Re: [boost] C++ millennium challenge solved? (was: lvalues and rvalues)

From: Daryle Walker (darylew_at_[hidden])
Date: 2005-12-18 16:12:11

On 12/17/05 12:17 AM, "Eric Niebler" <eric_at_[hidden]> wrote:

> template<typename T>
> struct probe
> {
>    operator T ();
>    operator T volatile const &() const;
> };
> 
> template<typename T>
> probe<T> make_probe(T const &);
> 
> template<typename T>
> char is_volatile(T const &);
> 
> template<typename T>
> double is_volatile(T const volatile &);
> 
> #define IS_RVALUE(x) \
>    (\
>    sizeof(char) == \
>    sizeof(is_volatile(true?make_probe(x):(x)))\
>    )

Other versions of the "choose via 'sizeof' comparisons" trick, including
ones I've seen in Boost, use types like "char[1]" and "char[2]".  Why?
Because there is no ban on _all_ built-in types being the same size(*).  So
the assumption of "sizeof(char) != sizeof(double)" can fail.

* Which would have to be at least 32 bits (or 64 in C99).

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