Re: UDA initialize a user defined type attribute field

[Ali Çehreli]

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