Re: [Boost-users] generic programming : variadic virtual function

Subject: Re: [Boost-users] generic programming : variadic virtual function
From: Stephen Nuchia (snuchia_at_[hidden])
Date: 2009-08-26 11:18:07

> From: Bruno Lalande [mailto:bruno.lalande_at_[hidden]] 
> Wouldn't the following give the desired result?

Yes, in fact after I found no answers in my mailbox this morning I
noodled on the problem some more and came up with that solution.  The
difficulty is that there is a lot of non-variadic cruft common to the
general and specialized template classes. I didn't want to repeat all of
that.

My solution was to insert a new intermediate template class in the
hierarchy.  But that would scale poorly if there were a richer set of
variadic member enabling conditions.  And maintaining extra layers of
forwarding constructor definitions can be tedious and error-prone.  Oh
well, one can't have everything.

To summarize the issue: boost::enable_if does not seem to work for
selectively declaring member function signatures so if you have members
with signatures that vary in a complex way depending on template
parameters you have to use class template specialization.  To avoid
repeating non-variadic member declarations, factor them into a common
public base class.

Here's my class hierarchy now:

template<S,W> class pre_base; // common thing interface elements
template<S,W=void> class base : public pre_base<S,W> // general thing
interface
template<S> class base<S,void> : public pre_base<S,void> // thing
interface with no W
...
// a thing_list is a compound thing (exposes a thing interface) 
template <S,W> thing_list_base : public base<S,W> // common
implementation for a list of things
template <S,W=void> thing_list : public thing_list_base<S,W> // general
impl. for list of general things
template <S> thing_list<S,void> : public thing_list_base<S,void> //
impl. for list of things w/o Ws.

The repeated specialization pattern is what I was missing; the concrete
class template and the interface templates use the same pattern.  I had
the interface declarations in place but was struggling with the concrete
class templates.

Thank you!
-swn