Help with Template Code

[Daniel Keep]

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

-- 
int getRandomNumber()
{
    return 4; // chosen by fair dice roll.
              // guaranteed to be random.
}

http://xkcd.com/

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