Boost logo

Boost :

From: Eric Niebler (eric_at_[hidden])
Date: 2004-12-05 02:54:06

Eric Niebler wrote:
> Here is an interesting way to "distinguish" between L-values and
> R-values.

I just committed to boost-sandbox an experimental new version of
BOOST_FOREACH that uses the technique I described to correctly handle
even const rvalue container expressions. I was unable to find a way to
detect const r-value expressions at compile-time, so instead I use a
runtime check along with boost::variant. It's fully inlineable, so the
runtime check should be optimized away, although the use of variant will
incur a small perf hit (not yet measured).

The r-value detection stuff is rather subtle, and it requires a very
compliant compiler. (In fact, I haven't yet convinced myself it's
entirely kosher.) It works on gcc 3.3.3, but not on VC++, so the word to
the wise is: don't use BOOST_FOREACH with const r-value container
expressions on VC++. No doubt there are other compilers which are also
broken with regard to the const r-value detection stuff.

If anybody is interested, grab the code and try it with your favorite
compiler and let me know how it works out. If it doesn't compile, try
#defining BOOST_FOREACH_NO_CONST_RVALUE_DETECTION and see if that fixes
the problem.

It lives in boost-sandbox at ROOT/boost/foreach.hpp. I'm also attaching
it here along with a simple driver.

Eric Niebler
Boost Consulting

Boost list run by bdawes at, gregod at, cpdaniel at, john at