Re: [Boost-users] [mpl]... is there an mpl::string

Subject: Re: [Boost-users] [mpl]... is there an mpl::string
From: Eric Niebler (eric_at_[hidden])
Date: 2009-04-08 14:16:51

Noah Roberts wrote:
> Eric Niebler wrote:
>> I was on the fence myself about 
>> making c_str a class static of mpl::string. My reason for doing so was 
>> that getting a C-style string at compile time is really the only 
>> reason to use mpl::string, and so accessing it shouldn't incur an 
>> extra template instantiation.
> 
> Well it wouldn't exactly be 'extra' would it?  The mpl::string itself 
> would then never be instantiated since it contains nothing but typedefs 
> and such.  You might need to make "size" an enum value...but it should 
> be at least possible to make string uninstantiated.

I doubt any interesting metaprogram that uses mpl::string could avoid 
instantiating mpl::string. What I don't know is whether the templates 
needed to initialize mpl::string::c_str[] are instantiated even if 
c_str[] is never referenced. If so, those instantiations are wasted. 
That would be an argument in favor of moving c_str outside mpl::string.

There's another consideration that I've been glossing over. mpl::string 
isn't *really* random access. Since mpl::string<'a','b','c'> designates 
the same character sequence as mpl::string<'abc'>, it takes O(N) 
template instantiations to find the N-th element in the sequence, at 
least in the current implementation. I'd like to fix that, but I don't 
know how (yet). It'd also be nice to be able to initialize the c_str[] 
array without instantiating a pile of templates.

>> Anyway, I've invested enough time in this already and am happy with 
>> the result. If you'd like to see mpl::c_str<>, would you care to 
>> submit a patch? Or at least open a feature-request Trac ticket?
> 
> I could see about doing it at home but I'm notorious for not doing so. 
> Interesting problem though, I might get to it.

Yep, interesting. There are no small projects, it seems. <sigh>

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com