On Thursday, 1 November 2018 at 16:14:45 UTC, Neia Neutuladh
wrote:
The spec says that a user-defined attribute must be an
expression, but DMD accepts a wide range of things as UDAs:
struct Foo { string name = "unknown"; }
@Foo int bar;
`bar` has the *type* Foo as an attribute. It's not an
*instance* of Foo. So if I try to look at the UDAs:
static foreach (uda; __traits(getAttributes, bar))
{
static if (is(typeof(uda) == Foo))
{
pragma(msg, "bar is @Foo");
}
}
That just doesn't work; typeof(Foo) isn't anything, so
is(typeof(Foo) == Foo) is false.
I can change my code to read `static if (is(uda == Foo))`. But
that obviously fails:
@Foo("customName") int bar2;
What do you do to handle this?
Check if an UDA is a type?.. As in, not just `is(uda == Foo)`,
but simply `is(uda)`:
```
struct Foo { string name = "unknown"; }
@Foo @string @Foo("hello") int bar;
static foreach (uda; __traits(getAttributes, bar)) {
static if (is(uda)) {
// if `uda` is a type...
static if (is(uda == Foo)) {
pragma(msg, "bar is @Foo!!!");
} else {
pragma(msg, "bar is "~uda.stringof);
}
} else {
// if `uda` is not a type...
pragma(msg, "bar is "~uda.stringof);
}
}
```
