When a user defined type is used as an attribute like this:
// the attribute
struct MyAttrib
{
    immutable string name;
    imuttable int sz;
    immutable string defaultvalue;
}

// using the attribute
class Data
{
    // field 'id' has attribute MyAttrib with name and sz set to 'fancy name'
    // and 10 resp.
    @MyAttrib( "fancy name", 10 )
    long id;
    ...
}


But how do I only set the MyAttrib.defaultvalue?

class OtherData
{
    //@MyAttrib( ??defaultvalue = "null" ?? )
    //@MyAttrib( string.init, int.init, "null" ) //seems to work?
    long id;
    ...
}

MyAttrib instances may be initialized like this:

auto myattrib = { defaultvalue:"null" };

But this seems not possible when using MyAttrib as attribute.

On Tuesday, 18 February 2014 at 23:43:56 UTC, Arjan wrote:
> But how do I only set the MyAttrib.defaultvalue?

You could do it by adding a constructor to MyAttrib that only takes one string and fills it in that way.

On Tuesday, 18 February 2014 at 23:46:28 UTC, Adam D. Ruppe wrote:
> On Tuesday, 18 February 2014 at 23:43:56 UTC, Arjan wrote:
>> But how do I only set the MyAttrib.defaultvalue?
>
> You could do it by adding a constructor to MyAttrib that only takes one string and fills it in that way.

But what happens than when only the first field 'name' is used in MyAttrib like this:

class OtherData
{
    @MyAttrib( "fancy name" )
    long id;
    ...
}

I guess the field MyAttrib.defaultvalue will be assigned the value "fancy name"?

Arjan:

> MyAttrib instances may be initialized like this:
>
> auto myattrib = { defaultvalue:"null" };
>
> But this seems not possible when using MyAttrib as attribute.

This could become a D enhancement request.


> I guess the field MyAttrib.defaultvalue will be assigned the value "fancy name"?

Unfortunately D doesn't have named arguments.

You can use chains, returning this from some methods:

MyAttrib().setName("Foo").setSz(10).setDefaultValue("Bar")

Bye,
bearophile

On 02/18/2014 03:43 PM, Arjan wrote:
> When a user defined type is used as an attribute like this:
> // the attribute
> struct MyAttrib
> {
>      immutable string name;
>      imuttable int sz;
>      immutable string defaultvalue;
> }
>
> // using the attribute
> class Data
> {
>      // field 'id' has attribute MyAttrib with name and sz set to 'fancy
> name'
>      // and 10 resp.
>      @MyAttrib( "fancy name", 10 )
>      long id;
>      ...
> }
>
>
> But how do I only set the MyAttrib.defaultvalue?
>
> class OtherData
> {
>      //@MyAttrib( ??defaultvalue = "null" ?? )
>      //@MyAttrib( string.init, int.init, "null" ) //seems to work?
>      long id;
>      ...
> }
>
> MyAttrib instances may be initialized like this:
>
> auto myattrib = { defaultvalue:"null" };
>
> But this seems not possible when using MyAttrib as attribute.

This problem has nothing to do with UDAs and can be solved similar to regular cases, e.g. by using a factory function:

// the attribute
struct MyAttrib
{
    immutable string name;
    immutable int sz;
    immutable string defaultvalue;
}

MyAttrib makeMyAttribWithDefaultValue(string defaultValue)
{
    return MyAttrib(MyAttrib.name.init, MyAttrib.sz.init, defaultValue);
}

// using the attribute
class Data
{
    // field 'id' has attribute MyAttrib with name and sz set to 'fancy name'
    // and 10 resp.
    @MyAttrib( "fancy name", 10 )
    long id;

    @makeMyAttribWithDefaultValue("hello")
    int i;
}

void main()
{
    pragma(msg, __traits(getAttributes, Data.id));
    pragma(msg, __traits(getAttributes, Data.i));
}

Ali