Re: [boost] [type_erasure] Review started (July 18-27, 2012)

7 Aug 2012

...
-----Original Message-----
From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Steven
Watanabe
Sent: Monday, August 06, 2012 1:22 PM
To: boost@lists.boost.org
Subject: Re: [boost] [type_erasure] Review started (July 18-27, 2012)
AMDG
On 08/06/2012 02:55 AM, Paul A. Bristow wrote:
...
template <typename K, typename T>
std::ostream& operator<< (std::ostream& os, std::pair<K, T>& p) { /*!
Output a pair of values.
      \details For example: "1.23 , 3.45".
    */
   os << p.first << ", " << p.second;
   return os;
}
And I can echo the input as expected
1, 9.9000
  2, 8.8000
but when I try to use the sequence printer
p2.print(std::cout, my_map);
the compiler (VC10) complains the there is no operator<<
Yep.  It can't be found by ADL unless it's declared in namespace std, which is forbidden.
You can solve this by specializing
boost::type_erasure::ostreamable instead,
I've confirmed that it works in the forbidden namespace std,

but despite some splinters from head scratching, I have concluded that this is not in my skill set
:-(

This specialization of ostreamable:

template<>
struct ostreamable<class Os, const std::pair<const int, double>& >
{
    static void apply(Os& out, const std::pair<const int, double>& arg)
    {
      out << arg; // No operator<< of type 'Os'
   }
};

still doesn't resolve the requirement.

type_erasure_print_map.cpp(140): error C2678: binary '<<' : no operator found which takes a
left-hand operand of type 'Os' (or there is no acceptable conversion)
          type_erasure_print_map.cpp(86): could be 'std::ostream &boost::type_erasure::operator
<<<const int,double>(std::ostream &,const std::pair<_Ty1,_Ty2> &)'
          with
          [
              _Ty1=const int,
              _Ty2=double
          ]
          type_erasure_print_map.cpp(96): or       'std::ostream &boost::type_erasure::operator
<<(std::ostream &,const std::pair<_Ty1,_Ty2> &)'
          with
          [
              _Ty1=const int,
              _Ty2=double
          ]
          while trying to match the argument list '(Os, const std::pair<_Ty1,_Ty2>)'
          with
          [
              _Ty1=const int,
              _Ty2=double
          ]

(Of course I really want the pairs types to be templated for types T1, and T2, but I thought I'd try
to make it simpler to start).

What am I still doing wrong?

Thanks.

Paul

PS a full (not-)working example attached.

---
Paul A. Bristow,
Prizet Farmhouse, Kendal LA8 8AB  UK
+44 1539 561830  07714330204
pbristow@hetp.u-net.com