hasUDA with this

[Timoses]

On Thursday, 19 July 2018 at 19:18:43 UTC, jmh530 wrote:
> I wanted to create a struct with a member function whose 
> behavior was different depending on whether the struct instance 
> had a particular UDA.
>
> However, it seems like hasUDA doesn't seem to produce the 
> result I would have expected here. I tried using getAttributes, 
> but that didn't work either.
>
> Am I missing something, or should I submit an enhancement 
> request?
>
>
>
> import std.traits : hasUDA;
>
> enum test;
>
> struct Foo
> {
>     int x;
>
>     bool checkUDA()
>     {
>     	static if (hasUDA!(this, test))
>         {
>         	return true;
>         }
>         else
>         {
>         	return false;
>         }
>     }
> }
>
>
> void main()
> {
>     import std.stdio : writeln;
>
>     @test Foo foo = Foo(1);
>     static assert(hasUDA!(foo, test));
>
>     writeln(foo.checkUDA()); //prints false, expected true
> }

I haven't worked with UDAs myself yet, but I believe that when 
you apply something like

@test Foo foo;

then you apply the UDA to `foo` and not to `Foo`.

See below:

enum test;

struct Foo
{}

bool checkUDA(alias f)()
{
     static if (hasUDA!(f, test))
     {
         return true;
     }
     else
     {
         return false;
     }
}

void main()
{
     @test Foo foo;
     // same as `hasUDA!(foo, test)`
     assert(checkUDA!foo); //prints true
}

Now I'm really not sure how UDAs are best utilized, so above 
example might not be how it should be done.. However, you could 
make a templated version of checkUDA depending on what calls it. 
E.g.

bool checkUDA(T, alias f)(T t)
{
     // ...
}

although this leads to awkward calls like

foo.checkUDA!foo

which doesn't seem right either. Perhaps wait for somebody more 
knowledgeable to answer your question : D.