Range based for with pair<Iterator,Iterator>

Question

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);

Answer 1

Iterator pairs are not ranges by design! The idea was specifically rejected from the language and library specification. See e.g.

• paper n2995.pdf Pairs do not make good ranges

If you "find it tiresome" to write the tie() workaround, just use

for (auto& edge : make_iterator_range(boost::edges(_graph)))
    /*do something with edge*/;

You could alias boost::make_iterator_range something shorter, but my editor¹ suggests make_iterator_range as completion when I type mir. This is plenty speedy for me

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¹ of course, that editor is Vim