On Monday, 23 July 2012 at 13:32:54 UTC, Philippe Sigaud wrote:
As an aside to a reflection-related train of thought, I tired
again to get a templated type's parameter list.
I mean, given
class C(T) { T t; }
Drat, 'tab' is dangerous with the forum interface, sorry for that.
So given C above and
auto c = new C!(int)();
I want a template that, given typeof(c), gives me back 'C' and
'int'.
is() is our friend here, notably the new T... extension:
import std.typetuple;
template Composition(T) if (isComposite!T)
{
static if (is(T comp == Name!(U), alias Name, U) || is (T
comp == Name!(U), alias Name, U...))
alias TypeTuple!(Name, U) Composition;
}
(isComposite!T is the same check with is())
Works beautifully:
auto t = tuple(1,"abc", 'c');
writeln(Composition!(typeof(t)).stringof); // ("Tuple(Specs...)",
int, string, char)
So, I get "Tuple(Specs...)" (the template name) and the
arguments. Nice!
Except, this does not work for 2-args templates. I thought the
U... branch would get them, but apparently, U... means 'with a
template param tuple' very strictly. No problem, I'll just add
the 2, 3 and 4-args cases, that should be enough.
template Composition(T) if (isComposite!T)
{
static if (is(T comp == Name!(U), alias Name, U) || is (T
comp == Name!(U), alias Name, U...))
alias TypeTuple!(Name, U) Composition;
else static if (is(T comp == Name!(U,V), alias Name, U, V))
alias TypeTuple!(Name, U,V) Composition;
else static if (is(T comp == Name!(U,V,W), alias Name, U, V,
W))
alias TypeTuple!(Name, U,V,W) Composition;
...
}
(and isComposite accordingly).
Right. Except, this only gets template instantiations with pure
types as parameters. Things like std.algorithm.map which take a
function by name are out.
OK... So, I'll add also alias overloads. Hmm, Pure Type / Alias :
2 possibilities, so for 3 args that's 8 different tests to do + 4
for 2-params, + 2 for the single param and the tuple...
Except, you can have a tuple also in 2-params and 3-params
templates.
Also, what about value template parameters? I tested, this does
not work when passed strings or ints, or whatever. I should test
for them also.
Argh, so it's more 10 different possibilities per slot, the n-th
power for n args.
No way am I generating a template with more than a thousand(!)
is() tests in it!
Can someone offer me some advice on this? My old version that
cheated with the type .stringof suddenly seems so much better.
Philippe
