Built-in array opSliceAssign

[Eduard Staniloiu]

On Thursday, 25 October 2018 at 12:38:44 UTC, Paul Backus wrote:
> On Thursday, 25 October 2018 at 12:25:37 UTC, Eduard Staniloiu 
> wrote:
>> As I wrote in the comments above, I was expecting `a[] = b[]` 
>> to iterate the slices and assign the elements of b into a.
>>
>> What really happens is a memcpy: as you can see from godblot 
>> [0], this gets lowered to a call to `_d_arraycopy`, in 
>> druntime.
>
> In D, when you assign one aggregate to another, opAssign is 
> only called for the aggregate, not for any of its elements. 
> However, postblit constructors are called for both. Example: 
> https://run.dlang.io/is/XfDaWw
>

Can you, please, give me a link to where it says this in the 
specs?

Based on the example, I would expect that the code gets lowered 
to some version of `_d_arrayassign` [0]. I still think that this 
is problematic, as it's unexpected to the user: you're expecting 
the assignment operator to be called, not the postblit.

I know that the compiler can and will create an opAssign if a 
postblit or dtor is defined, as you can write the assignment as a 
this._dtor; blit rhs into this.

This being said, I think that if the user took the time to define 
opAssign, it should be called, because he might want to do 
something extra when an assignment occurs: an ex. having 
different logs "creating new obj" vs "changing existing obj".


> Your example will work as expected if you change the opAssign 
> to a postblit constructor: https://run.dlang.io/is/HBbGO2