C++ Boost: Any gotchas with BOOST_FOREACH?

Question

This one is for Boost experts. Are there any gotchas or details that the programmer needs to be aware of before he goes in and replaces all his old C/C++ style loops with the lean-and-mean-looking BOOST_FOREACH?

(This question is partly derived from here.)

score 10 · Accepted Answer · answered Apr 04 '09 at 10:34

10

Take a look at:

The Conditional Love article for an understanding of the rationale
The Pitfalls section of documentation
The Portability section just in case you are developing cross-platform products
The bugs page for BOOST_FOREACH

answered Apr 04 '09 at 10:34

dirkgently

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Is point 1 an issue for BOOST_FOREACH? As I understood it BOOST_FOREACH, evaluated each parameter once and only once. No? "It evaluates its arguments exactly once, leading to no nasty surprises" --docs – Catskul Jun 16 '11 at 17:47

bayda · Answer 2 · 2009-04-04T10:46:08.313

9

BOOST_FOREACH - macro, I don't like macroses and prefer to use STL algorithms + lambda + bind.

Also C++0x will contain for-loop similar on BOOST_FOREACH:

int my_array[5] = {1, 2, 3, 4, 5};
for(int &x : my_array)
{
  x *= 2;
}

it is one additional reason for don't use partialy dead BOOST_FOREACH.

edited Apr 04 '09 at 10:46

answered Apr 04 '09 at 10:37

bayda

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Actually, this is the reason I started using BOOST_FOREACH. The syntax is similar enough so that once the C++0x construct is available, you can update your code with a simple regex search-and-replace operation. – Ferruccio Apr 04 '09 at 11:01
1

I'm skeptic, very skeptic. tr1 is still not available everywhere and that was a long time ago. C++09 will be fully usable by 2020 at this rate, if C++ doesn't die from bloat. – Robert Gould Apr 04 '09 at 12:42
Yes, some peoples don't use/like STL for this time and will not use new C++0x features to 2020. But facts says next: VC and GCC compillers already supports some C++0x features. – bayda Apr 04 '09 at 13:03
That's the problem it's just "some", and that some is the low hanging fruit :/ – Robert Gould Apr 04 '09 at 13:40

score 8 · Answer 3 · answered Apr 04 '09 at 10:35

8

As it's just a macro, you can't use commas in typenames, so
BOOST_FOREACH(pair<int,int> A, mapB){}
won't work.
For other disadvantages I'd consult the BOOST_FOREACH() documentation.

answered Apr 04 '09 at 10:35

tstenner

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I believe that BOOST_FOREACH( (pair A), mapB) should work. Did not check though. – David Rodríguez - dribeas Apr 04 '09 at 11:47
Won't work, and I spent half a day trying to figure it our, until it suddenly strict me it was a macro. Macro's don't understand templates and they think the comma is just another argument to replace – Robert Gould Apr 04 '09 at 12:38
`auto` is your friend! `BOOST_FOREACH(auto a, mapB) {...}` – Gabriel Mar 22 '11 at 10:39
@Gabriel Won't work unless you have a very recent compiler. At work we still have GCC 4.1 on some machines. – tstenner Mar 22 '11 at 10:54
@tstenner Works at least with VS2010. – Gabriel Mar 23 '11 at 00:54
@Gabriel That what I meant with 'very recent' ;) – tstenner Mar 23 '11 at 11:06
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The simple solution is to use `typedef`, for example `typedef pair MyIntPair; BOOST_FOREACH(MyIntPair A, mapB)`. – Lindydancer Jan 12 '12 at 15:58
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The simplest solution is to use a tie, explained here: http://stackoverflow.com/a/8859736/375343 – Paul Fultz II Jan 24 '12 at 01:36

score 7 · Answer 4 · answered Apr 05 '09 at 06:29

I profiled BOOST_FOREACH versus a hand-coded loop. BOOST_FOREACH was about 30% slower in a simple loop that incremented the elements of a vector of size 100,000. So, if you are coding a small loop, it is not going to be as fast. Once your loop does major processing, Amdahl's Law kicks in and the loss due to BOOST_FOREACH is negligible.

score 4 · Answer 5 · answered Apr 04 '09 at 10:34

4

Take a look at the source of the BOOST_FOREACH macro (in foreach.hpp) - it's not what I would call "lean and mean" :-)

answered Apr 04 '09 at 10:34

C++ Boost: Any gotchas with BOOST_FOREACH?

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