Structure initialization

[Sheff]

Daniel Keep Wrote:

> 
> 
> Sheff wrote:
> > Olli Aalto Wrote:
> > 
> >> Sheff wrote:
> >>> Hi, everyone!
> >>> Can anybody tell me how to partially initialize a structure in place, i.e:
> >>> I have a structure:
> >>>
> >>> struct Props
> >>> {
> >>> int flag = 1;
> >>> int state = 0;
> >>> }
> >>> and a function:
> >>> void f(Props p);
> >>>
> >>> And I want to pass an instance of Props to f with only some fields initialized, right now I have to write like this:
> >>> f(Props(1,1000));
> >>> but I want to be able to write something like this:
> >>> f(Props(state:1000));
> >>> But that doesn't work, is there a way I can make it work ?
> >> You might want to try something like this:
> >>
> >> module props;
> >>
> >> import tango.io.Stdout;
> >>
> >> struct Props
> >> {
> >>      int flag = 1;
> >>      int state = 0;
> >>
> >>      public static Props opCall(int state)
> >>      {
> >>          Props p;
> >>          p.state = state;
> >>          return p;
> >>      }
> >> }
> >>
> >> int f(Props props)
> >> {
> >>      return props.state;
> >> }
> >>
> >> void main()
> >> {
> >>       int state = f(Props(3));
> >>       Stdout(state).newline;
> >> }
> >>
> >> O.
> > 
> > Hm, that's not exactly what I meant, you see, there may be hundreds of fields in a structure, and I want to initialize only some of them, for example:
> > struct A
> > {
> > int f1 = default_1;
> > int f2 = default_2;
> > ....
> > int f100 = default_100;
> > }
> > and what I want is to write 
> > f(A(f3:some_other_value,f50:some_another_value));
> > instead of
> > f(A(default_1, default_2, some_other_value, default_4, ..., default_49, some_another_value, ..., default_100));
> > 
> > I mean, there may be different combinations, I want to be able to write like this:
> > f(A(f3:some_other_value,f50:some_another_value));
> > or like this:
> > f(A(f12:some_other_value,f7:some_another_value,f13:some_another_value_2));
> > or etc...
> 
> The only way I can think of doing what you want is to make a template
> for it.
> 
> T make(T)()
> {
>     return T.init;
> }
> 
> T make(T, argsT...)(argsT args)
> {
>     assert( args.length & 1 == 0 );
>     T result;
> 
>     foreach( i,_ ; args )
>         static if( i & 1 == 0 )
> 	        mixin(`result.`~args[i]~` = args[i+1];`);
> 
>     return result;
> }
> 
> ...
> 
> f(make!(A,"f3","f50")(some_other_value,some_another_value));
> 
> WARNING: I just woke up, and the above has *not* been tested.  YMMV :)
> 
> 	-- Daniel
> 
> -- 
> int getRandomNumber()
> {
>     return 4; // chosen by fair dice roll.
>               // guaranteed to be random.
> }
> 
> http://xkcd.com/
> 
> v2sw5+8Yhw5ln4+5pr6OFPma8u6+7Lw4Tm6+7l6+7D
> i28a2Xs3MSr2e4/6+7t4TNSMb6HTOp5en5g6RAHCP  http://hackerkey.com/
Ok, thanks for your reply, I'll check this out.