Custom attributes (again)

[deadalnix]

Le 06/04/2012 11:41, Johannes Pfau a écrit :
> Am Fri, 06 Apr 2012 00:48:34 -0700
> schrieb Walter Bright<newshound2 at digitalmars.com>:
>
>> On 4/6/2012 12:35 AM, Alex Rønne Petersen wrote:
>>> It actually can be a problem. In .NET land, there are many
>>> attributes across many projects (and even in the framework itself)
>>> with the same names. It turns out that regular namespace lookup
>>> rules alleviate this problem.
>>
>>
>> Perhaps a better scheme is:
>>
>>      enum foo = 3;
>>
>>      ...
>>
>>      @attr(foo) int x;
>>
>> That way, foo will follow all the usual rules.
>>
>
> The last time custom attributes where discussed, a C# like model was
> proposed. Is there a good reason why we should deviate from the C#
> implementation?
>
> C#:
> http://msdn.microsoft.com/en-US/library/48zeb25s(v=vs.80).aspx
> http://msdn.microsoft.com/en-US/library/sw480ze8(v=vs.100).aspx
> http://msdn.microsoft.com/en-US/library/z919e8tw(v=vs.80).aspx
>
> (C# attributes can be applied to almost everything, therefore sometimes
>   "disambiguating targets" is necessary. Probably not needed in D):
> http://msdn.microsoft.com/en-US/library/b3787ac0(v=vs.80).aspx
>
> Syntax in D would be different of course, but I see absolutely no need
> for the redundant (and ugly) @attr.
>
> Declaring a custom attribute:
> ---------
> module std.something;
>
> struct Author
> {
>      string name;
>      public this(string name)
>      {
>          this.name = name;
>      }
> }
> ---------
>
> Using it:
> ---------
> import std.something; //Usual namespace lookup rules apply to attributes
>
> /*
>   * @Author(param) calls the constructor of the Author struct and
>   * attaches the struct instance to test. Probably @Author (without
>   * parenthesis) coud be made to mean std.something.Author.init
>   */
> @Author("Johannes Pfau") int test;
> ---------
>
> Attaching attributes multiple times as in C# should be possible.
>
> Using reflection to get that attribute:
> ---------
> if(__traits(hasAttribute, test, std.something.Author))
> {
>      Author[] authors = __traits(getAttribute, test,
>          std.something.Author);
> }
> ---------
>
> An array is used here to support attaching the same attribute multiple
> times. Of course "auto authors = ..." should be usable here too.
>

That is a really nice proposal !