Structure initialization

[Daniel Keep]

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/

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