Resize vector given an iterator in C++

Question

Suppose we are given std::vector<T> V and an iterator p to a position within the vector.

Q1: What is a good way to return a new vector w for which the given iterator p is std::end(w)?

I could create a new vector w, move the elements from std::begin(v) to p and assign p = std::end(w).

Q2: Is there a way to do what I want but keeping p const ?

The origin of my question is the following: I have a vector for which I applied std::remove(std::begin(v),std::end(v), elem). This should put all the elements that are not equal to elem and return an iterator to the end of that range. I would like to clip the vector there.

You cannot return a *new* vector, because the iterator is only valid for the given vector. If you're happy to modify the existing vector, though, you can simply call `V.erase(p, V.end())`. — Kerrek SB, Jun 13 '16 at 17:10
@LightnessRacesinOrbit Will expand the question with the reasons — myfirsttime1, Jun 13 '16 at 17:11
What's wrong with the erase/remove idiom? Also, `remove` does not move the elements to the end. The end elements have unspecified values. — Nicol Bolas, Jun 13 '16 at 17:18

Kerrek SB · Accepted Answer · 2017-01-26T23:05:58.013

7

You can do this:

std::vector<T> w = v;
v.erase(p, v.end());
w.swap(v);

Now p is an iterator into w, p == w.end(), and v is the original vector. Note that swapping two dynamic containers (with appropriate allocators) makes iterators of one container become iterators of the other container.

This doesn't quite work for returning values from functions, because there is no similar guarantee for move construction, nor is copy elision mandatory.

However, given that you say that p is the result of remove, maybe a simpler solution would be the typical remove-erase idiom:

v.erase(std::remove(std::begin(v),std::end(v), elem), std::end(v));

edited Jan 26 '17 at 23:05

answered Jun 13 '16 at 17:13

Kerrek SB

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[Demo](http://ideone.com/7jsSOK) – Kerrek SB Jun 13 '16 at 17:15
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To do this in-place: `v.resize(std::distance(v.begin(), p));` – Kyle McDonald Jan 26 '17 at 22:39

Resize vector given an iterator in C++

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