this r-value optimizations

[Moritz Maxeiner via Digitalmars-d-learn]

On Tuesday, 1 August 2017 at 22:47:24 UTC, Nordlöw wrote:
> Given the `struct S` with lots of data fields, I've written the 
> following functional way of initializing only a subset of the 
> members in an instance of `S`:
>
> struct S
> {
>     [...]
> }
>
> Now the question becomes: will the S-copying inside `withF` be 
> optimized out in the case when `this` is an r-value such as in 
> the call
>
>     auto y = S(32).withF(42.0);
>
> ?

You're going to have to be specific about optimized out by whom. 
By the frontend? Doesn't seem that way to me by looking at the 
`-O0` assembly generated by ldc [1].

>
> If not, one solution of doing this manually is to write `withF` 
> as a free function
>
> [...]
>
> Is this the preferred way of solving this until we (ever) get 
> named parameters in D?

Preferred by whom? The people who want named parameters in D seem 
to be a minority in the community from my personal observation, 
so you would most likely get personal preferred way as an answer 
(instead of "the" unanimous preferred way).
In any case, this looks like a case of evil early optimization to 
me, because it's statistically unlikely that this is going to be 
the bottleneck of your program (though profiling would be in 
order to confirm / disprove that assumption for your specific use 
case).

[1] https://godbolt.org/g/Htdtht