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

[Jon D via Digitalmars-d-learn]

On Monday, 23 November 2015 at 15:19:08 UTC, Steven Schveighoffer 
wrote:
> On 11/21/15 10:19 PM, Jon D wrote:
>> On Sunday, 22 November 2015 at 00:31:53 UTC, Jonathan M Davis 
>> wrote:
>>>
>>> Honestly, arrays suck as output ranges. They don't get 
>>> appended to;
>>> they get filled, and for better or worse, the documentation 
>>> for copy
>>> is probably assuming that you know that. If you want your 
>>> array to be
>>> appended to when using it as an output range, then you need 
>>> to use
>>> std.array.Appender.
>>>
>> Hi Jonathan, thanks for the reply and the info about 
>> std.array.Appender.
>> I was actually using copy to fill an array, not append. 
>> However, I also
>> wanted to preallocate the space. And, since I'm mainly trying 
>> to
>> understand the language, I was also trying to figure out the 
>> difference
>> between these two forms of creating a dynamic array with an 
>> initial size:
>>
>>     auto x = new int[](n);
>>     int[] y;  y.reserve(n);
>
> If you want to change the size of the array, use length:
>
> y.length = n;
>
> This will extend y to the correct length, automatically 
> reserving a block of data that can hold it, and allow you to 
> write to the array.
>
> All reserve does is to make sure there is enough space so you 
> can append that much data to it. It is not relevant to your use 
> case.
>
>> The obvious difference is that first initializes n values, the 
>> second
>> form does not. I'm still unclear if there are other material
>> differences, or when one might be preferred over the other :) 
>> It's was
>> in this context the behavior of copy surprised me, that it 
>> wouldn't
>> operate on the second form without first filling in the 
>> elements. If
>> this seems unclear, I can provide a slightly longer sample 
>> showing what
>> I was doing.
>
> extending length affects the given array, extending if 
> necessary. reserve is ONLY relevant if you are using appending 
> (arr ~= x). It doesn't actually affect the "slice" or the 
> variable you are using, at all (except to possibly point it at 
> newly allocated space).
>
> copy uses an "output range" as it's destination. The output 
> range supports taking elements and putting them somewhere. In 
> the case of a simple array, putting them somewhere means 
> assigning to the first element, and then moving to the next one.
>
> -Steve

Thanks for the reply. And for your article (which Jonathan 
recommended). It clarified a number of things.

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;

I can imagine that the first might be faster, but otherwise there 
appears no difference. As the article stresses, the question is 
the ownership model. If I'm understanding, both cause an 
allocation into the runtime managed heap.

--Jon