Re: Traits for character underlying types

From: Daryle Walker (darylew_at_[hidden])
Date: 2001-02-21 14:45:34

on 2/20/01 3:36 PM, Howard Hinnant at hinnant_at_[hidden] wrote:

> Daryle Walker wrote on 2/20/01 2:48 PM
>> The "char" type is supposed to be like either "signed char" or "unsigned
>> char," which one is implementation defined.  Similarly, the "wchar_t" type
>> is supposed to be like one of the other integral types, which one is
>> implementation defined.  Could we have some sort of structure indicating
>> those types' base types?  I guess it would be something like:
[SNIP example]

>> How would we figure out the types?  I guess something using <climits> macros
>> and/or comparisons from <limits> values would be used.  What header would
>> this go under, "type_traits.hpp" or a new one?
> 
> I'm sure this could be done with <climits> macros.  But the following
> might be more fun! :-)
[SNIP big example with a bunch of old and hopeful class templates]

I don't necessarily need something big & fun, maybe something quick & dirty
will do -_^.  This is what I have so far:

//=========================================================================
template <class T, class B = ::boost::detail::empty_base>
struct character_convertible : B
{
  operator char() const
  {
#if CHAR_MIN == 0
    return static_cast< unsigned char >( static_cast<const T&>(*this) );
#else
    return static_cast< signed char >( static_cast<const T&>(*this) );
#endif
  }

#ifndef BOOST_NO_INTRINSIC_WCHAR_T
  operator wchar_t() const
  {
    // The underlying type here is a complete guess.  Help!
    return static_cast< unsigned short >( static_cast<const T&>(*this) );
  }
#endif
};
//=========================================================================

(It's from an upcoming version of the "operators.hpp" header.)

BTW, where does the BOOST_LONGLONG_SUPPORT come from?  I didn't see it in
the latest "config.hpp" on CVS.  Is this also a wish?  For now, I #include
<climits> and check if ULLONG_MAX and/or LLONG_MAX exist.

> Seems like I've seen someone talk about a "integral best fit" lib that
> might simplify all the if'ing I'm doing in the wchar_t specialization,
> but alas I can't remember who or where.

Could you be talking about the "/boost/integer.hpp" header, which lets an
user pick the best integral type for a certain bit size?  I started that
discussion for the CRC stuff.  I've thought of it, but I would have to
somehow incorporate the signed/unsigned determination.

> After all that, this is not a comment on your first question of whether
> such a struct should be part of boost.  It is simply a sample
> implementation.

If it's just for my quick needs above, it could just stay in that header.  I
guess that someday someone else might need that information.

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com