Re: [boost] Re: mpl/loki

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-07-13 08:44:45

Howard Hinnant wrote:
> On Sunday, July 13, 2003, at 08:49  AM, Peter Dimov wrote:
>
>> Maybe the problems are caused by overloading void_. I haven't looked
>> at MPL
>> recently, but as a general observation I have identified at least
>> three uses
>> of a void_-like entity.
>>
>> 1. A type parameter used to emulate a variable argument template. I
>> use '_missing' for this purpose (leading underscore for
>> implementation details.)
>>
>> template<class A1 = _missing, class A2 = _missing, ...> struct F;
>>
>> 2. An optional parameter that, when not supplied, has a reasonable
>> (dependent) default. I use 'unspecified'.
>>
>> template<class R = unspecified, class F> ... bind(F f);
>>
>> 3. A type that is guaranteed to be distinct from all other useful
>> types.
>> 'nil' is what Lisp calls it; void_ is fine, too.
>
> Another possible spelling for this animal is:
>
> class nat {nat();};
>
> Inspired from nan.  In this case means Not A Type.  It is nice and
> short which comes in handy for when there are a lot of template
> parameters to default.

Yep. But I don't think defaulting to 'nat' is correct; defaults in this case
are 'missing'.

You can make a list of nat's, and this is not an empty list. IOW with:

template<class A1 = nat, class A2 = nat, ...> struct list;

you can't tell the difference between list<>, list<nat>, list<nat, nat>...

In Lisp this would correspond to () (which is also nil there), (nil), (nil
nil), ...