Re: [Boost-users] Re: Two issues with lexical_cast

30 Jan 2005

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From: "Yuval Ronen" 
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Terje Slettebø wrote:
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I've brought this thread to Kevlin Henney's attention, as I've tended to
take care of lexical_cast related questions on the list (including
forwarding things needing his consideration, such as any changes to
lexical_cast, to him). I'm replying here to let you know that your
report is
being taken care of.
Many, many thanks!
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1. lexical_cast(Source arg) takes the source argument by value. Why not
by const& ? Calling lexical_cast(some_basic_string) now makes a copy of
the source argument for no reason.
I heard from Kevlin that the reason was that there originally were some
issues with binding to string literals. Changing it to use const
reference
in my local copy still gives a lot of error messages, having to do with
std::numeric_limits being used in the implementation. If we find a way
to
fix this (yet letting it still work on compilers with no class template
partial specialisation, like MSVC 6), without breaking existing
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then apparently, this change should be ok. If anyone makes a patch for
Thanks, yourself. :) Feedback is always welcome (regardless who's library it
is); it's an opportunity to improve it.

programs,
this,
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it would be welcome.
Hmmm, interesting. I'll give it a look. Maybe I'll be lucky enough to
think of something...
I've been looking at this, as well, and after trying one approach (doing a
compile-time switch to select between the ordinary, and a "dummy"
std::numeric_limits (if the source type is an array)), I bumped into another
problem: the specialisations for wchar_t, which assumes "const wchar_t *",
not "const wchar_t[...]". This might again be fixed, using partial
specialisation, to work with array, but another approach could be to "go
back to square one" and do the array-to-pointer decay "manually":

    template
    Target lexical_cast(const Source &arg)
    {
        // Array-to-pointer decay

        typedef typename mpl::if_<
          is_array<Source>,
          add_pointer::type>::type,
          Source
        >::type NewSource;

        detail::lexical_stream interpreter;
        Target result;

        if(!(interpreter << arg && interpreter >> result))
            throw_exception(bad_lexical_cast(typeid(NewSource),
typeid(Target)));
        return result;
    }

This passes all tests on Intel C++ 7.1, MSVC 7.1 and g++ 3.2. I've sent it
as a suggestion to Kevlin, as well, I think he's pretty busy with other
things. Anyone having a better suggestion than the above is of course
welcome to contribute it. One thing about the above change is that it
requires a few more includes, but this is really the simplest way I found to
be able to use pass by const reference for lexical_cast.

The additional includes are:

#include 
#include 
#include 
#include 
#include 

Regards,

Terje