[boost] lexical_cast "Future directions"

From: Early Ehlinger (earlye_at_[hidden])
Date: 2002-12-27 19:34:50

On the boost site for lexical_cast, a future direction is listed:

"Optimize the use of a stream away for identity conversions."

I recently encountered a situation where this optimization is not only nice
from a performance perspective, but is also necessary from a functional
perspective.

Deep inside of some nested templates, I had some code that effectively did
this due to the way the template parameters happened to play out in the
client code:

std::string source = "Hello there";
std::string dest = boost::lexical_cast< std::string >( source );

Normally, this would be a silly thing to do, but there are valid reasons
that the template code in place ended up doing this.  (Please take my word
for it.  It would likely take a month to go through the details of how I got
there...)  Anyway, the surprising result of this was that the space in
"Hello there" caused the interpreter.eof() check in lexical_cast to fail.
So if that check were not present, dest would have been assigned the value
"Hello".  Fortunately, the check was there, and instead I got a
bad_lexical_cast exception.

Is this the way lexical_cast is intended to work?  I don't know, but it
certainly isn't what I found to be behavior that "feels right".  So I
whipped up a modification to lexical cast that uses a template class and
overloaded static member functions to optimize calls like this.  Basically,
I've got the current lexical_cast code inside of
boost::detail::lexical_caster<>::do_it<>, and provide a form of
boost::detail::lexical_caster<>::do_it that is not a template and accepts
the same type as the target of the cast.

lexical_cast returns the result of boost::detail::lexical_caster< Target
>::do_it( object ).  If object is of the type Target, then the non-template
overload of ...::do_it is called, otherwise, the behavior is identical to
the current behavior.

I don't know if the community wants the new behavior or would prefer to keep
the old (and in my humble opinion, surprising) behavior.  In either case, I
thought I'd share my efforts.  Any feedback is greatly appreciated.

--
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begin 666 lexical_cast.hpp
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`
end