Re: [Boost-users] Problem with boost::transform_iterator, std::pairs and templates.

on Thu Jul 19 2007, Manuel Holtgrewe wrote:

Hi,

I have a problem using the Boost.Iterators library correctly. The following minimal example does neither compile with GCC 4.2 nor with a current ICC 10.0 version.

It seems that putting a boost::transform_iterator inside a pair makes it impossible to retrieve its value again using std::iterator_traits.

That's not the conclusion I would draw from what you show below.

I would welcome a proposal how to resolve the problem. I get the bug when using transform_iterator together with the MCSTL (Multi Core STL) and thus cannot change the internals. I would thus like to know if and how I can make the following work without touching the inner() and outer() functions themselves since I do not have the possibility to change the library code itself.

Am I using boost::transform_iterator incorrectly?

I think so.

Kind Regards,

Manuel

== File Source ==

#include <iostream> #include <vector> #include <functional>

#include

class plus_two : public std::unary_function { public: int operator()(int x) const { return x + 2; } };

template <class Pair> void inner(Pair pair) { typedef typename Pair::first_type Iterator; typedef typename std::iterator_traits<Iterator>::value_type T;

Pair p = pair;

T *x = &p.first[0];

OK, first of all, if p.first is a transform_iterator, then the result of p.first[0] is not a reference as you might expect It's a temporary proxy object (see http://www.boost.org/libs/iterator/doc/iterator_facade.html#operator), which explains this warning and error:

conflict.cpp: In function 'void inner(Pair) [with Pair = std::pair, boost::transform_iterator >]': conflict.cpp:21: warning: taking address of temporary conflict.cpp:21: error: cannot convert 'boost::detail::operator_brackets_proxy >*' to 'int*' in initialization

Secondly, even if it could be a reference, (or you used the otherwise-equivalent &*p.first) your transform function returns by value, not by reference. So that too would be a reference to a temporary. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com The Astoria Seminar ==> http://www.astoriaseminar.com