[boost] Re: Meta function variants of mpl::apply_if neccesary?

From: Dirk Gerrits (dirk_at_[hidden])
Date: 2002-09-20 17:17:35

Terje Slettebø wrote:
>>From: "Aleksey Gurtovoy" <agurtovoy_at_[hidden]>
> 
>>Terje Slettebø wrote:
>>In the posting with my "factorial" metafunction
>>(http://aspn.activestate.com/ASPN/Mail/Message/1316005), I
>>propose to make the metafunctions evaluate their own arguments
>>(apply "::type" to them), just like it happens in run-time functions.
>>With that, the factorial example becomes (quoting from my posting):
>>
>>
>>>"value" is another new metafunction. It's a way to provide a generic
>>>constant, of the type given in the first argument, and the
>>>value given in the second. Thus, if "Value" is int_c<...>, then
>>>"value<Value,1>::type" is int_c<1>.
>>
>>mpl::integral_c<..>?
> 
> I understand you haven't had a chance to read the post-review postings, as
> you say below here, because in the one where I introduced "value," I
> explained that it was a way to provide a generic constant, so that a
> metafunction could use any value type (integral_c, int_c, rational_c,
> fixed_c, etc.), rather than having to write one for each, and it still
> wouldn't work for any new type.
> 
> It works like this: If Type is e.g. integral_c<...>, then
> "value<Type,0>::type" gives integral_c<0>. If Type is rational_c, then it
> gives rational_c<0,1>, etc.

Please forgive the insolence of a MPL newbie, but I thought integral_c 
did exactly what your value is supposed to do? And I think that is what 
Aleksey was trying to explain as well.

Correct me if I'm wrong though.

Dirk Gerrits