[boost] L-values and R-values (was: BOOST_FOREACH questions)

2 Dec 2004


      Jonathan Turkanis wrote:
...
"Eric Niebler" <eric@boost-consulting.com> wrote in:
...
Seems to me that Arkadiy has run up against the same problem with
BOOST_TYPEOF_PRESERVE_LVALUE, which also gets this case wrong (according
to the docs).
Yeah, that's what I was thinking about last night. I was trying to see how close
I could approximate decltype.
Here is an interesting way to "distinguish" between L-values and 
R-values. The following program uses the properties of the conditional 
operator to implement an IS_LVALUE macro that detects the value-ness of 
an expression and prints its determination. It works with gcc 3.3.3 and 
with Comeau online, but vc7.1 doesn't like it. Alas, I haven't yet 
figured out a way to use this technique to get a compile-time constant. 
Maybe someone more clever than myself can figure that out.

#include <stdio.h>

struct foo
{
     template<typename U>
         operator U () const
     {
         throw "R-value";
     }

     template<typename V>
         operator V & () volatile const
     {
         throw "L-value";
     }
};

foo const f = {};

#define IS_LVALUE(x) \
     try { (true? f : (x)); } \
     catch( char const *value ) { printf("%s\n",value); }

struct bar {};

bar         baz1() { static bar b; return b; }
bar const   baz2() { static bar b; return b; }
bar &       baz3() { static bar b; return b; }
bar const & baz4() { static bar b; return b; }

int main()
{
     bar b;
     bar const cb = {};

     // l-values
     IS_LVALUE(b);
     IS_LVALUE(cb);

     // r-values
     IS_LVALUE(baz1());
     IS_LVALUE(baz2());

     // l-values
     IS_LVALUE(baz3());
     IS_LVALUE(baz4());

     return 0;
}


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