on Mon Aug 13 2007, "Martin Schulz" <Martin.Schulz-AT-synopsys.com> wrote:
Giovanni, sorry for the late reply.
Consider this:
std::vector<int> x = .... for_each(filter(x, some_filter_op), do_something);
'Filter' will obviously return a proxy for 'x' (actually wrapping x iterators with boost::filter iterators) as a temporary object. for_each will bind the temporary to a const reference (at least until we get rvalue references).
The fact that temporary objects cannot be passed in as non-const references had been decided to avoid programming accidents in that modifications to the temporary object were assumed to be meaningfull and should be preserved. If you want this object as non-const nevertheless, simply give a name to it.
That's less simple than you make it sound. It could have a very complicated type. One good way to deal with this is to make the proxy object return a constant object. Then it will bind to a T& parameter. Furthermore, the constness of the proxy should not affect the constness of its access to elements of x. That essentially means returning a different proxy type depending on whether the incoming container is const or not. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com The Astoria Seminar ==> http://www.astoriaseminar.com