Re: [Boost-users] [mpl] insert not inserting?

From: Scott Meyers (usenet_at_[hidden])
Date: 2007-03-29 00:54:22

David Abrahams wrote:
> Scott, I hate to say this, but I really think your first problem
> demonstrated that mpl::set is too broken to use.

Okay, I was pretty much coming to that conclusion myself.  For the time being, 
I'll use vector and be careful about avoiding duplicate entries (practical in my 
case, because all I'm trying to do is develop a proof of concept).  I really 
wish I could use what used to be referred to as typedef templates (and which now 
has a more general name and utility in C++0x, as I recall), but I'll swallow my 
desire for abstraction.  I'll just hardwire vector and move on.

>>> Well, excuse me if the following is obvious to you,
>>> but ::type is to a metafunction what actual invocation is
>>> to a run-time function. So, mpl::end<Constraints> refers
>>> to the name of the entity, but does not actually compute
>>> its "return value" unless you add the ::type suffix.
>> I don't think this always holds.  
> 
> Yeah, it always holds.

Thanks for the detailed elaboration, which I'll study when I have a bit more 
time.  You understand this stuff better than I ever will, but I have to say that 
as a naive new user,

   BOOST_MPL_ASSERT(( mpl::equal<mpl::set<A,B>, mpl::set<A,B> > ));

sure looks like I'm invoking mpl::equal and asserting that the result is true.

I will remark that after reading your book and spending probably between 1 and 2 
dozen hours trying to write a simple MPL-based program, the rules for when to 
use the ::type suffix are a complete mystery to me.  Originally I just omitted 
it unless my code didn't compile (assuming, of course, I wanted it to compile -- 
sometimes the goal of TMP is to prevent something from compiling), but now I've 
advanced to throwing it in if my MPL program doesn't behave the way I want it 
to.  It's still a black box to me, so if I'm mystified (not uncommon), I try my 
code both with and without the black box.  The result is awful, I'm sure, but 
speaking only for myself, I think this is a necessary stage in my scramble up 
the learning cliff.

Scott