[boost] Re: looping construct - testers wanted ;)

From: Eric Niebler (eric_at_[hidden])
Date: 2004-05-07 12:37:16

David Abrahams wrote:
> 
> It surprises me that BOOST_FOREACH is ever measurably different from
> a hand-coded loop.  Can anyone explain that?
> 

I could hazard a guess. BOOST_FOREACH uses two nested for loops. The 
sole purpose of the inner loop is to rebind the loop variable in case it 
is a reference. There is some fancy footwork to make sure the inner loop 
executes exactly once for each iteration of the outer loop. In addition, 
if the inner loop is exited with a "break," that information must be 
relayed to the outer loop. This is all accomplished with the help of a 
hidden bool, which is set, checked and reset at each iteration. It's the 
only overhead.

For all but the most trivial loops, the cost of reading and writing a 
bool is lost in the noise. IMO, being able have reference loop variables 
is worth the very small price.

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