Re: [Boost-bugs] [Boost C++ Libraries] #3996: Enable use of BOOST_FOREACH over const-ref of noncopyable ptr_containers

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #3996: Enable use of BOOST_FOREACH over const-ref of noncopyable ptr_containers
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-06-25 01:03:29

#3996: Enable use of BOOST_FOREACH over const-ref of noncopyable ptr_containers
--------------------------------------------------------+-------------------
  Reporter:  Kazutoshi Satoda <k_satoda@…>              |       Owner:  nesotto                     
      Type:  Patches                                    |      Status:  new                         
 Milestone:  Boost 1.43.0                               |   Component:  ptr_container               
   Version:  Boost 1.42.0                               |    Severity:  Problem                     
Resolution:                                             |    Keywords:  BOOST_FOREACH is_noncopyable
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Comment (by eric_niebler):

 1) A type !ThirdParty is defined in !ThirdParty.h. The user can't edit
 this file because it's not hers. It does not specialize is_noncopyable
 because it knows nothing about BOOST_FOREACH.

 2) A user creates her own !ThirdPartyForEach.h that #includes
 !ThirdParty.h and specializes is_noncopyable. Where ever she wants to use
 !ThirdParty together with BOOST_FOREACH, she includes !ThirdPartyForEach.h

 3) In Unit1.cpp, the user instantiates ptr_vector<!ThirdParty>. She
 includes !ThirdParty.h instead of !ThirdPartyForEach.h because in this
 translation unit she is not using BOOST_FOREACH.

 4) In Unit2.cpp, she also instantiates ptr_vector<!ThirdParty>. She *does*
 include !ThirdPartyForEach.h in this translation unit because it uses
 BOOST_FOREACH.

 5) Undefined behavior.

 The problem stems from the fact that the user cannot possibly know that
 she needs to include !ThirdPartyForEach.h EVEN IN TRANSLATION UNITS THAT
 DON'T USE BOOST_FOREACH. This is a recipe for ODR violations.

 Contrast this with my suggested solution. is_noncopyable will *only* be
 instantiated in the translation units that actually use BOOST_FOREACH. In
 these source files, it's natural to expect people to include the header
 that correctly implements the foreach hooks.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3996#comment:4>
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