copy and array length vs capacity. (Doc suggestion?)

[Steven Schveighoffer via Digitalmars-d-learn]

On 11/23/15 7:29 PM, Ali Çehreli wrote:
> On 11/23/2015 04:03 PM, Steven Schveighoffer wrote:
>  > On 11/23/15 4:29 PM, Jon D wrote:
>
>  >> In the example I gave, what I was really wondering was if there is a
>  >> difference between allocating with 'new' or with 'reserve', or with
>  >> 'length', for that matter. That is, is there a material difference
>  >> between:
>  >>
>  >>      auto x = new int[](n);
>  >>      int[] y; y.length = n;
>  >
>  > There is no difference at all, other than the function that is called
>  > (the former will call an allocation function, the latter will call a
>  > length setting function, which then will determine if more data is
>  > needed, and finding it is, call the allocation function).
>
> Although Jon's example above does not compare reserve, I have to ask:
> How about non-trivial types? Both cases above would set all elements to
> ..init, right? So, I think reserve would be faster if copy() knew how to
> take advantage of capacity. It could emplace elements instead of
> copying, no?

I think the cost of looking up the array metadata is more than the 
initialization of elements to .init. However, using an Appender would 
likely fix all these problems.

You could also use 
https://dlang.org/phobos/std_array.html#uninitializedArray to create the 
array before copying. There are quite a few options, actually :)

A delegate is also surprisingly considered an output range! Because why 
not? So you can do this too as a crude substitute for appender (or for 
testing performance):

import std.range; // for iota
import std.algorithm;

void main()
{
    int[] arr;
    arr.reserve(100);

    iota(100).copy((int a) { arr ~= a;});
}

-Steve