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### 1246. `vector::resize()` missing efficiency guarantee

Section: 24.3.11.3 [vector.capacity] Status: NAD Submitter: David Abrahams Opened: 2009-10-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

If `v` is a `vector`, I think repeated calls to `v.resize( v.size() + 1 )` should be amortized O(1), but it's not clear that's true from the text of the standard:

>     void resize(size_type sz);

> > Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructed elements to the sequence.

Seems to me if we used `push_back` instead of appends, we might be giving the guarantee I'd like. Thoughts?

[ 2009-11-10 Howard adds: ]

> Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below.

Proposed resolution:

In 24.3.11.3 [vector.capacity]/10, change

>     void resize(size_type sz);

> > Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructed elements to the sequence equivalent to `sz - size()` consecutive evaluations of `push_back(T())`.

Rationale:

The description in terms of `push_back` led some to believe that one could expect the exact same growth pattern from both `resize` and `push_back` (e.g.) which could lead to sub-optimal implementations. Additionally, 24.3.11 [vector], p1 includes a statement that this container "supports (amortized) constant time insert and erase operations at the end;", therefore addressing the concern of this issue.