lazy facet instantiation

[Howard Hinnant]

On Nov 15, 2005, at 3:48 PM, Martin Sebor wrote:

>> It also affords one other really nice extension:
>
> We had this in an early version of our library but ended up
> taking it out. IIRC, one problem was that for the trick to
> work everything had to be a template or inline which would
> tend to have unpleasant consequences for compilation times.

This is a real issue.  But one that I think is manageable.

> Another more serious issue with this had to do with the fact
> that user-defined facets need not have any particular kind
> of a constructor (default or otherwise), and use_facet can't
> just make an assumption that it does.

Here I think we have a minor misunderstanding.  I'm only proposing  
special treatment for standard facets instantiated on "reasonable  
types".  For example, the same restriction on character types as  
basic_string imposes for charT.  Likewise for iterators.  No special  
treatment for user-defined facets.

-Howard