Re: [Boost-users] [mpl] Alternative to for_each that passes the function object by reference (WAS: Doc for for_each)

From: Hendrik Belitz (hbelitz_at_[hidden])
Date: 2007-03-29 05:09:40

Dear fellow boost users,

I'd like to address a problem I just encountered on using mpl::for_each
(Boost 1.33). Similar to Scott, I implemented a stateful functor that should
be called for all types in a mpl::vector<>. for_each is no option, since it
passes the functor by value (Maybe someone could enlighten me in stating why
this is a wanted behaviour? According to the meta-programming book, for_each
takes a functor to be able to store the results of a computation which is
not possible if the functor is copied?). Are there any alternatives to
for_each for that case?

Regards
 Hendrik

-----Ursprüngliche Nachricht-----
Von: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] Im Auftrag von Aleksey Gurtovoy
Gesendet: Donnerstag, 29. März 2007 11:48
An: boost-users_at_[hidden]
Cc: dave_at_[hidden]; Scott Meyers
Betreff: Re: [Boost-users] [mpl] Doc for for_each

Scott,

Scott Meyers writes:
> Having written code using a stateful functor with mpl::for_each and 
> observing that side effects were not taking place, I went to 
> http://www.boost.org/libs/mpl/doc/refmanual/refmanual_toc.html to 
> confirm my suspicion that the functor was being passed by value. But 
> there appears to be no documentation for mpl::for_each (neither in the 
> HTML version of the reference manual nor in the PDF version). I 
> ultimately confirmed my suspicion by looking at the header file, but 
> is there a reason why for_each seems to be documented only in the 
> printed book?

It was (accidentally) missed out. It's fixed in 1.34 release; meanwhile, the
page can be accessed online at http://tinyurl.com/2f3zvv
(http://boost.cvs.sourceforge.net/*checkout*/boost/boost/libs/mpl/doc/refman
ual/for-each.html?revision=1.1.2.1&pathrev=RC_1_34_0)

HTH,
-- 
Aleksey Gurtovoy
MetaCommunications Engineering
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