From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2003-01-04 04:08:22
Jaap Suter wrote:
> > Nifty! You're using one of my favorite metaprogramming tricks, the
> > default template parameter which allows you to avoid creating a
> > separate implementation template. I almost forgot about that one,
> > it's been so long since I've been able to use it.
> Funny you mention that. I've been meaning to ask for its
> appropriateness on this list. After all, we are exposing
> implementation details in the interface which is not something
> I would recommend on a daily basis.
At least one has to be careful :). There are at least two situations when
default template parameters affect the observable library behavior:
1) adding default _non-type_ template parameters "disqualifies" a
metafunction from participating in lambda expressions:
, int one_before_previous_ = 0
, int previous_ = 1
, plus< _1, fibonacci<_2> >
>::type sum; // compilation error here!
BOOST_STATIC_ASSERT(sum::value == 88);
2) adding a default template parameter to a metafunction which number of
parameters is already equal to BOOST_MPL_METAFUNCTION_MAX_ARITY results in
the same negative effect:
// BOOST_MPL_METAFUNCTION_MAX_ARITY == 5
, typename N2
, typename N3 = integral_c<int,0>
, typename N4 = integral_c<int,0>
, typename N5 = integral_c<int,0>
, typename result_ = integral_c<int,0>
>::type sum; // error!
Otherwise, it's more or less safe :).
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