Re: [Boost-bugs] [Boost C++ Libraries] #5475: [Foreach] rvalue reference binding of temporary ranges

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5475: [Foreach] rvalue reference binding of temporary ranges
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-11-08 20:47:08

#5475: [Foreach] rvalue reference binding of temporary ranges
--------------------------------------+-------------------------------------
  Reporter:  mimomorin@…              |       Owner:  eric_niebler  
      Type:  Patches                  |      Status:  reopened      
 Milestone:  To Be Determined         |   Component:  foreach       
   Version:  Boost Development Trunk  |    Severity:  Problem       
Resolution:                           |    Keywords:  foreach, C++0x
--------------------------------------+-------------------------------------

Comment (by Michel Morin <mimomorin@…>):

 Finally, I came up with workarounds for gcc's auto type deduction bugs.
 Now BOOST_FOREACH consists of three implementations:
 C++03 BOOST_FOREACH, C++11 BOOST_FOREACH and workarounds for gcc bugs.
 All the existing tests and new tests (involving function templates and
 C++11 lambdas)
 ran successfully on gcc and clang.

 To workaround gcc bugs, I used a hybrid of C++11 and C++03 codes:
 1. Bind a const reference to a range using `auto const&`. If the range is
 non-const lvalue, constness of the reference is stripped off via
 `const_cast` when accessing the reference.
 2. `begin` and `end` iterators are stored as `auto_any_t` to avoid the
 bugs.
 This workaround code is enabled in C++11 modes of gcc 4.4-4.6.

 On MSVC++ 10, I changed to use the C++03 BOOST_FOREACH code.
 Since this compiler doesn't support iteration over noncopyable rvalue
 ranges,
 it's not worth taking a risk for regressions.

 I also added a few tests:
 * noncopyable_rvalue_*.cpp: This test emits expected-failures if
 `BOOST_FOREACH_USE_CXX11` or `BOOST_FOREACH_USE_CXX11_GCC_WORKAROUND` is
 not defined.
 * cxx11_gcc_workaround_*.cpp: This test needs support of rvalue
 references. On compilers without rvalue reference support, this test
 succeeds unconditionally.
 * cxx11_lambda_*.cpp: This test needs support of C++11 lambdas. On
 compilers without C++11 lambdas support, this test succeeds
 unconditionally.
 * char_array_fail_*.cpp: A test case that should fail to compile
 (`compile-fail` is specified in Jamfile). This test tries to iterating
 over a char array.
 * rvalue_nonconst_access_fail_*.cpp: A test case that should fail to
 compile (`compile-fail` is specified in Jamfile). This test tries to
 capture elements in an rvalue range by non-const references.

 Patches for `boost/foreach.hpp` and `libs/foreach/test/` are attached.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5475#comment:34>
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