Appender is ... slow

[Brad Anderson via Digitalmars-d-learn]

On Thursday, 14 August 2014 at 19:10:18 UTC, Jonathan M Davis 
wrote:
> I've never really tried to benchmark it, but it was my 
> understanding that the idea behind Appender was to use it to 
> create the array when you do that via a lot of appending, and 
> then you use it as a normal array and stop using Appender. It 
> sounds like you're trying to use it as a way to manage reusing 
> the array, and I have no idea how it works for that. But then 
> again, I've never actually benchmarked it for just creating 
> arrays via appending. I'd just assumed that it was faster than 
> just using ~=, because that's what it's supposedly for. But 
> maybe I just completely misunderstood what the point of 
> Appender was.
>
> - Jonathan M Davis

I too have trouble understanding what Appender does that 
supposedly makes it faster (at least from the documentation). My 
old, naive thought was that it was something like a linked list 
of fixed size arrays so that appends didn't have to move existing 
elements until you were done appending, at which point it would 
bake it into a regular dynamic array moving each element only 
once looking at the code it appeared to be nothing like that (an 
std::deque with a copy into a vector in c++ terms).

Skimming the code it appears to be more focused on the much more 
basic "~= always reallocates" performance problem. It seems it 
boils down to doing essentially this (someone feel free to 
correct me) in the form of an output range:

auto a = /* some array */;
auto b = a;
a = a.array();
for(...)
   b.assumeSafeAppend() ~= /* element */;


(assumeSafeAppend's documentation doesn't say whether or not 
it'll reallocate when capacity is exhausted, I assume it does).