[boost] Re: L-values and R-values

From: Eric Niebler (eric_at_[hidden])
Date: 2004-12-02 10:05:27

David Abrahams wrote:
> David Abrahams wrote:
> 
>> Still no dice for vc7.1, though.
> 
> 
> Hum, that was too easy:
> 

For every difficult problem, there is a solution that is simple, elegant 
... and wrong. Correct version below fails. :( You can't use int for 
this test because the "const" is ignored in "int const const_rvalue()". 
You need to use UDTs.

#include <boost/static_assert.hpp>

typedef char yes;
typedef char (&no)[2];

template <class T>
yes is_nonconst_rvalue(T const&, ...);

template <class T>
no is_nonconst_rvalue(T&, int);

struct foo
{
#ifndef BOOST_MSVC
     template<typename U>
         operator U() const;

     template<typename V>
         operator V& () volatile const;
#else
     template<typename U>
         operator U() volatile;

     template<typename V>
         operator V& () const;
#endif

     static foo const instance;
     static bool select;
};

#define IS_RVALUE(x)                 \
     (sizeof(is_nonconst_rvalue((foo::select ? foo::instance : (x)), 0)) 
== sizeof(yes))

struct bar {};

bar rvalue();
bar const const_rvalue();
bar& lvalue();
bar const& const_lvalue();

bar volatile volatile_rvalue();
bar const volatile const_volatile_rvalue();
bar volatile&  volatile_lvalue();
bar const volatile& const_volatile_lvalue();

BOOST_STATIC_ASSERT(IS_RVALUE(rvalue()));
BOOST_STATIC_ASSERT(IS_RVALUE(const_rvalue()));
BOOST_STATIC_ASSERT(!IS_RVALUE(lvalue()));
BOOST_STATIC_ASSERT(!IS_RVALUE(const_lvalue()));

BOOST_STATIC_ASSERT(IS_RVALUE(volatile_rvalue()));
BOOST_STATIC_ASSERT(IS_RVALUE(const_volatile_rvalue()));
BOOST_STATIC_ASSERT(!IS_RVALUE(volatile_lvalue()));
BOOST_STATIC_ASSERT(!IS_RVALUE(const_volatile_lvalue()));

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com