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The ideas for BOOST_FOREACH began life in the Visual C++
group at Microsoft during the early phases of the design for C++/CLI. Whether
to add a dedicated "foreach" looping construct to the language was
an open question at the time. As a mental exercise, Anson Tsao sent around
some proof-of-concept code which demonstrated that a pure library solution
might be possible. The code was written in the proposed C++/CLI dialect of
the time, for which there was no compiler as of yet. I was intrigued by the
possibility, and I ported his code to Managed C++ and got it working. We worked
together to refine the idea and eventually published an article about it in
the November 2003 issue of the CUJ.
After leaving Microsoft, I revisited the idea of a looping construct. I reimplemented
the macro from scratch in standard C++, corrected some shortcomings of the
CUJ version and rechristened it BOOST_FOREACH. In October
of 2003 I began a discussion about it on the Boost developers list, where it
met with a luke-warm reception. I dropped the issue until December 2004, when
I reimplemented BOOST_FOREACH yet again. The new version
only evaluated its sequence expression once and correctly handled both lvalue
and rvalue sequence expressions. It was built on top of the recently accepted
Boost.Range library, which
increased its portability. This was the version that, on Dec. 12 2004, I finally
submitted to Boost for review. It was accepted into Boost on May 5, 2005.
Thanks go out to Anson Tsao of Microsoft for coming up with the idea and demonstrating
its feasibility. I would also like to thank Thorsten
Ottosen for the Boost.Range
library, on which the current version of BOOST_FOREACH is
built. Finally, I'd like to thank Russell Hind, Alisdair Meredith and Stefan
Slapeta for their help porting to various compilers.
For more information about how BOOST_FOREACH works, you
may refer to the article “Conditional
Love” at The C++
Source.