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

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-29 12:21:00

on Thu Mar 29 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:

> 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.

You're passing a predicate nullary metafunction to the library and
asking MPL to invoke it.  It's sort of like, at runtime,

   int f(int, int); # akin to mpl::equal

   template <class NullaryFunction>
   int invoke(NullaryFunction g) { return g(); }

   int result = invoke( boost::bind(f, 3, 5) );
                        ^^^^^^^^^^^^^^^^^^^^
                        the nullary function, akin to 
                        mpl::equal<mpl::set<A,B>, mpl::set<A,B> >

The nice thing about this nullary metafunction is that its type name,
"mpl::equal<mpl::set<A,B>, mpl::set<A,B> >" is descriptive of what
you're testing, so BOOST_MPL_ASSERT can arrange to have it show up
prominently in the error message if the test fails.

> 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.

:) Not much of an advance.  Try "studying my detailed elaboration" and
let me know if that's still the case.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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