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

30 Jan 2005

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From: "Terje Slettebø" <tslettebo@broadpark.no>
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From: "Yuval Ronen" <ronen_yuval@yahoo.com>
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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.
template<typename Target, typename Source>
    Target lexical_cast(const Source &arg)
    {
        // Array-to-pointer decay
typedef typename mpl::if_<
          is_array<Source>,
          add_pointer<add_const<remove_bounds<Source>::type>::type>::type,
          Source
        >::type NewSource;
detail::lexical_stream<Target, NewSource> interpreter;
        Target result;
if(!(interpreter << arg && interpreter >> result))
            throw_exception(bad_lexical_cast(typeid(NewSource),
typeid(Target)));
        return result;
    }
The additional includes are:
#include <boost/type_traits/is_array.hpp>
#include <boost/type_traits/remove_bounds.hpp>
#include <boost/type_traits/add_pointer.hpp>
#include <boost/type_traits/add_const.hpp>
#include <boost/mpl/if.hpp>
Come to think of it, it can be done simpler and clearer, and avoiding any
extra includes, like this:

    namespace detail
    {
        template<class T>
        struct array_to_pointer_decay
        {
            typedef T type;
        };

        template<class T, size_t N>
        struct array_to_pointer_decay<T[N]>
        {
            typedef const T * type;
        };
    }

    template<typename Target, typename Source>
    Target lexical_cast(const Source &arg)
    {
        typedef typename detail::array_to_pointer_decay<Source>::type
NewSource;

        detail::lexical_stream<Target, NewSource> interpreter;
        Target result;

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

This requires support for partial specialisation, but so does remove_bounds
used in the first suggestion, so it doesn't change the compiler
requirements. Besides, the current version of lexical_cast in CVS doesn't
work with MSVC 6, anyway. Of course, if this was very important, I guess it
would be possible to fall back to pass by value for MSVC 6, and possibly fix
other problems with it. I also see that MSVC 6 is no longer part of the
Boost regression test.

Regards,

Terje