BOOST_FOREACH versus for loop

Question

I would like to have your advice regarding the usage of BOOST_FOREACH.

I have read around it is not really recommended in terms of performance being a very heavy header.

Moreover, it forces the use of "break" and "continue" statements since you can't really have an exit condition driven by a boolean and I've always been told that "break" and "continue" should be avoided when possible.

Of course, the advantages are that your are not dealing directly with iterators which ease the task of iterating through a container.

What do you think about it?

Do you think that if used it should be adopted systematically to guarantee homogeneity in a project or its use is recommended only under certain circumstances?

"and I've always been told that "break" and "continue" should be avoided when possible." Can you ellaborate please? — utnapistim, Feb 21 '13 at 10:49
IIRC There were reasons to avoid break and continue back in the good old days where people did not have RAII patterns and optmimizers might get into problems when loops would get too "jumpy". Those were the days of the "single entry single exit" dogma where functions had only one return statement, tons of if's and some goto's. And there are people who learned those dogmas eons ago, never questioned them and are spreading them to youngsters in times where they don't make any sense but lots of trouble. — Arne Mertz, Feb 21 '13 at 11:07
While I agree with you, I have to say that there could be value in not using break or continue in for loops. If you reserve those keywords to while loops and agree not to use them in for loops, the intent becomes clearer. But this is just too much IMHO, I wouldn't advise it. — MatiasFG, Dec 10 '14 at 12:58

score 28 · Answer 1 · edited Aug 18 '20 at 15:23

28

I would say C++ range based loops supercede it. This is an equivalent of this BOOST_FOREACH example:

std::string hello( "Hello, world!" );
for (auto c : hello)
{
  std::cout << c;
}

I never found I needed to use it in ++03.

Note when using the range based loop over containers with expensive to copy elements, or in a generic context, it is best to use const& to those elements:

SomeContainerType<SomeType> v = ....;
for (const auto& elem : v)
{
  std::cout << elem << " ";
}

Similarly, if you need to modify the elements of the container, then use a non-const & (auto& elem : v).

edited Aug 18 '20 at 15:23

stephanmg

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answered Feb 21 '13 at 10:51

juanchopanza

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I think you should use [const] auto& most of the time - otherwise you get a copy – Daniel Feb 21 '13 at 11:41
more detailed description: http://msdn.microsoft.com/en-us/library/vstudio/dd293667.aspx (References and cv-qualifiers) – Daniel Feb 21 '13 at 11:47
@Daniel yes, but int his case we have single `char`s, and I was trying write the equivalent of the `BOOST_FOREACH` example. – juanchopanza Feb 21 '13 at 11:58
was not really a critique about your answer - just an extension because alot of people get confused when using auto – Daniel Feb 21 '13 at 12:02
2

@Daniel I added a note about that. – juanchopanza Feb 21 '13 at 12:06

score 10 · Answer 2 · edited Feb 12 '15 at 00:10

In programming, clarity is trump. I've always used boost foreach in C++03, found it much more readable than the hand-written loop, the header size won't kill you. As @juanchopanza rightly noted, of course, this question is obsolete in C++11.

Your concerns with break and continue are unfounded and probably counterproductive. With the traditionally long for-loop headers of C++03, people tend to not read the loop header and to overlook any condition variables that hide in the loop header. Better make your intent explicit with break and continue.

If you have decided to use boost foreach, use it systematically. It is supposed to be used to replace the bread-and-butter loops, after all.

score 5 · Answer 3 · answered Nov 27 '17 at 18:54

5

I just replaced a use of BOOST_FOREACH with a simple for loop and got a 50% speedup, so I would say it is definitely not always the best thing to use. You will also not get a loop counter (e.g. "i") which sometimes you actually need. Personally I'm not a fan but YMMV if it suits your style better.

BTW - a "heavy header" won't affect performance of your program, only the compilation time.

answered Nov 27 '17 at 18:54

RobinP

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Can you please share actual code example: before, after? Devil is the details – Niki Aug 13 '18 at 12:29
@Niki good point but it's too long ago now (yes I checked my version control system but can't find that change). – RobinP Aug 14 '18 at 14:13

BOOST_FOREACH versus for loop

3 Answers3