UDA usage

[Matt Soucy]

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 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;
>> }
>>
>>
> 
> 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
>>
>> 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
> 
> 
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...

-- 
Matt Soucy
http://msoucy.me/

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