[Bug c++/41803] New: Compiler forcing iterator to const

[Bug c++/41803] New: Compiler forcing iterator to const

[dont_spam_james at yahoo dot com]

In the source code there are two classes, Point and Edge.  A series of points
are stored in a std::multiset.  Edges are formed between the points, referenced
by multiset iterators.  The points are given pointers to the edge they are a
part of.

In the function Edge::splitEdge, an edge is broken up by a new point.  The
point is added to the multiset, a new edge is created, the edge's iterators are
reset, and the Edge pointers are reset.

When resetting the Edge pointers, the compiler is insisting that the data
contained in std::multiset<Point>::iterator is a const Point.

If I change the Point member data Edge *dp_edge to mutable and make the
Point::setEdge function const, the code compiles.


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           Summary: Compiler forcing iterator to const
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dont_spam_james at yahoo dot com
 GCC build triplet: x84_64-unknown-linux-gnu
  GCC host triplet: x84_64-unknown-linux-gnu
GCC target triplet: x84_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803

[Bug c++/41803] Compiler forcing iterator to const

[dont_spam_james at yahoo dot com]

------- Comment #1 from dont_spam_james at yahoo dot com  2009-10-22 22:47 -------
Created an attachment (id=18873)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18873&action=view)
Simple C++ file showing the build error.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803

[Bug c++/41803] Compiler forcing iterator to const

[dont_spam_james at yahoo dot com]

------- Comment #2 from dont_spam_james at yahoo dot com  2009-10-22 22:52 -------
Created an attachment (id=18875)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18875&action=view)
.ii file

Obtained using

g++ -v -save-temps -m32 -o -Wall -Werror -pedantic testLinuxBuildError.cpp


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803

[Bug libstdc++/41803] Compiler forcing iterator to const

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2009-10-22 22:56 -------
Looks like a bug in multiset:
      typedef typename _Rep_type::iterator const_iterator;


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |libstdc++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803

[Bug libstdc++/41803] Compiler forcing iterator to const

[chris at bubblescope dot net]

------- Comment #4 from chris at bubblescope dot net  2009-10-22 23:14 -------
The reason that in general changing the Point a std::multiset<Point>::iterator
refers to is forbidden is that doing so will break the invariant on which the
multiset is based, resulting in random crashes occuring later on.

The standard does say, in 23.1.4 paragraph 5 "Keys in an associative container
are immutable.". Seeing as you know that actually your changes won't break the
ordering, what you are doing is legal, and can be done in two ways:

1) Use mutable, as you were already suggesting.
2) Add a const_cast to the 'const point&' you get out.

I realise this is a little annoying, but it was considered the alternative was
too buggy, as most people would just changes things in (multi)sets without
realising in most cases it would lead to horribly broken code.


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chris at bubblescope dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |chris at bubblescope dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803

[Bug libstdc++/41803] Compiler forcing iterator to const

[paolo dot carlini at oracle dot com]

------- Comment #5 from paolo dot carlini at oracle dot com  2009-10-22 23:21 -------


*** This bug has been marked as a duplicate of 14410 ***


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paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41803