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 ?

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.

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...

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/

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.