Re: How to resolve ADL(?) issue using std::copy and std::ostream_iterator

From:
cbarron3@ix.netcom.com (Carl Barron)
Newsgroups:
comp.std.c++
Date:
Mon, 3 Jul 2006 15:22:48 GMT
Message-ID:
<1hhvdtx.fgmb3q1zpofsN%cbarron3@ix.netcom.com>
Chris Johnson <devcjohnson@gmail.com> wrote:

My knowledgeable friends over at comp.lang.c++ directed me to post this
message here. I am pasting the message here as originally asked.
Apologies if this comes off as spamming/cross-posting to either group
(not the intent):

Greetings all. I am really stuck on this one as I can't seem to grok if
I am abusing the C++ language or if I am simply using components of the
C++ Standard Library incorrectly. Here is the code:
     
[sample code attempting to use std::ostream_iterator<std:pair<T1.T2> >.


Looking at the definition of std::output_iterator<T,Ch,Tr>
[the last to args default to char and std::char_traits<char>]

you will find it evaluates the expression *it = x as os << x, in effect
so we must as you show as NON_COMPILANT, devise an operator << () for
std::ostream_iterator to compile. When the compiler looks for operator
<< it looks in namespace std since ostream is there, it looks whereever
the type T is as well so std::ostream_iterator<std::pair<T1,T2> > will
ultimately look for the operator in only namespace std. But if T is not
in namespace std, sucn as its in the global namespace, it will look for
the operator in the global namespace as well. Thats the theory poorly
explained. The following OutProxy solves the problem:

// general case
   template <class T>
   struct OutProxy
   {
       const T &t;
       OutProxy(const T &a):t(a){}
       std::ostream & put (std::ostream &os) const
       { return os << t;}
   };

 // specialize for Pair<T1,T2>
   template <class T1,T1> struct OutProxy<std::pair<T1,T2> >
   {
      const std::pair<T1,T2> &p;
      OutProxy(const std::pair<T1,T2> &a):p(a)
      std::ostream & put(std::ostrwam &os) const
      {
         return os << p.first << ' ' << p.second;
      }
   };
// template <class T>
   std::ostream & operator << (std::ostream &os, OutProxy<T> const &p)
   { return p.put(os);}

 std::copy<test.begin(),test.end(),
      std::ostream_iterator<OutProxy<MapType::value_type> >(std::cout));

 note this place the type used by ostream_itertor into the global
namespce and this it finds operator << (). There is an implicit
conversion used to convert std::pair<T1,T2> to OutProxy<std::pair<T1,T2>

.


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