Why is it difficult to reference arrays in structs?

[mipri]

On Thursday, 3 October 2019 at 04:32:52 UTC, Brett wrote:
> struct Y { double q; }
> struct X { Y[] a; }
>
> X x;
>
> auto r = x.a;
>
> r is not a reference?!?!

Arrays are (ptr,length) pairs. r is a copy of them:

   #! /usr/bin/env rdmd
   import std.stdio;

   struct X { int[] a; }

   void main() {
       auto test = X([0]);
       test.a.reserve(10); // for later
       auto r = test.a;
       writeln(test.a.ptr == r.ptr); // true
       writeln(test.a.length == r.length); // true
   }

Mutate it and they're still using the same ptr:

       r ~= [1];
       writeln(test.a.ptr == r.ptr); // true
       writeln(test.a.length == r.length); // false

But r is still a copy of the ptr,length pair, and
its length is updated separately.

Consider:

   #! /usr/bin/env rdmd
   import std.stdio;

   struct X { int[] a; }

   void main() {
       X test;
       test.a.reserve(3);
       auto a = test.a;
       a ~= [1, 2, 3];
       auto r = test.a.ptr[0..3];
       writeln(r); // output: [1, 2, 3]
   }

I have to slice the ptr directly as array operations are aware
of all this. Consider:

   #! /usr/bin/env rdmd
   import std.stdio;

   struct X { int[] a; }

   void main() {
       X test;
       test.a.reserve(3);
       auto a = test.a;
       a ~= [1, 2, 3];
       test.a.length = 3;
       writeln(test.a); // [0,0,0]
       writeln(a); // [1,2,3]
   }

What you can do copy the (ptr,length) pair, do what you want with
it, and then assign it back to the struct. Don't worry about 
tricky
stuff.

> auto r = &x.a;
>
> Now r is a reference, great!.

look at it:

     writeln(typeid(a)); // output: int[]*

It's a pointer to an int[]. It behaves like a pointer.

> I want to have a slice of an array that I can append to as a 
> sort of temporary and have it append to the main array(the end 
> of the slice always is the end of the main array)..
>
> I've tried assumeSafeAppend but it does nothing to help.

Learning D goes into tons of detail about slices and appending to
copies and when you'd want assumeSafeAppend and so on.