Cost of .dup vs. instantiation

[Chris via Digitalmars-d-learn]

On Wednesday, 28 May 2014 at 17:33:19 UTC, monarch_dodra wrote:
> On Wednesday, 28 May 2014 at 14:36:25 UTC, Chris wrote:
>> I use Appender to fill an array. The Appender is a class 
>> variable and is not instantiated with each function call to 
>> save instantiation. However, the return value or the function 
>> must be dup'ed, like so:
>>
>> Appender!(MyType[]) append;
>> public auto doSomething() {
>>  scope (exit) { // clear append }
>>  // ... do something
>>  append ~= item;
>>  return (append.data).dup
>> }
>>
>> My question is whether I save anything with Appender as a 
>> class variable here. I have to .dup the return value (+ clear 
>> the Appender). If I had a new Appender with each function 
>> call, it might be just as good.
>>
>> public auto doSomething() {
>>  Appender!(MyType[]) append;
>>  // ....
>>  return append.data.
>> }
>>
>> Right or wrong?
>
> You might save a little because you avoid the cost of "growing" 
> your appender repeatedly: Once the appender has come to 
> "maturity", it will very likely stop growing.
>
> At that point, you only pay for *1* allocation per call to 
> doSomething. Further advantages include:
>  - dup has "APPENDABLE" info (whereas appender.data does not).
>  - less wasted memory: dup uses no more memory than it has to, 
> whereas Appender may over-allocate, depending on how you fill 
> it.
>
> The "downside" to your approach is that you keep a handle on a 
> buffer that can grow, but never shrink. If a at a certain 
> point, you have to process some particularly large input, then 
> you'll consume excessive amounts of memory.

monarch_dodra:
Hm. This last point might be an issue. If I process a large input 
(text in this case) then I might run into trouble with "append" 
as a class variable. I also had a weird bug, because I didn't 
clear the memory for overwrite.

TheFlyingFiddle:
"append" is a class variable in _my_ program, not Appender, like 
so:

class A {
   Appender!(MyType[]) append;
   // ...
}