On 10/1/20 10:36 AM, Meta wrote:
On Thursday, 1 October 2020 at 09:49:36 UTC, Mathias LANG wrote:
Author here. The most complete way to know would be to read the
changelog: https://dlang.org/changelog/2.094.0.html#preview-in
The TL;DR is that, in addition to `const scope`, `in` now
automatically behaves as `ref` when "it makes sense" such as large
value types or presence of destructors / postblit (more details in the
changelog!) and will accept rvalues, unlike other ref parameters.
Why was this added when we already have `auto ref`? Yes, it makes the
function a template, but if `in` can automatically choose whether the
variable is ref or not, then auto ref could easily do the same.
There is a difference. `in` is choosing it based on the type, not
whether it's an rvalue or lvalue. auto ref doesn't care whether it's an
int or a 1k-sized struct, if it's an lvalue, it's ref, and if it's an
rvalue, it's non-ref.
Not only that, but every auto-ref parameter is another template
parameter varying on the usage. So calling on an lvalue and rvalue will
generate 2 separate mostly-identical functions.
With -preview=in, only one function is generated per type.
-Steve
