Trying to understand something about UDAs.

[Adam D. Ruppe via Digitalmars-d-learn]

On Monday, 12 December 2016 at 19:37:42 UTC, Kevin Balbas wrote:
> I'm trying to figure out how to test whether a UDA struct was 
> initialized with arguments (and change the behavior of the 
> surrounding code if it was.

Easy solution: test the type. @Attr is setting the type as a UDA, 
@Attr("test") is setting a *value* of type Attr.

static if(is(UDA)) {
   // UDA is a type, so it was @UDA
} else {
   // it is something else (do further tests to see if value, 
etc). For example, test is(typeof(UDA) == Attr) to see if it is a 
value of type Attr.
}


> I can rationalize what the error means -- the compiler wasn't 
> given a string so it requires a concrete object to get the 
> member from.  What I don't understand is why *both* of those 
> static ifs passed.  If it has the member and I can't get it, 
> and if getting the member compiles yet at the same time it 
> doesn't, I'm not sure how I can actually make this test work.

So you can get the member of a type, but not the value of that 
member. getMember itself succeeds - you can pass the result to 
further reflection things (test its type, offset, etc.), but you 
cannot get the value, which is what your code was trying to do 
there (that is like if you wrote `"foo";` in the middle of 
nowhere. It treats the string as an orphaned value).

Basically, getMember is legal, just not in the context you were 
trying it, thus the __traits(compiles) works but the next line 
doesn't.


But the static if(is(UDA)) is probably what you actually want.