Cost of .dup vs. instantiation

[TheFlyingFiddle via Digitalmars-d-learn]

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?

When it comes to optimizations it's hard to say. Benchmarking is 
better than relying advice/opinions on the internet in any case.

That being said i doubt that the instantiation cost of the 
Appender is relevant. (Btw the appender is not a class variable! 
It is a struct with reference semantics). Reusing an appender is 
more for those cases where you want to reuse the underlying 
memory of the appender itself.