Range based for with pair<Iterator,Iterator>

I have a question with respect to the following answer:

https://stackoverflow.com/a/15828866/2160256

As stated there, we cannot use range based for with BGL like this:

   for(auto e : boost::edges(g))
       // do something with e

However, here it states, that we can overload the begin() and end() functions that are required to use range based for semantics. So I tried:

   template<class I>
   I begin(std::pair<I,I>& p)
   { return p.first;}

   template<class I>
   I end(std::pair<I,I>& p)
   { return p.second;}

However, the compiler still complains:

error: no matching function for call to ‘begin(std::pair<some_really_ugly_type,some_really_ugly_type>&)’

What am I doing wrong? Does the name lookup not work? Or is this not possible after all? I also found this answer, which works, but shouldtn't it be possible with the begin/end free function overlods as well? Regards, Marti

BTW: I find it really tiresome to write

   typename Graph::edge_iterator ebegin, eend;
   std::tie(ebegin,eend) = boost::edges(_graph);
   std::for_each(ebegin,eend,[&](const edge_descriptor& e){/*do something with e*/;});

UPDATE: C++17 should now allow the following :-)

auto [ebegin,eend] = boost::edges(_graph);

Solution

In a range-based for loop, name lookup for non-member begin() and end() uses ADL only. It doesn't perform ordinary unqualified lookup. §6.5.4 [stmt.ranged]/p1.3:

if _RangeT is a class type, the unqualified-ids begin and end are looked up in the scope of class _RangeT as if by class member access lookup (3.4.5), and if either (or both) finds at least one declaration, [...]

otherwise, begin-expr and end-expr are begin(__range) and end(__range), respectively, where begin and end are looked up in the associated namespaces (3.4.2). [ Note: Ordinary unqualified lookup (3.4.1) is not performed. —end note ]

Hence, your begin() and end() overloads are not found.