On 8/3/18 12:17 PM, aliak wrote:
Hi
Is there a way to tell if an alias is to a template?
I'm writing some algorithms and I need to distinguish between a binary
predicate that provides "less than" and one that provides "equal to"
semantics. So I have these two templates:
template eq(alias pred) {
alias eq = pred;
}
template lt(alias pred) {
alias lt = pred;
}
Then in some place:
static if (is(pred == eq)) {
return eq(a, b);
} else static if (is(pred == lt)) {
return !lt(a, b) && !lt(b, a); // equality with less than predicate
} else {
// default assumptions about predicate
}
Then I can use it like:
auto a = S!(eq!((a, b) => a == b)) ;
auto a = S!(lt!((a, b) => a < b));
Once you have an alias, it's the original thing in all respects. So
there's no way to get the specific alias that was used. That's not a
bug, but a feature.
I've tried std.traits.TemplateOf and __traits(isSame and also variations
of typeof and also traits.isInstanceOf. I wonder if I have to
parameterize eq and lt over a compile time sequence instead?
Any tips to get this working?
I'd focus on encoding the lt/gt into a type. Much easier to test types.
Perhaps a UDA works on an alias? I'm not sure.
-Steve
