David Abrahams wrote: [...]
The question is, what should we do about this? It turns out to be surprisingly hard to write the declarations of these temporary variables so they work in all cases. For example, v had to be a const& because in one case index_result<...>::type turned out to be
char const[4]
and when the default is used, it's
double
with p[value || ...] returning a double rvalue (by value).
Thoughts?
Make the user add reference to the Default type instead of index_result<>::type? template<class Params> void f_impl(const Params& p) { typename boost::parameter::index_result< Params, struct name_>::type n = p[name]; typename boost::parameter::index_result< Params, struct value_, double >::type v = p[value || boost::bind(&value_default) ]; typename boost::parameter::index_result< Params, struct index_, int >::type i = p[index | 999]; g(n,v,i); } So index_result<Params, Keyword, Default>::type would be either Params[Keyword]::type& or Default. And maybe add another metafunction that returns Params[Keyword]::type without a reference. -- Daniel Wallin