Re: Weird behavior with UDAs
On Saturday, 13 June 2020 at 13:08:29 UTC, realhet wrote: How can be a string represented with 'null' by default instead on `""`. Unless I state it explicitly with name="" ? o.O Because string is simply `alias string = immutable(char)[]`, and default initializer for arrays is null.
Re: Weird behavior with UDAs
On Saturday, 13 June 2020 at 12:55:36 UTC, realhet wrote:
Hello, I have a problem I can't even understand with the code
For the first I realized that an UDA can be a type too, and come
up with this:
template getUDA(alias a, U){
enum u = q{ getUDAs!(a, U)[$-1] };
static if(hasUDA!(a, U) && !is(mixin(u)))
enum getUDA = mixin(u);
else
enum getUDA = U.init;
}
So I always get an instance of the UDA with the default values.
But the second question is still beyond me:
How can be a string represented with 'null' by default instead on
`""`. Unless I state it explicitly with name="" ? o.O
Re: Weird behavior with UDAs
On Saturday, 13 June 2020 at 12:55:36 UTC, realhet wrote:
My first question is, how to avoid that error with A.i4? Why
is there a difference between @UNIFORM and @UNIFORM(), do the
first returns a type and the later returns a value?
Basically yeah. a UDA in D is just whatever you write gets
directly attached - it does no additional processing. So if you
give it a type, it keeps a type. If a value, it keeps the value.
The simplest answer is probably "don't do that"; if it is meant
to be a value, always give it a value.
But you could also write your own get UDA thing that recognizes
the type (the check: static if(is(attribute)) for a type vs
static if(is(typeof(attribute))) for the value) and returns the
init value if you want in your code.
My second quertion is, why the UNIFORM struct with
uninitialized string producing UNIFORM(null).
How can be a difference when I say name="", and it's just the
same as the default string initializer, and then it produce
UNIFORM("")?
null and "" can be used interchangeably and sometimes yield the
same thing, but they aren't exactly the same.
Since you specified it there in the definition as a default value
in a struct the compiler used that distinction. I'd suggest you
write what you mean even in cases where it is the same so you
cover the bases.
If you specifically want null, check `str is null` and use ` =
null`. If either is fine, just use `str.length == 0`.
Weird behavior with UDAs
Hello, I have a problem I can't even understand with the code
below:
https://run.dlang.io/is/7yXAEA
import std.stdio, std.range, std.algorithm, std.traits, std.meta;
struct UNIFORM{ string name; }
struct A{
int i1;
@UNIFORM() int i2;
@UNIFORM("fuzz") int i3;
@UNIFORM int i4;
}
template getUDA(alias a, U){
static if(hasUDA!(a, U)) enum getUDA = getUDAs!(a, U)[$-1];
else enum getUDA = U.init;
}
pragma(msg, getUDA!(A.i1, UNIFORM)); //UNIFORM(null)
pragma(msg, getUDA!(A.i2, UNIFORM)); //UNIFORM(null)
pragma(msg, getUDA!(A.i3, UNIFORM)); //UNIFORM("fuzz");
pragma(msg, getUDA!(A.i4, UNIFORM)); //Error: initializer must be
an expression, not UNIFORM
struct UNIFORM2{ string name=""; }
struct B{ @UNIFORM2() int i1; }
pragma(msg, getUDA!(B.i1, UNIFORM2)); //UNIFORM("") instead if
UNIFORM(null)
//
My first question is, how to avoid that error with A.i4? Why is
there a difference between @UNIFORM and @UNIFORM(), do the first
returns a type and the later returns a value?
My second quertion is, why the UNIFORM struct with uninitialized
string producing UNIFORM(null).
How can be a difference when I say name="", and it's just the
same as the default string initializer, and then it produce
UNIFORM("")?
Thank You!
