Feature suggestion: in-place append to array

[grauzone]

Steven Schveighoffer wrote:
> On Thu, 04 Mar 2010 11:43:27 -0500, grauzone <none at example.net> wrote:
> 
>> Some sort of "resetAndReuse" function to clear an array, but enabling 
>> to reuse the old memory would be nice:
>>
>> int[] a = data;
>> a = null;
>> a ~= 1; //reallocates (of course)
>> a.length = 0;
>> a ~= 1; //will reallocate (for safety), used not to reallocate
>> resetAndReuse(a);
>> assert(a.length == 0);
>> a ~= 1; //doesn't reallocate
>>
>> This can be implemented by setting both the slice and the internal 
>> runtime length fields to  0.
>>
>> Additionally, another function is necessary to replace the old 
>> preallocation trick:
>>
>> //preallocate 1000 elements, but don't change actual slice length
>> auto len = a.length;
>> a.length = len + 1000;
>> a.length = len;
>>
>> As I understood it, this won't work anymore after the change. This can 
>> be implemented by enlarging the array's memory block without touching 
>> any length fields.
>>
>> I'm sure the function you had in mind does one of those things or both.
> 
> proposed usage (as checked in a couple days ago):
> 
> int[] a;
> a.setCapacity(10000); // pre-allocate at least 10000 elements.
> foreach(i; 0..10000)
>    a ~= i; // no reallocation
> a.length = 100;
> a.shrinkToFit(); // resize "allocated" length to 100 elements
> a ~= 5; // no reallocation.

What shrinkToFit() does is not really clear. Does it reallocate the 
memory block of the array such, that no space is wasted? Or does it 
provide (almost) the same functionality as my resetAndReuse(), and make 
the superfluous trailing memory available for appending without 
reallocation?

I think a resetAndReuse is really needed. I have found it can prevent 
"GC thrashing" in many cases. E.g. when caching frequently re-evaluated 
data in form of arrays (free previous array, then allocate array that 
isn't larger than the previous array).