[Boost-users] Ideas for something that looks like metaprogramming

Subject: [Boost-users] Ideas for something that looks like metaprogramming
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2009-04-23 19:58:48

I'm revamping my atomic_counter template (see <
http://www.dlugosz.com/Repertoire/refman/Classics/atomic_counter_whitepa
per.html#toptop>) and plan on exposing all the available "magic" (e.g.
things you can't do in C++) op-codes in the atomic read-modify-write
department, as primitives.  Then, use those in the template class.

But, I have gained a deeper appreciation for some of the LOCKable
instructions I never thought were interesting.  For example, you can
LOCK ADD, which changes the value in memory but doesn't return back what
it was or became.  What good is that?  Directly translating to C++, none
at all.  But... you do get back some information.  In assembly language,
you see the flags.  So, a thread can add something to the location, and
although doesn't know what it became in general, at least know if this
thread is the one that made it zero.  Or, overflowed, or whatever.
Those flags are the basis of if-statements in assembly language.  So,
given primitive calls that return the whole set of flags, it occurs to
me to be able to harness that with a metaprogramming-like interface.

For example, I'm hesitant to implement something called operator &= that
doesn't return the new value, but "something else" instead.  I think the
only interesting flag here is whether the result is zero.  So bool
operator&=(left,right) returning whether the result was non-zero would
be very confusing, even if it did work right when used in an 'if'
condition!

So, just brainstorming, x&=y would return something not directly usable,
but does work with an overloaded ==, so if you were to write (x&=y)==0
the result would be what you expect from C++.  Maybe it's not worth the
effort.  But if anyone wants to discuss it...

--John
(sorry about the footer; it's not my idea)

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