Re: [Boost-users] mpl documentation question

Subject: Re: [Boost-users] mpl documentation question
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-04-07 14:17:50

AMDG

On 04/07/2012 12:05 PM, Robert Ramey wrote:
> 
> That's interesting and useful.  So I guess my problem is with the usage
> of the term "equivalent".  I had interpreted this as "can be substituted 
> for".
> 
> So I'm now thinking that my original concern is justified - that the 
> document
> doesn't accurately convey what "less<c1, c2>" really is.

Yes.  I agree that the documentation needs
some work here.

>  This has always
> been confusing to me - and still is. I totally get what "less<c1, c2>::type" 
> is
> supposed to mean but the meaning and intended usage of "less<c1, c2>"
> is still lost on me.
> 

Okay, I'll try to explain more clearly.
mpl::less<c1, c2>::type is the result of invoking
the metafunction.  This is the way all metafunctions
work in MPL so no surprises here.

Now, mpl::less<c1, c2> is itself an MPL Integral
Constant.  This works because MPL Integral Constant
is a Concept rather than a specific template.
In other words, if you only care about
the *value* of the result, and not the *exact type*,
then you can use mpl::less<c1, c2> instead of
typename mpl::less<c1, c2>::type.  Just think
of this as a convenient short-cut for arithmetic
metafunctions.

In Christ,
Steven Watanabe