I have the following code example:

struct A{}

A[5] a;
ulong[] idx = [1,3,4];

auto get()
{
    return idx.map!(_ => a[_]);
}

foreach(i; 0 .. a.length)
    write(&a[i], " ");
writeln;

foreach(ref b; get())
    write(&b, " ");
writeln;

How can I change get function so it returns the references to the content in a?

Is there a good documentation describing how to avoid copying of structs in the code?

On Thursday, 4 November 2021 at 11:26:30 UTC, Andrey Zherikov wrote:

struct A{}

A[5] a;
ulong[] idx = [1,3,4];

auto get()
{
    return idx.map!(_ => a[_]);
}

foreach(i; 0 .. a.length)
    write(&a[i], " ");
writeln;

foreach(ref b; get())
    write(&b, " ");
writeln;

Have the lambda return by reference:

auto get()
{
    return idx.map!(ref (i) => a[i]);
}

On Thursday, 4 November 2021 at 13:03:54 UTC, Paul Backus wrote:

auto get()
{
    return idx.map!(ref (i) => a[i]);
}

That works, thanks!

On Thursday, 4 November 2021 at 13:03:54 UTC, Paul Backus wrote:

struct A{}

A[5] a;
ulong[] idx = [1,3,4];

auto get()
{
    return idx.map!(_ => a[_]);
}

foreach(i; 0 .. a.length)
    write(&a[i], " ");
writeln;

foreach(ref b; get())
    write(&b, " ");
writeln;

auto get()
{
    return idx.map!(ref (i) => a[i]);
}

Making this example a bit complex: I want get to return additional data to the reference. How should I change the lambda then?

auto get()
{
    return idx.map!(ref (i) => tuple(a[i], i)); // return ref and index for simplicity
}

On Thursday, 4 November 2021 at 18:21:06 UTC, Andrey Zherikov wrote:

auto get()
{
    return idx.map!(ref (i) => a[i]);
}

auto get()
{
    return idx.map!(ref (i) => tuple(a[i], i)); // return ref and index for simplicity
}

Since there are no such thing as ref variables in D, you can't store a reference inside a tuple. So in this case, you will have to use a pointer:

auto get()
{
    return idx.map!((i) => tuple(&a[i], i));
}

On Thursday, 4 November 2021 at 11:26:30 UTC, Andrey Zherikov wrote:

[...]

[...]

return idx.map!(_ => a[_]);

How can one make the type of idx's elements portable (compile with -m32 to see what the problem is)? This seems to be an surprisingly nontrivial issue at least in C [1].

[1] https://stackoverflow.com/questions/3174850/what-is-the-correct-type-for-array-indexes-in-c

On Friday, 5 November 2021 at 19:49:50 UTC, Ali Çehreli wrote:

Ah, good point. Thanks!

On Friday, 5 November 2021 at 20:13:02 UTC, Andrey Zherikov wrote:

Adding a bit of CTFE:

struct A {}
struct B
{
    A[] ar = [A.init];
    size_t[] idx = [0];

    A*[] get()
    {
        return idx.map!((idx) => &ar[idx]).array;  // Error: couldn't find field `ar` of type `A[]` in `MapResult([0LU], null)`
    }
}

auto foo()
{
    B b;
    return b.get();
}

void main()
{
    writeln(foo());       // This works
    pragma(msg, foo());   // This doesn't work
}

How to make this work?