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

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.
<snip>

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
www.boost-consulting.com