"Eric Niebler" <eric@boost-consulting.com> writes:

Imagine a template defined like this:

template< class T, class U = void > struct foo {};

I would like to turn this into a metafunction class F such that:

mpl::apply< F, int >::type ==> foo<int, void>

and

mpl::apply< F, int, float >::type ==> foo<int, float>

Can this be done? If F is foo< _1, _2 >, then the first apply above won't compile because it hasn't been given enough arguments.

template <class L, class D> struct default2nd { template <class A0, class A1 = D> struct apply : mpl::apply<L,A0,A1> {} }; default2nd< foo<_,_>, void> ? -- Dave Abrahams Boost Consulting www.boost-consulting.com