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Subject: [Bug c++/42032] Aliasing errors in stl_tree.h
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From: "craig dot schlenter at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #7 from craig dot schlenter at gmail dot com  2009-11-16 04:37 -------
(In reply to comment #3)
> I brought this up on the Google-internal C list.  They reduced it further:
> 
> $ cat main.cc
> #include <map>
> 
> int main(void)
> {
>    typedef std::map<int, char*> MyMap2;
>    MyMap2 map2_;
>    MyMap2::iterator map_iter2 = map2_.find(5);
>    return *map_iter2->second;
> }
> $ g++ -O3 -Wall -c main.cc
> main.cc: In function 'int main()':
> main.cc:8: warning: dereferencing pointer '<anonymous>' does break
> strict-aliasing rules
> /opt/local/include/gcc44/c++/bits/stl_tree.h:179: note: initialized from here

Hironori Bono spotted btw. that if the key for the map is changed from an int
to a std::string, then the aliasing warning disappears:

#include <map>
#include <string>

int main(void)
{
  typedef std::map<std::string, char*> MyMap2;
  MyMap2 map2_;
  MyMap2::iterator map_iter2 = map2_.find("hello");
  return *map_iter2->second;
}

This is rather strange and confusing.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42032