Re: [boost] [lexical_cast] char types and UDTs

Subject: Re: [boost] [lexical_cast] char types and UDTs
From: Eric Niebler (eric_at_[hidden])
Date: 2012-04-11 13:12:23

On 4/11/2012 9:36 AM, Olaf van der Spek wrote:
> On Wed, Apr 11, 2012 at 5:18 PM, Eric Niebler <eric_at_[hidden]> wrote:
>>> You have to construct a string somewhere, don't you?
>>> If you have one already, you could use iterator_range<const char*>
>>> instead to avoid a copy.
>>
>> Yes, I see that lexical_cast has optimizations for
>> iterator_range<wchar_t const *> and a few other, sufficiently
>> 'string-like' types. But sub_match essentially *is* a string-like
>> iterator_range. (It's a std::pair of iterators.) I'm genuinely surprised
> 
> Is it? Does it have begin() and end() for example?
> Why not use std::iterator_range instead of std::pair?

Because the C++ standard says sub_match should inherit from std::pair.
But does it matter? There are other 3rd party types that I'm sure users
would like to adapt to lexical_cast. The docs just say a type needs a
stream insertion operator. Shouldn't that be sufficient?

Besides, using std::iterator_range would not solve the problem because
lexical_cast only knows about boost::iterator_range. Do you see?

>> there's no way to tell lexical_cast that. Instead I have to just know
>> (a) which are the magical types lexical_cast is optimized for and (b)
>> for which it can determine the correct underlying stream character type
>> (hint: the docs are unclear or out of date), and massage my type into
>> one of those before calling lexical_cast. Why?
> 
> How could it automatically determine the necessary character type?

It can't be done automatically, but lexical_cast can expose the
stream_char trait and make it a documented part of the interface. Users
can specialize it for their types.

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