Re: [boost] Named template parameters

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-03-05 22:43:39

Hmm, I think I missed a turn in the road earlier on ;)

Are you suggesting the following?

template <class Base,
          class Policies,
          class ValueOrNamedParameters = boost::detail::default,
          class ReferenceOrNamedParameters = boost::detail::default,
          ...>
class iterator_adaptor;

and could then be used like the old way:

iterator_adaptor<int*, default_iterator_policies, int, int&, ...>

and could also be used like this:

iterator_adaptor<int, default_iterator_policies,
  iterator_traits_builder
  ::reference<int>
  ::iterator_category<std::input_iterator_tag>

On Mon, 5 Mar 2001, David Abrahams wrote:

abraha> 
abraha> ----- Original Message -----
abraha> From: "Jeremy Siek" <jsiek_at_[hidden]>
abraha> 
abraha> > A good goal... but I don't know how to achieve it.
abraha> 
abraha> The rough outline goes like this:
abraha> 
abraha> 1. Instead of the current default values for e.g., Reference, use some tag
abraha> type, say boost::detail::default.
abraha> 2. Write a template is_named_parameters<T> that can detect whether a type is
abraha> built using the named parameters technique. Shouldn't be too hard, I think.
abraha> 3. Write a template select_named_parameters<T,U,V,W,X>::type that selects
abraha> the named parameter type among its arguments**. If there are no named
abraha> parameters in the args, returns a traits type where everything is
abraha> boost::detail::default.
abraha> 4. Synthesize the reference type something like this (pseudocode):
abraha> typedef typename select_named_parameters<Value, Reference, Category,
abraha> Distance, Pointer>::type named_params;
abraha> typedef (named_params::reference == default && Reference == default) ?
abraha>     iterator_defaults<Base,Value>::reference
abraha>     :   named_params::reference != default
abraha>         ? Reference != default ? ERROR : named_params::reference
abraha>         : Reference;
abraha> 
abraha> Make any sense?
abraha> 
abraha> -Dave
abraha> 
abraha> 
abraha> **You could also write merge_named_parameters<T,U> and chain them for
abraha> completeness, but this is probably good enough, and simpler.
abraha> 
abraha> 
abraha>  
abraha> 
abraha> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ 
abraha> 
abraha> 
abraha> 

----------------------------------------------------------------------
 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate                  email: jsiek_at_[hidden]
 Univ. of Notre Dame         work phone: [phone]
----------------------------------------------------------------------