Re: [boost] [regex] format_perl conundrum

From: Eric Niebler (eric_at_[hidden])
Date: 2007-03-19 15:58:17

John Maddock wrote:
> Eric Niebler wrote:
>> the RegexTraits concept doesn't have
>> toupper() and tolower() functions. I suggest adding them.
> 
> Right, but format_perl isn't part of TR1, so this is all in the realms of 
> vendor-specific extensions.  I added some *optional* extra members to the 
> traits class to deal with this: the code detects at compile time whether the 
> member are there, and uses them if they are, otherwise uses some sensible 
> defaults.

Ah. I didn't know that format_perl didn't make the cut. But it's still a 
problem for RegexTraits::value(), format_default ("$1") and format_sed 
("\\1").

To be honest, I find the need for the hacks you describe above to be a 
bit distasteful. Any vendor who wants format_perl-like behavior will 
route around the damage in the RegexTraits concept. A better (?) design 
might have to been to go whole-hog with locales and facets, and define 
some standard regex traits facets that basic_regex<> and match_results<> 
can query for at runtime. Eg.: If a regex-trait version-2 facet isn't 
installed, try for version-1, then defer to the ctype and collate 
facets, etc. And the traits doesn't need to be part of the basic_regex<> 
type (it isn't in xpressive). C-compatibility could have been maintained 
by providing a ctype facet that is implemented in terms of the global C 
locale. The whole RegexTraits concept and version tags seem like wheel 
reinvention to me.

Just thinking out loud,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com