lazy facet instantiation

[Howard Hinnant]

Has the locale implementation technique of "lazy facet instantiation"  
been kicked around?  I searched the archive but came up empty.  Imho  
this is a critical technique for reducing the code size triggered by  
casual use of C++ I/O.

The basic idea is that has_facet and use_facet are rigged to  
recognize "required" facets.  For example if has_facet is called for  
a "required facet", it just answers true without checking (and that's  
a compile-time-if which checks if the facet is standard, so there is  
very little code generated.  Similarly use_facet is specialized on  
this "required facet" tag, checks the locale to see if it exists, and  
if the facet doesn't, just adds it on the spot.  The big advantage  
here is that a facet never gets instantiated unless client code  
specifically calls:

use_facet<some_standard_facet>(loc);

The default locale ctor does nothing - adds no facets, not even  
instantiates them.

For example, uses of the message, money, collate and time facets are  
rare.  Why pay for them if you don't use them?

My apologies in advance if I've simply misunderstood the current  
implementation.

-Howard