range-based for on STL containers

Question

I am using heavily BOOST_FOREACH for iteration over containers, and since I moved to c++0x recently, I thought I could replace BOOST_FOREACH with range-based for construct. The following piece of code

#include<vector>
#include<boost/shared_ptr.hpp>
#include<boost/range.hpp>
using std::vector; using boost::shared_ptr;
class Node;
int main(void){
   vector<shared_ptr<Node>> nodes;
   for(const shared_ptr<Node>& n: nodes);
}

does not compile with gcc 4.6, leading to

error: call of overloaded 'end(std::vector<boost::shared_ptr<Node> >&)' is ambiguous
note: candidates are:
/usr/include/c++/4.6/bits/range_access.h:78:5: note: decltype (__cont->end()) std::end(const _Container&) [with _Container = std::vector<boost::shared_ptr<Node> >, decltype (__cont->end()) = __gnu_cxx::__normal_iterator<const boost::shared_ptr<Node>*, std::vector<boost::shared_ptr<Node> > >]
/usr/include/c++/4.6/bits/range_access.h:68:5: note: decltype (__cont->end()) std::end(_Container&) [with _Container = std::vector<boost::shared_ptr<Node> >, decltype (__cont->end()) = __gnu_cxx::__normal_iterator<boost::shared_ptr<Node>*, std::vector<boost::shared_ptr<Node> > >]
/usr/include/boost/range/end.hpp:103:47: note: typename boost::range_iterator<const T>::type boost::end(const T&) [with T = std::vector<boost::shared_ptr<Node> >, typename boost::range_iterator<const T>::type = __gnu_cxx::__normal_iterator<const boost::shared_ptr<Node>*, std::vector<boost::shared_ptr<Node> > >]
/usr/include/boost/range/end.hpp:92:41: note: typename boost::range_iterator<C>::type boost::end(T&) [with T = std::vector<boost::shared_ptr<Node> >, typename boost::range_iterator<C>::type = __gnu_cxx::__normal_iterator<boost::shared_ptr<Node>*, std::vector<boost::shared_ptr<Node> > >]

Is there a way to avoid such ambiguity, or is range-based for simply unusable in such situation?

MSalters · Accepted Answer · 2011-07-29 09:53:40Z

3

Tricky. You're pulling in std::end and boost::end because the associated namespaces of std::vector<boost::shared_ptr> are both std and boost. Both are templates that match.

However, a non-template end() would be an even better match. So, just provide your own:

inline std::vector<boost::shared_ptr<Node> >::iterator
   end(std::vector<boost::shared_ptr<Node> > vsn&)
{
  return std::end(vsn); 
}

answered Jul 29, 2011 at 9:53

MSalters

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7 Comments

eudoxos Over a year ago

Wow, thanks! I tried a minimal exmaple and it breaks only if I #include<boost/range.hpp>. I will edit the question to show it.

eudoxos Over a year ago

Hm... It would be better to have something more universal, as here for each containers type 4 iterators (begin, const begin, end, const end) have to be wrapped that way.Hopefully boost::serialization will support std::shared_ptr soon and I will be able to abandon boost::shared_ptr :-|

Georg Fritzsche Over a year ago

@eudoxos: This issue will be fixed in future Boost releases.

Kerrek SB Over a year ago

Was that original problem only because of the using directives, or would that always have been resolved wrongly no matter what?

Howard Hinnant Over a year ago

Additional information: gcc 4.6 is in error. The range-based for-loop was changed to avoid this problem just this past March (probably after gcc 4.6 was set in concrete). The compiler should first check vector for member begin and end and use those if found (which it will in this example). Only if the member begin and end are not found will it resort to ADL.

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