Help with Template Code

[John Demme]

Daniel Keep wrote:

> 
> 
> John Demme wrote:
>> Jarrett Billingsley wrote:
>> 
>>> "John Demme" <me at teqdruid.com> wrote in message
>>> news:eukg7o$m31$1 at digitalmars.com...
>>>> Hey all!
>>>>
>>>> There's a particular problem I'm trying to solve using templates.  I
>>>> don't see a reason that the compiler couldn't do this, but I'm not
>>>> certain I can do it with templates yet.
>>>>
>>>> Here's a slightly simplified pseudo-code-ish version of what I want to
>>>> do:
>>>>
>>>> T inst(T : struct)(T.tupleof t);
>>>>
>>>> Yes- this makes no sense, so let me describe.  I want to create a
>>>> templated
>>>> function wherein the template argument is a struct... OK, that's easy.
>>>> Next, I want the parameters of the function to be the types in the
>>>> struct. For example, if I have the following struct:
>>>> struct Foo {
>>>>        int a;
>>>>        float b;
>>>> }
>>>>
>>>> then the following call:
>>>> Foo f = inst!(Foo)(5, 8.26)
>>>> would pass the Tuple!(int,float)(5, 8.26) into the inst function.  No,
>>>> it's
>>>> not OK to add stuff so the calling code, but the inst function can be
>>>> as ugly as necessary.
>>>>
>>>> I feel like this should be possible, but I don't know how... Any ideas?
>>>>
>>>> Thanks
>>>>
>>>> --
>>>> ~John Demme
>>>> me at teqdruid.com
>>>> http://www.teqdruid.com/
>>> Wow!
>>>
>>> struct S
>>> {
>>>     int x;
>>>     float y;
>>>     char[] z;
>>>
>>>     static S opCall(typeof(S.tupleof) args)
>>>     {
>>>         S s;
>>>
>>>         foreach(i, arg; args)
>>>             s.tupleof[i] = arg;
>>>
>>>         return s;
>>>     }
>>> }
>>>
>>> void main()
>>> {
>>>     S s = S(1, 2.3, "hi");
>>>     writefln(s.x);
>>>     writefln(s.y);
>>>     writefln(s.z);
>>> }
>>>
>>> I really didn't think I would be able to write that.
>> 
>> Ahh!!! typeof!  That does it for me... much thanks.  BTW, with your
>> example above, you could probably turn that opCall into a mixin... It'd
>> be a nice little mixin to have in Tango and/or Phobos.
>> 
>> I've got one more template problem, but I'm pretty sure I can't do this. 
>> I now want to access the names of the struct's fields so that I could,
>> for example, make a templated function that accepts a struct and prints
>> name:value pairs for all the fields.  Is there any way I can do this? 
>> (If so, I'm gonna be really, really impressed.)
>> 
>> Thanks again
> 
> Not directly.  The way I'm going to solve this problem is to have a
> convention that any struct whose I want to be able to access by name
> should have a 'fieldsof' property.  This will be a tuple of strings that
> name the fields in the order they appear in the struct.  So, for your
> example:
> 
> struct Foo
> {
>     alias Tuple!("a","b") fieldsof;
> 
>     int a;
>     float b;
> }
> 
> In that case, Foo.fieldsof[i] is the name of the field Foo.tupleof[i].
> 
> Would be nice to have this built-in, but it's not a big drama.
> 
> -- Daniel
> 

Yeah... I'm doing something similar now.  I guess I'll stick with that.

Thanks

-- 
~John Demme
me at teqdruid.com
http://www.teqdruid.com/