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digitalmars.D.learn - UDA usage

reply Matt Soucy <msoucy csh.rit.edu> writes:
So, I was away from active D development for a while, and I'm having
trouble finding info about how to use UDAs. I can see the basics, like
how to apply a UDA to a symbol, but now how to do specific things, like:
* Check to see if a specific attribute exists
* Check to see if an attribute of a specific type exists, something like:=

 Foo('a') int x;
enum hasAnyFoo =3D __traits(hasAttributeOfType, Foo, x);
* Create a function that can only be called if the first argument has a
UDA of a specific type

Are any of these doable?
Thank you,
-Matt Soucy
Oct 04 2013
parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
On Saturday, 5 October 2013 at 02:51:21 UTC, Matt Soucy wrote:

 * Check to see if a specific attribute exists
 * Check to see if an attribute of a specific type exists, 
 something like:


The helper functions from my web.d might help:

// checks to see if it has one of a type
// static if(hasAnnotation!(func, Foo)) { has  Foo }
// use if Foo is a simple enum.
template hasAnnotation(alias f, Attr) {
	bool helper() {
		foreach(attr; __traits(getAttributes, f))
			static if(is(attr == Attr) || is(typeof(attr) == Attr))
				return true;
		return false;

	}
	enum bool hasAnnotation = helper;
}

/// checks to see if it has an annotation with a value e.g. 
 Something(10)
template hasValueAnnotation(alias f, Attr) {
	bool helper() {
		foreach(attr; __traits(getAttributes, f))
			static if(is(typeof(attr) == Attr))
				return true;
		return false;

	}
	enum bool hasValueAnnotation = helper;
}

/// gets the thing you checked for above. so getAnnotation!(Func, 
Something) returns Something(10) (if Something is as struct)
template getAnnotation(alias f, Attr) if(hasValueAnnotation!(f, 
Attr)) {
	auto helper() {
		foreach(attr; __traits(getAttributes, f))
			static if(is(typeof(attr) == Attr))
				return attr;
		assert(0);
	}

	enum getAnnotation = helper;
}



 * Create a function that can only be called if the first 
 argument has a
 UDA of a specific type


Since the UDA is only available at compile time, you can't do 
that without some kind of template.
Oct 04 2013
parent reply Matt Soucy <msoucy csh.rit.edu> writes:
On 10/04/2013 11:04 PM, Adam D. Ruppe wrote:

 On Saturday, 5 October 2013 at 02:51:21 UTC, Matt Soucy wrote:

 * Check to see if a specific attribute exists
 * Check to see if an attribute of a specific type exists, something li=




ke:

=20
 The helper functions from my web.d might help:
=20
 // checks to see if it has one of a type
 // static if(hasAnnotation!(func, Foo)) { has  Foo }
 // use if Foo is a simple enum.
 template hasAnnotation(alias f, Attr) {
     bool helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(attr =3D=3D Attr) || is(typeof(attr) =3D=3D At=


tr))

                 return true;
         return false;
=20
     }
     enum bool hasAnnotation =3D helper;
 }
=20
 /// checks to see if it has an annotation with a value e.g.  Something(=


10)

 template hasValueAnnotation(alias f, Attr) {
     bool helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(typeof(attr) =3D=3D Attr))
                 return true;
         return false;
=20
     }
     enum bool hasValueAnnotation =3D helper;
 }
=20
 /// gets the thing you checked for above. so getAnnotation!(Func,
 Something) returns Something(10) (if Something is as struct)
 template getAnnotation(alias f, Attr) if(hasValueAnnotation!(f, Attr)) =


{

     auto helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(typeof(attr) =3D=3D Attr))
                 return attr;
         assert(0);
     }
=20
     enum getAnnotation =3D helper;
 }
=20
=20


Nice, thank you. These are really helpful!


 * Create a function that can only be called if the first argument has =




a

 UDA of a specific type


=20
 Since the UDA is only available at compile time, you can't do that
 without some kind of template.


Using the above tools I got what I wanted, so I'm satisfied for now.
Thank you, though!
-Matt Soucy
Oct 04 2013
parent reply Matt Soucy <msoucy csh.rit.edu> writes:
On 10/04/2013 11:50 PM, Matt Soucy wrote:

 On 10/04/2013 11:04 PM, Adam D. Ruppe wrote:

 On Saturday, 5 October 2013 at 02:51:21 UTC, Matt Soucy wrote:

 * Check to see if a specific attribute exists
 * Check to see if an attribute of a specific type exists, something l=






ike:

 The helper functions from my web.d might help:

 // checks to see if it has one of a type
 // static if(hasAnnotation!(func, Foo)) { has  Foo }
 // use if Foo is a simple enum.
 template hasAnnotation(alias f, Attr) {
     bool helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(attr =3D=3D Attr) || is(typeof(attr) =3D=3D A=




ttr))

                 return true;
         return false;

     }
     enum bool hasAnnotation =3D helper;
 }

 /// checks to see if it has an annotation with a value e.g.  Something=




(10)

 template hasValueAnnotation(alias f, Attr) {
     bool helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(typeof(attr) =3D=3D Attr))
                 return true;
         return false;

     }
     enum bool hasValueAnnotation =3D helper;
 }

 /// gets the thing you checked for above. so getAnnotation!(Func,
 Something) returns Something(10) (if Something is as struct)
 template getAnnotation(alias f, Attr) if(hasValueAnnotation!(f, Attr))=




 {

     auto helper() {
         foreach(attr; __traits(getAttributes, f))
             static if(is(typeof(attr) =3D=3D Attr))
                 return attr;
         assert(0);
     }

     enum getAnnotation =3D helper;
 }


=20
 Nice, thank you. These are really helpful!
=20

 * Create a function that can only be called if the first argument has=






 a

 UDA of a specific type


 Since the UDA is only available at compile time, you can't do that
 without some kind of template.


=20
 Using the above tools I got what I wanted, so I'm satisfied for now.
 Thank you, though!
 -Matt Soucy
=20
=20


Update on this - unfortunately, I discovered that this doesn't seem to
work for any members, due to requiring a specific "this". I can't quite
figure out why though...

--=20
Matt Soucy
http://msoucy.me/
Oct 08 2013
parent "Adam D. Ruppe" <destructionator gmail.com> writes:
On Tuesday, 8 October 2013 at 20:59:04 UTC, Matt Soucy wrote:

 Update on this - unfortunately, I discovered that this doesn't 
 seem to work for any members, due to requiring a specific 
 "this". I can't quite figure out why though...


Try adding static to the helper functions in the templates. I 
just did that and this worked:

struct Whoa {
         int a;
}

class Foo {
          lol void argh() {}
          Whoa(12) int keanu;
}

void main() {
         pragma(msg, hasAnnotation!(Foo.argh, lol));
         pragma(msg, getAnnotation!(Foo.keanu, Whoa));
}


~$ dmd test34
true
Whoa(12)




The change to get it to compile was just this:

template hasValueAnnotation(alias f, Attr) {
         static bool helper() { // added static


and so on for all three of the helper functions, just make them 
all static.
Oct 08 2013