Custom attributes (again)

[Manu]

On 6 April 2012 12:41, Johannes Pfau <nospam at example.com> wrote:

> 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.
>

+1
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