[Boost-users] Re: C++ Template Metaprogramming - factorial example

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2005-01-24 15:49:58

Brad Austin wrote:
> I wrote:
>> I recently purchased David Abrahams' and Aleksey Gurtovoy's new book
>> on the MPL, "C++ Template Metaprogramming" (obviously recently since
>> it's only been out a couple of weeks), and am working through some of
>> the examples.  I'm having trouble with the factorial example in
>> section
>> 8.3.1, page 160:
>>
>>     #include <boost/mpl/int.hpp>
>>     #include <boost/mpl/multiplies.hpp>
>>     #include <boost/mpl/equal.hpp>
>>     #include <boost/mpl/eval_if.hpp>
>>     #include <boost/mpl/prior.hpp>
>>     #include <boost/static_assert.hpp>
>>
>>     namespace mpl = boost::mpl;
>>
>>     template <class N>
>>     struct factorial
>>       : mpl::eval_if<
>>             mpl::equal_to<N,mpl::int_<0> >  // check N == 0
>>           , mpl::int_<1>                    // 0! == 1
>>           , mpl::multiplies<                // N! == N * (N-1)!
>>                 N
>>               , factorial<typename mpl::prior<N>::type>
>>             >
>>         >
>>     {
>>         BOOST_STATIC_ASSERT(N::value >= 0); // for nonnegative N
>>     };

With the modified include directive, the code compiles fine for me on GCC
(MinGW) 3.2.x, 3.3.x and 3.4.x.

> Anyway, is this mailing list an appropriate place to bring up issues
> like this?  I know David and Aleksey are members, but I'm not clear
> on whether their book qualifies as an official Boost product.

This is as good a place as any to bring up the question. You posted on a
weekend; now that it's over you should get an anser soon.

Jonathan