, such as (`multi`)`set` or (`multi`)`map`, or the new unordered containers.

Add below `a.erase(q)`, `a.extract(q)`, with the following notation:

`a.extract(q)>`, Return type `pair<key, iterator>` Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists,returns `a.end()`.

[ Summit: ]

> We look forward to a paper on this topic. We recommend no action until a paper is available. The paper would need to address exception safety.

[ Post Summit Alisdair adds: ]

> Would `value_type` be a better return type than `key_type`?

[ 2009-07 post-Frankfurt: ]

> Leave Open. Alisdair to contact Chris Jefferson about this.

[ 2009-09-20 Howard adds: ]

> See the 2009-09-19 comment of 839 for an API which accomplishes this functionality and also addresses several other use cases which this proposal does not.

[ 2009-10 Santa Cruz: ]

> Mark as NAD Future. No consensus to make the change at this time.

Original resolution [SUPERSEDED]:

> In 24.2.7 [associative.reqmts] Table 85, add:

> > Table 85 -- Associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `a.erase(q)` | ... | ... | ...
> > `a.extract(q)` | `pair<key_type, iterator>` | Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

> In 24.2.8 [unord.req] Table 87, add:

> > Table 87 -- Unordered associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `a.erase(q)` | ... | ... | ...
> > `a.extract(q)` | `pair<key_type, iterator>` | Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

[08-2016, Post-Chicago]

Move to Tentatively Resolved

Proposed resolution:

This functionality is provided by P0083R3