Slice allocation after appending

[Steven Schveighoffer]

On 12/22/20 5:12 PM, Rekel wrote:
> According to the D slice article 
> (https://dlang.org/articles/d-array-article.html), slices do not care 
> where they start, only where they end, when checking whether expanding 
> in place is permitable, or at least that is what I understood regarding it.
> 
> Now I'm unsure how to check this, I tried to a bit using the online 
> editor and a bit of pointer usage which seemed to confirm my suspicion, 
> but does this mean that taking a (small) slice at the end of a 
> (possibly) very large dynamic array can lead to problematic behavior?

Problematic in what way? It will not stomp on data that is live. So I'd 
say it's the opposite of problematic.

You can check whether an append will reallocate or not by checking the 
capacity. If arr.capacity <= arr.length, then an append will reallocate.

> For example;
> 
>> int[] a = new int[10]; // Imagine this is very large
>> int[] b = a[$-1..$];   // Small slice at the end
>> b ~= 2;                // b extends, possibly in place
>> a ~= -1;               // a no longer can, so the entire array needs 
>> reallocating
> (instead of be reallocating & a growing in place)

For sure a will need reallocating. The runtime cannot know that you will 
append with a, so it uses the space for b.

If you'd rather b reallocate, you can use the concatenation operator on 
it instead:

b = b ~ 2;

Or you can manage the array allocations yourself without using the 
runtime (see std.array.Appender).

> 
> Again I'm not very certain I fully understood how slices are 
> implemented, but is this example, and the problem I imagine it leading 
> to, valid?

Is the description you have valid? yes. Is it a problem with the 
implementation? I'd say no. If you change your expectations, you can 
avoid this situation easily.

-Steve