Iterate an std::vector by capacity

Question

When I fill an std::vector knowing in advance the final size, I usually reserve its capacity in advance to avoid reallocation of its content (which on C++03 involves calling copy constructors for each object already stored in the vector).

This is a trivial example:

std::vector<std::string> v;
v.reserve(10);    

for( std::vector<std::string>::size_type i = 0, capacity = v.capacity();
     i < capacity;
     ++i )
{
    v.push_back(std::to_string(i));
}

There's a better way (less C-style) to loop around the std::vector capacity?

I'm looking for both C++03 and C++11 answers.

Edit: I rewrote the sample because all the answers and comments were going off topic, concerning only filling the std::vector with an array, which is not the point of the question.

A better as opposed to what? What is the problem with the current `fill` you want to avoid? — SergeyA, Dec 31 '15 at 18:29
May be you just want to convert an array into a vector ? http://stackoverflow.com/questions/8777603/what-is-the-simplest-way-to-convert-array-to-vector — mikedu95, Dec 31 '15 at 18:30
@SergeyA: better in the sense "more C++ style"... this is very much C style. — lornova, Dec 31 '15 at 18:32
@mikedu95: this is just an example, I'm looking for a generic answer on how to loop around an std::vector capacity — lornova, Dec 31 '15 at 18:34
Please do not mix size and capacity. Notice: vector::size() <= vector::capacity() (capacity is a size threshold for reallocation) — , Dec 31 '15 at 18:42
@SergeyA: it means looping not for the size() but for the capacity(). For the size you have the iterators, for the capacity you don't. — lornova, Dec 31 '15 at 19:58
I suppose `while (v.size() < v.capacity()) { ... }` is not what you are looking for. But I don't know what else could be the answer since using a reference to a as-yet-unconstructed element of the vector is UB even if space has been reserved. — rici, Dec 31 '15 at 23:04
@rici: I'm looking for something more object oriented... like iterators or range-based for()... these methods work with allocated items of the std::vector, but I have not been able to find something for reserved items. — lornova, Dec 31 '15 at 23:07
...Of course, you could assign through a `std::vector::back_insert_iterator` instead of using `std::vector::emplace_back`. I really don't see the advantage, though, and the back_insert_iterator cannot be compared with a std::vector::iterator, so it's not going to be much use for loop control. But, fwiw, it's an iterator :) — rici, Dec 31 '15 at 23:23
What *is* the point of the question? What do you actually want to achieve? — Richard Hodges, Jan 01 '16 at 01:08
@Lorenzo: Note that after your `std::vector::reserve(n)`, capacity may be greater than `n`, so you fill your vector for a size which is implementation specific... — Jarod42, Jan 01 '16 at 04:45

score 6 · Answer 1 · edited Dec 31 '15 at 20:13

6

You don't really need a loop at all. There is an overload of std::vector constructor that takes two iterators. So just use std::begin and std::next to get the start and end of the passed-in array and make a vector from that.

template<typename T>
std::vector<T> fill(const T* t, size_t size)
{
    return {std::begin(t), std::next(std::begin(t), size)};
}

Similarly you could use pointer arithmetic, which is what the above method essentially does under the hood:

template<typename T>
std::vector<T> fill(const T* t, size_t size)
{
    return {t, t + size};
}

edited Dec 31 '15 at 20:13

lornova

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answered Dec 31 '15 at 18:30

Cory Kramer

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There is still a loop under the hood, and since the question starts with 'reserve', I have the feeling OP is talking about performance?... Not sure still. – SergeyA Dec 31 '15 at 18:36
2

@Sergey - If the vector constructor does any worse than using a reserve, you should ask for you money back from the library developer. – Bo Persson Dec 31 '15 at 18:44
@BoPersson, would be hard given I got it for free :) But this is not what I meant. I was trying to say that my first impression was that OP is trying find a more performant solution, than reserve + loop. Possibly I am wrong. I am still not sure what they want. – SergeyA Dec 31 '15 at 18:47
@CoryKramer: thanks for the answer but it is off topic... I'm not looking at a way to fill a vector with an array (that was just an example, now I changed the code to better reflect my question), the question is about iterating the `capacity()` of an `std::vector`. – lornova Dec 31 '15 at 20:15

score 0 · Answer 2 · answered Dec 31 '15 at 19:08

In c++11 or better, if you fill a vector knowing its size std::initializer_list will be your best bet.

You probably want to avoid initialising through a pointer to an array of const objects because that almost guarantees a copy. I appreciate that this is necessary in c++03, which should be one of your main motivations to move to a newer compiler if possible.

so:

std::vector<Thing> mythings { Thing(1), Thing(2), Thing(3) };

Is about as fast as you can get it. (not that there is no need for a fill function).

If you compile this on a reasonably modern compiler with optimisations on, you'll find that the Things will actually be created in-place within the vector. There is no faster way.

Iterate an std::vector by capacity

2 Answers2