Re: A problem with non const iterators that behave like const ones

Pete Becker <>
Fri, 24 Aug 2007 17:35:19 CST
On 2007-08-24 11:41:02 -0400, Bart van Ingen Schenau
<> said:

Ema wrote:

I'm facing a strange issue. I'm using composed types (like std::map(s)
of std::set(s)) and everything works fine, but when I try to access
structures stored in these variables to modify them, the gcc
compiler(s) (from gcc 3.2 to gcc 4.x) refuse to compile it while msvc
Here's an easy snippet:

<snip - code & errors>

I really don't understand why the compiler thinks that data member
Foo::Bar::b is in read only structure when I'm using ::iterator and
not ::const_iterator.

Actually, g++ is right and MSVC is wrong.

Actually, they're both right.

The elements of a std::set<> are not modifiable, because that could
cause a violation of the invariants of the std::set<> class.
The elements of std::set<> are always stored in increasing order
(according to the comparator). This is a class invariant of std::set<>.
As std::set<> does not know which parts of the contained objects affect
the ordering, the conservative assumption is that all parts do so.
For this reason, even the regular iterators of std::set<> do not allow
modification of the item they refer to.

That's the explanation for the change to set's iterator requirements
made for C++0x. In C++ 2003 there is no such requirement.

Roundhouse Consulting, Ltd. ( Author of "The
Standard C++ Library Extensions: a Tutorial and Reference

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