How to get rid of VC warning C4239?

Hi, there,

I am writing a C++ wrapper for different underlying xml libraries. In
order to minimize the use of pointers, i provide a class template to
wrap an xml node object pointer, using the pimpl idiom:

template <class XmlNode>
class basic_xml_node {

     typedef basic_xml_node<XmlNode> node_type;

public:

     ...

     // copy constructor: accept a non-const RHS
     basic_xml_node(node_type& rhs): pimpl_(rhs.pimpl_) { }

     // copy assignment: accept a non-const RHS
     node_type& operator=(node_type& rhs) {
         pimpl_ = rhs.pimpl_;
         return *this;
     }

     // non-const member function.
     void set_localname(std::string const& localname) {
         pimpl_->set_localname(localname);
     }

     ...

private:
     boost::shared_ptr<XmlNode> pimpl_;
};

The copy constructor accepts a non-const RHS to avoid the following
misuse:

typedef basic_xml_node<libxml2_node> xml_node;
void foo(xml_node const& node) {
     xml_node copy = node;
     copy->set_localname("toto");
}

Since the basic_xml_node class template holds a pimpl to the real xml
node object, if its copy constructor/assignment accepts a const RHS,
the code above will compile and run, and the effect is that the local
name of the const node reference parameter will be changed after the
function call. If the copy constructor/assignment accepts a non-const
RHS, the code above will no longer compile (which is what i expect).

Unfortunately, when i use the class template, VC++ complains a lot on
"nonstandard extension used" (warning C4239). Details:

conversion from basic_xml_node<libxml2_node> to
basic_xml_node<libxml2_node>&: A reference that is not to 'const'
cannot be bound to a non-lvalue.

I think such warnings are benign, but even if the warning id is
disabled in my xml header files, such warnings still appear in client
code, as long as the client code uses (implicitly) the copy
constructor or copy assignment.

I know that std::auto_ptr class template has the same problem. So i
just would like to know if there is a way to get rid of this warning?
I do not want to disable this warning globally, or turn down the
warning level.

Thanks in advance!

ZHENG Zhong

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