On Monday, 11 November 2019 at 21:52:12 UTC, Jonathan M Davis
wrote:
On Monday, November 11, 2019 12:17:37 PM MST Bastiaan Veelo via
Digitalmars- d-learn wrote:
[...]
I could use some help in rewriting the code below so that arr1
and arr2 each have their own data; ideally with minimal
changes so that I can make the transcompiler do the right
thing.
Thanks!
Bastiaan.
void main()
{
import std.stdio;
WrapIntegerArray arr1;
arr1[0] = 42;
WrapIntegerArray arr2;
writeln(arr2[0]); // 42, not 0.
writeln("arr1.wrap.arr.ptr = ", arr1.wrap.arr.ptr);
writeln("arr2.wrap.arr.ptr = ", arr2.wrap.arr.ptr); //
identical
assert(arr2[0] == 0); // fails
}
struct IntegerArray
{
int[] arr;
alias arr this;
this(int l)
{
arr = new int[l];
}
}
struct WrapIntegerArray
{
auto wrap = IntegerArray(5); // This is CTFE! :-(
alias wrap this;
}
All struct and class members which are directly initialized
must have their values known at compile-time. For structs,
that's what goes in the init value for the type. A side effect
of this is that it's usually a bad idea to directly initialize
dynamic arrays which are member variables. You need to do the
initialization in a constructor. And for structs, if you need a
no-arg constructor, then you'll need to use a factory function
(since structs can't have no-arg constructors). e.g.
struct WrapIntegerArray
{
IntegerArray wrap;
alias wrap this;
this(int len)
{
wrap = IntegerArray(len);
}
}
or
struct WrapIntegerArray
{
IntegerArray wrap;
alias wrap this;
static make()
{
WrapIntegerArray retval;
retval.wrap = IntegerArray(5);
return retval;
}
}
So, you could then have something like
auto arr1 = WrapIntegerArray(5);
arr1[0] = 42;
or
auto arr1 = WrapIntegerArray.make();
arr1[0] = 42;
but if you use the init value (which is what you get if you let
the type be default-initialized), then you'll have to first do
something to allocate the dynamic array if you want to be able
to index it, since if you don't give it a value at
compile-time, it's null (and you don't want to give it a value
at compile-time, because then every default-initialized struct
of that type will refer to the same dynamic array). Of course,
you could always just append values, and the dynamic array will
be allocated and grow accordingly, but that's obviously not the
same as allocating it up front to have a specific length.
- Jonathan M Davis
Thank you Jonathan. A factory function seems to be what I need,
then. It'll be an interesting challenge to have my transpiler
detect when a factory function is needed, and making sure that
they are called at the right places. But this might escalate
since structs can have members that are structs that have a
factory function. So the enclosing struct also needs a factory
function. And so forth.
My problem is the translation of so called schematic arrays, a
type that I have only seen in Extended Pascal. A schematic array
is a type in which the length of the array is a parameter of the
type. Extended Pascal does not have dynamic arrays, so schematic
arrays are its attempt at improving a bit on just plain static
arrays. The length parameter can be constant, in which it behaves
akin to templates (which it doesn't have either), but it can also
be set at run time during initialization (so I can't translate it
to a template).
Maybe I can omit the translation of schematic arrays
(IntegerArray in this case) altogether and instead detect where
and how they are instantiated. Then replace them with a static
array whenever the length is known at compile time, and a dynamic
array otherwise. A static array is not nice, but at least they
can be members of structs without a factory function. Either way,
my transpiler needs to smarten up...
Bastiaan.
