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

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-29 22:02:34

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

> David Abrahams wrote:
>> 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> >
>
> I find it a lot easier to understand if I focus on the fact that, unlike runtime 
> asserts, which conceptually take boolean values, BOOST_MPL_ASSERT takes a 
> function, which it itself invokes.  This is, to me, a *major* change.    It's 
> the difference between
>
> bool f();  // some function
>
> assert( f() );  // the way asserts work now
>
> assert( f );    // the way asserts would work if they worked like MPL asserts

Right.

> I think a lot of the trouble I have is due to a lack of
> documentation explaining the differences between what I'm used to
> with runtime C++ and how MPL does things.  IME, it's not that the
> MPL documentation is incorrect, it's just that the reference manual
> is, well, a reference manual.  So I skim over the fact that
> BOOST_MPL_ASSERT takes a metafunction, because I'm thinking, "I
> already know how asserts work." But I *don't* know how MPL asserts
> work, because they use a completely different conceptual model, yet
> they use the same vocabulary.
>
> I'm not blaming the reference manual here.  It does what it should
> do.  And I'm not blaming your book.  But note in your book how on
> page 160 you show that BOOST_STATIC_ASSERT takes an *expression* and
> on page 162 you show how BOOST_MPL_ASSERT takes a *metafunction*,
> yet the only thing you explicitly call the reader's attention to is
> the need for double parentheses in the MPL assert.  At least in my
> case, it was really easy to fail to note the shift from an
> expression (what I was used to) to a function (quite a different
> thing).

Well, you have a very good point there.  It would have been good to
point that out more explicitly.

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