• Arjan (29/29) Feb 18 2014 When a user defined type is used as an attribute like this:
• Adam D. Ruppe (3/4) Feb 18 2014 You could do it by adding a constructor to MyAttrib that only
• Arjan (11/15) Feb 18 2014 But what happens than when only the first field 'name' is used in
• bearophile (7/12) Feb 18 2014 Unfortunately D doesn't have named arguments.
• =?UTF-8?B?QWxpIMOHZWhyZWxp?= (31/60) Feb 18 2014 This problem has nothing to do with UDAs and can be solved similar to

"Arjan" <arjan ask.me.to> writes:

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.

"Adam D. Ruppe" <destructionator gmail.com> writes:

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.

"Arjan" <arjan ask.me.to> writes:

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"?

"bearophile" <bearophileHUGS lycos.com> writes:

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

=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:

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