On Monday, 22 August 2016 at 00:22:48 UTC, ag0aep6g wrote:
On 08/22/2016 12:06 AM, Engine Machine wrote:
T!()'s "data" is specified in the class just like all the other
derivations. I don't want to have to specify an external base
class as
in your InstaniateOrBase. Why? Because!!! (There should be no
need to,
and if one goes this route of creating classes, it should be
all or
nothing, else there is no real benefit)
You make it sound like I came up with the external base class,
but I just took that from your code.
No, what I meant was I don't want a base. I only added it in my
code so it would compile/do what I want.
[...]
It seems that when one uses `class T(A...) : X`, one can't,
for some X,
not have inheritance. The compiler expects X to be something
inheritable
from no matter what.
I didn't know it, but your code shows that X can also be an
empty compile time list (Seq). I have no idea if this is in the
spec, or if it just happens to work with dmd.
If it's valid D, you just need a template that goes from T and
A to T!(A[0 .. $ - 1]), or if A is already empty, to the empty
Seq.
Like so:
----
template InstantiateOrEmptySeq(alias tmpl, args...)
{
alias Seq(T...)=T;
static if (args.length > 0)
alias InstantiateOrEmptySeq = tmpl!(args[0 .. $ - 1]);
else
alias InstantiateOrEmptySeq = Seq!();
}
class T(A...) : InstantiateOrEmptySeq!(T, A)
{
...
}
----
Yes! I though I tried that from Timon's original solution.
The following code works and does what I want!
template InstantiateOrEmptySeq(alias tmpl, args...)
{
alias Seq(T...)=T;
static if (args.length > 0)
alias InstantiateOrEmptySeq = tmpl!(args[0 .. $-1]);
else
alias InstantiateOrEmptySeq = Seq!();
}
class T(A...) : InstantiateOrEmptySeq!(T, A)
{
static if (A.length == 0)
{
// Base class
int x;
} else
static if (A[$-1] == "Animal")
{
int y;
} else
static if (A[$-1] == "Dog")
{
int z;
} else
static if (A[$-1] == "Pug")
{
int s;
} else static assert(A[$-1]~" not a defined class of
"~this.stringof);
}
The only down side is that the static if's are flat. They don't
show the relationship between, say, Dog an Animal, that a nested
set of static if's would show, but I think this is not possible
due to the recursion and shadowing(I imagine one could test if
the variables exist but that is messy).
Anyways, Thanks for the help.
I think this is just Timons answer anyways. What the original
confusion was, was the shadowing, which I thought would not
happen(or at least expected an error), Lodovico solved that.
