cloning arrays

[Jonathan M Davis via Digitalmars-d-learn]

On Monday, September 12, 2016 07:56:26 Russel Winder via Digitalmars-d-learn 
wrote:
> On Sat, 2016-09-10 at 15:54 -0700, Jonathan M Davis via Digitalmars-d-
>
> learn wrote:
> > […]
> >
> > dup creates an array with mutable elements that are copies of what
> > was in
> > the original array. How deep a copy that is depends on the type. It
> > does
> > exactly as deep a copy as simply copying the element would do. e.g.
> >
> > […]
>
> Which raises the question of whether:
>
>   x[]
>
> and
>
>   x.dup
>
> have the same result.

They don't. For dynamic arrays,

auto y = x;

and

auto y = x[];

are identical save for constness. The first one results in y being a dynamic
array referring to exactly the same memory as x with exactly the same
constness, whereas the second results in y referring to the same memory, but
it's a tail-const version of x. e.g.

    auto x = cast(immutable)[1, 2, 3, 4];
    auto y = x;
    auto z = x[];
    static assert(is(typeof(x) == immutable(int[])));
    static assert(is(typeof(y) == immutable(int[])));
    static assert(is(typeof(z) == immutable(int)[]));

But regardless of the constness, x, y, z refer to exactly the same memory.
No memory is allocated, and none of the elements are copied.

If you do something like

y[] = x[];

then that would be equivalent to

y[0 .. $] = x[0 .. $];

and it would copy all of the elements of x into y, but it would require that
x and y be the same length, and it would not allocate any memory. It's just
copying the elements.

dup specifically allocates a new buffer, copies each of the elements of the
original array into that buffer, and then returns a dynamic array which is a
slice of that buffer referring to those elements. It does essentially the
same thing as

auto y = new int[](x.length);
y[] = x[];

except that that default-initializes all of the elements in y first, and
then copies over them, whereas dup should just copy them without doing the
default initialization first.

- Jonathan M Davis