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

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-12-17 12:37:45

On Dec 17, 2005, at 12:17 AM, Eric Niebler wrote:

> Here's my latest attempt. This works on gcc 3.4.4 (cygwin), but not on
> VC7.1 or (sadly) Comeau Online. Language lawyers, start your engines!
>
> 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)))\
>      )

That's very cool!

Just as a thought experiment, I tried to re-implement your IS_RVALUE  
macro in (what I hope to be) C++0X.  My purpose was mainly just to  
amuse myself, but also as a sanity check that C++0X will actually  
make things like this easier.  I'm aware that the motivation for  
IS_RVALUE is also greatly reduced in C++0X.  I'm just exploring.   
Fwiw I came up with:

#include <type_traits>

template<typename T> T&& test(T&&);
#define IS_RVALUE(x) (std::is_rvalue_reference<decltype(test(x)) 
 >::value)

I only spent a couple of minutes on it, so I wouldn't take it to the  
bank.  But it is lightly tested.

-Howard