passing member.member alias to mixin template

[ag0aep6g via Digitalmars-d-learn]

On 09/03/2017 08:54 PM, Eric_DD wrote:
> *** This works:
> 
> struct Array {
>      void foo() { writeln("foo"); }
> }
> 
> mixin template arrayOperations(arrays...) {
>      void foo() {
>          foreach(ref a; arrays) a.foo();
>      }
> }
> 
> class Thing {
>      Array data1;
>      Array data2;
>      mixin arrayOperations!(data1, data2);
> }
[...]
> ***
> 
> But if I wrap Array in a S, then I get a "need this for data of type Array"
> Is there a way (without an alias this in S) to get the following working?
> 
> 
> *** Non working code:
[...]
> struct S {
>      Array data;
> }
[...]
> class Thing {
>      S s1;
>      S s2;
>      mixin arrayOperations!(s1.data, s2.data);
> }

As far as I understand, the problem is that an alias of a member does 
not carry a `this` reference. It's added only when you use the alias in 
a method of the aggregate.

That means, s1.data is not an alias of s1's `data` field, but an alias 
of `S.data`. And so is `s2.data`. They're effectively the same alias.

It's the same with `data1` and `data2`. But in that case the aliases 
work because `foo` provides the correct `this` reference.

An example of what I mean:

----
import std.stdio;

class C
{
     int field;
     void method() { writeln(f); }
}

C c1;
C c2;

alias f = c1.field;

void main()
{
     c1 = new C;
     c2 = new C;

     c1.field = 1;
     c2.field = 2;

     c1.method(); /* prints "1" */
     c2.method(); /* prints "2" */

     version (none) writeln(f); /* Error: need 'this' */
}
----

Note that `c2.method()` prints "2", even though the alias f has been 
made from c1. The alias doesn't refer to c1's specific field, but to the 
generic field of the C class. The alias can only be used in methods of 
C, because they provide the needed `this`.