id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
9737,[spirit][classic] returning reference to temporary when using position_iterator with istreambuf_iterator<char>,Julien Charbon <jcharbon@…>,Joel de Guzman,"In boost spirit classic, when using position_iterator with istreambuf_iterator<char>, the underlying iterator_adaptor::deference() might return a reference to temporary:

For example with clang34/libc++:

{{{
$ clang++34 -O2 -Wall test-boost.cpp -o test-boost
source/boost/boost/iterator/iterator_adaptor.hpp:310:18: warning: returning reference to local temporary object [-Wreturn-stack-address]
        { return *m_iterator; }
                 ^~~~~~~~~~~
}}}

or with gcc47/libstdc++ in c++11 mode:

{{{
$ g++47 --std=c++11 -O2 -Wall test-boost.cpp -o test-boost
source/boost/boost/iterator/iterator_adaptor.hpp:310:19: warning: returning reference to temporary [enabled by default]
}}}

but not in default gcc47 default code (gnu++98):

{{{
$ g++47 -O2 -Wall test-boost.cpp -o test-boost
}}}

We tracked down this issue to  istreambuf_iterator<char>::reference and istreambuf_iterator<char>::operator*() definition:

* On clang3.4/libc++:

istreambuf_iterator<char>::reference type is 'char' and istreambuf_iterator<char>::operator*() also returns a 'char'. 

* On gcc47/libstdc++ in c++11 mode:

istreambuf_iterator<char>::reference type is 'char' and istreambuf_iterator<char>::operator*() also returns a 'char'. 

See: istreambuf_iterator::reference changes from _CharT& to _CharT:
http://gcc.gnu.org/wiki/Cxx11AbiCompatibility 

* However on gcc47/libstdc++ in default mode:

istreambuf_iterator<char>::reference type is 'char &' where istreambuf_iterator<char>::operator*() returns a 'char'. 

 And position_iterator being built from iterator_adaptor with:

{{{
    typedef boost::iterator_adaptor<
        main_iter_t,
        ForwardIterT,
        const_value_type,
        boost::forward_traversal_tag
    > type;
}}}

 there is no 5th template type which defines iterator_adaptor 'Reference':

{{{
  // Iterator Adaptor
  // ...
  // 
  //   Reference - the reference type of the resulting iterator, and in
  //      particular, the result type of operator*(). If not supplied but
  //      Value is supplied, Value& is used. Otherwise
  //      iterator_traits<Base>::reference is used.
}}}

 Thus by default, iterator_adaptor::deference() from a position_iterator built of istreambuf_iterator<char> returns a &char, which is fine only when istreambuf_iterator<char>::reference type is also 'char &'.",Bugs,closed,To Be Determined,spirit,Boost 1.55.0,Problem,fixed,,