Re: std::set and std::multiset element mutability

From:

David Wilkinson <no-reply@effisols.com>

Newsgroups:

microsoft.public.vc.language,microsoft.public.vc.stl

Date:

Wed, 23 Dec 2009 07:39:53 -0500

Message-ID:

<uFDtG08gKHA.3792@TK2MSFTNGP02.phx.gbl>

Leigh Johnston wrote:

VC9 allows you to modify a std::set and a std::multiset's elements.
This seems to be forbidden by the c++0x standard, so is this being fixed
in VC10 or left as it is in order not to break software which is c++03
standard's compliant (debatable) but not c++0x standard's compliant?

Following code compiles on VC9 but not VC10:

#include <set>

class A
{
    int m_n;
public:
    A(int n):m_n(n){}
    void Change(){m_n++;}
    bool operator < (const A& rhs) const {return m_n < rhs.m_n;}
};

int main()
{
    std::set<A> mySet;
    A a(1);
    mySet.insert(a);
    mySet.begin()->Change();
    return 0;
}

Error message on VC10 is

error C2662: 'A::Change' : cannot convert 'this' pointer from 'const A' to 'A &'
Conversion loses qualifiers.

So it seems this is fixed in VC10.

Code compiles on VC10 if you make A::m_n mutable and A::Change() const, but this
will break the set. This trick could be used to make inconsequential changes
(ones that did not change the ordering).

--
David Wilkinson
Visual C++ MVP