Re: User Defined Attributes (UDA) in Phobos/druntime

[Jacob Carlborg]

On 2013-06-12 14:29, Andrej Mitrovic wrote:

But without this you lose the ability to customize. I don't like just
"tagging" something, I like D's ability where you can actually
customize the UDA, for example:
-----
import std.typetuple;

struct Server
{
     string url;
}

struct Config
{
     @(Server("http://foo.bar";)) string name;
     @(Server("http://doo.bar";)) string[] items;
}

template GetAttributes(T...) if (T.length == 1)
{
     alias GetAttributes = TypeTuple!(__traits(getAttributes, T[0]));
}

void main()
{
     Config config;

     alias attribs = GetAttributes!(config.name);

     static if (attribs.length && is(typeof(attribs[0]) == test.Server))
     {
         pragma(msg, attribs[0]);
     }
}
-----

Thanks to CTFE we can tag a field with a struct that may have any
state. All you have to do in the library code is to first check if any
of the attributes is an "attribute type (a struct of some sort)" that
you recognize, and then read its state.

As far as I know other language allows UDA's to be of any type/value. 
Java uses the @annotation annotation on an interface and in C# classes 
with System.Attribute as the base class are used.

--
/Jacob Carlborg