I'm trying to declare format for database record in compile time. I consider to use attributes fo this. Can I do something similar to this code?

struct RecordFormat(T)
{}

alias Format = RecordFormat!( @("cool") int );

pragma( msg, Format );
	
void main()
{
}

Or I want something strange that is not supported by language? For now I don't understand whether UDA are a part of types or they are a part of variable instance definition or something different (3rd dimension)).

For example 'const int' is a type but what @attribute int is? Can it be considered as separate type or it could be binded to definition of variable only?
@attribute int number;

I failed to alias type
alias coolInt = @("cool") int;

But I can alias const int
alias constInt = const int;

I think I understood something wrong about attributes.

UDA's are compile-time metadata associated with a specific declaration.  So in something like:

@foo int x;

The @foo is attached to x, but is not part of the type.

On Wed, Jun 25, 2014 at 05:10:06PM +0000, Chris Nicholson-Sauls via Digitalmars-d-learn wrote:
> UDA's are compile-time metadata associated with a specific declaration.  So in something like:
> 
> @foo int x;
> 
> The @foo is attached to x, but is not part of the type.

The term "attribute" is a bit confusing, especially since "property" is also used in the language to refer to something completely different. A better term is perhaps "annotation". The @foo is an annotation on x, but its type is just int. Furthermore, the @foo annotation on it only exists at compile-type; it doesn't exist at runtime. The purpose is really to enhance compile-time introspection and metaprogramming; but what the OP wants seems to be something else altogether.

Sadly, the "attribute" terminology has stuck, and is unlikely to change.


T

-- 
What do you get if you drop a piano down a mineshaft? A flat minor.

On Wednesday, 25 June 2014 at 17:21:21 UTC, H. S. Teoh via Digitalmars-d-learn wrote:
>
> The term "attribute" is a bit confusing, especially since "property" is
> also used in the language to refer to something completely different. A
> better term is perhaps "annotation". The @foo is an annotation on x, but
> its type is just int.

Agree whole-heartedly.  I usually introduce people to the idea by referring to them as annotations, which they understand quickly, then later tell them that it's called an attribute because of reasons no one knows.  I get funny looks, but at least they comprehend it.  I'm not so sure it's too late to get the terminology changed, though, and I sincerely hope it will.

But if I write

@("hello") struct Hello {}

so all of the variables that have type Hello will have attribute @("hello") like come type qualifier because attribute is a part of declaration of Hello. Do I understand correctly?

On Thursday, 26 June 2014 at 07:11:03 UTC, Uranuz wrote:
> But if I write
>
> @("hello") struct Hello {}
>
> so all of the variables that have type Hello will have attribute @("hello") like come type qualifier because attribute is a part of declaration of Hello. Do I understand correctly?

No, it is only the symbol Hello that will have the attribute @("hello"). Attributes are attached to symbols, not types or values.