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### 2156. Unordered containers' `reserve(n)` reserves for `n-1` elements

Section: 24.2.8 [unord.req] Status: C++17 Submitter: Daniel James Opened: 2012-05-07 Last modified: 2017-07-30

Priority: 3

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Discussion:

I think that unordered containers' `reserve` doesn't quite do what it should. I'd expect after calling `x.reserve(n)` to be able to insert `n` elements without invalidating iterators. But as the standard is written (I'm looking at n3376), I think the guarantee only holds for `n-1` elements.

For a container with `max_load_factor` of `1`, `reserve(n)` is equivalent to `rehash(ceil(n/1))`, ie. `rehash(n)`. `rehash(n)` requires that the bucket count is `>= n`, so it can be `n` (Table 103). The rule is that `insert` shall not affect the validity of iterators if `(N + n) < z * B` (24.2.8 [unord.req] p15). But for this case the two sides of the equation are equal, so `insert` can affect the validity of iterators.

[2013-03-16 Howard comments and provides wording]

Given the following:

>     LF := load_factor()
>     MLF := max_load_factor()
>     S := size()
>     B := bucket_count()

>     LF == S/B

The container has an invariant:

>     LF <= MLF

Therefore:

>     MLF >= S/B
>     S <= MLF * B
>     B >= S/MLF

[2013-03-15 Issues Teleconference]

Moved to Open.

Howard to provide rationale and potentally revised wording.

[2012-02-12 Issaquah : recategorize as P3]

Jonathan Wakely: submitter is Boost.Hash maintainer. Think it's right.

Marshall Clow: even if wrong it's more right than what we have now

Geoffrey Romer: issue is saying rehash should not leave container in such a state that a notional insertion of zero elements should not trigger a rehash

AJM: e.g. if you do a range insert from an empty range

AJM: we don't have enough brainpower to do this now, so not priority zero

Recategorised as P3

[Lenexa 2015-05-06: Move to Ready]

Proposed resolution:

This wording is relative to N3485.

1. In 24.2.8 [unord.req] Table 103 — Unordered associative container requirements, change the post-condition in the row for `a.rehash(n)` to:

   > Post: `a.bucket_count() >= a.size() / a.max_load_factor()` and `a.bucket_count() >= n`.

2. In 24.2.8 [unord.req]/p15 change

   > The `insert` and `emplace` members shall not affect the validity of iterators if `(N+n) <= z * B`, where `N` is the number of elements in the container prior to the insert operation, `n` is the number of elements inserted, `B` is the container's bucket count, and `z` is the container's maximum load factor.