Re: [boost] [lexical_cast] Fast conversion from boost::iterator_range<std::string::iterator>?

Subject: Re: [boost] [lexical_cast] Fast conversion from boost::iterator_range<std::string::iterator>?
From: Michel Morin (mimomorin_at_[hidden])
Date: 2012-01-26 12:14:13

Hi Antony,

Thanks for your response.

Antony Polukhin wrote:
> There are a lot of libraries, that can have tuned conversions for
> lexical_cast. The bad thing, is that lexical_cast can be customized
> ONLY via overloading operator>>(stream&) and operator<<(stream&), and
> that is not very fast. That is the design. As simple, as possible.
> Multiple customization points will make the design obfuscated.
>
> It would be also a bad idea, to include a lot of different library
> headers to lexical_cast.hpp (will increase compilation times, add
> unnecessary dependencies...)

Makes sense. But, IMHO, it's worth optimizing codes for string ranges,
since string ranges are often used in string algorithms.

> If you assume, that it is safe to use &iter_rng.front() (extremely
> unsafe!),

IIUC, the main concern to implement optimized code is that
how to obtain begin and end pointer to CharT from `iter_rng` safely.
With C++11-conforming standard library, it is safe to implement

    bool operator<<(::boost::iterator_range<std::string::iterator> const& str)
    {
        if (!str.empty()) {
            start = const_cast<CharT*>(&str.front());
            finish = start + str.size();
        }
        else {
            start = 0;
            finish = 0;
        }
        return true;
    }

because `&*(s.begin() + n) == &*s.begin() + n` (`s` is an object of
std::basic_string<…>)
is guaranteed.

But, in C++03, the above code is not safe; the standard does not guarantee that
std::basic_string<…> is stored contiguously (though many standard library
implementations store it contiguously).

Regards,
Michel