"Vesa Karvonen" <vesa_karvonen@hotmail.com> writes:
David Abrahams
Terje Slettebø:
Yes, that is exactly what I mean above. *Systematic* use of lazy template metaprogramming would yield the following (unoptimized) factorial function:
template<class n> struct FACT : IF<IS_0<n>, INT<0>, TIMES<n, FACT<PRED<n> > > > {};
Elegant, indeed.
But it doesn't work ;-)
Yes, the code snippets in my message have several typos. Like I recall seeing certain B.S. saying in a USENET posting, I don't (always) compile my e-mails. :)
However, I have written a complete walkthrough of the factorial example that you can find in the library I just updated:
http://groups.yahoo.com/group/boost/files/LazyMPL/
[...]
In fact it would look almost the same in MPL, though ;-)
template<class n> struct fact : if_<equal_to<n, int_<0> >, int_<1>, // <== here's the fix ;-) multiplies<n, fact<typename prior<n>::type> > > {};
Yes, I realize that MPL contains *lots* of "glue" to make Strict TMP (strict template metaprogramming) as pleasant as possible.
However, are you sure that the above works? It seems to me that you are missing at least an invocation... ;--)
Heh, you're right ;-). Needs eval_if (nee apply if). Lazy is much nicer.
The one typename appears there because prior can work on iterators as well as numeric types. We can kill it this way:
template<class n> struct fact : if_<equal_to<n, int_<0> >, int_<1>, multiplies<n, fact<minus<n, int_<1> > > > > {};
And this particular case is especially friendly to MPL. ^^^^^^^^^^^^^^^^^^^^^^^^^^ Some of those advantages are lost when you're not manipulating numeric values.
I'm not sure what mean here. Please elaborate.
I mean that MPL doesn't do nearly as well as this, syntactically, on examples that don't involve numeric computation. You can always make a metafunction that always returns an integral constant wrapper look like an integral constant wrapper itself, which makes factorial favorable to MPL.
This FACT isn't generic (doesn't work for LONG<3>, for example; I assume you have LONG).
No. The library only has the INT-template, whose template argument is a long. Designing a complete numeric tower hasn't been a priority so far. (Actually, I think that supporting several primitive integer types may be an unnecessary overkill anyway
I always wondered about that myself.
but this is something I have no interest to discuss at this point. It would be a red-herring, IMO. The issue is orthogonal to the issues of Lazy TMP vs Strict TMP.)
Fair enough. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com