Re: Printing a map

Alan Johnson <>
Sun, 21 May 2006 22:11:57 -0700
Kai-Uwe Bux wrote:

Specifically for pairs, you could even provide a template (although some
might object to this):

  template < typename S, typename T >
  std::ostream & operator<< ( std::ostream & o_str,
                              std::pair< S, T > const & p )
    o_str << "( " << p.first << ", " << p.second << " )";
    return ( o_str );

You know, that was my first thought too, but consider the following program:

#include <map>
#include <algorithm>
#include <iterator>
#include <string>
#include <iostream>

template <typename K, typename T>
std::ostream & operator<<(std::ostream & os, const std::pair<K, T> & p)
     return os << p.first << " => " << p.second ;

int main()
     typedef std::map<std::string, int> map_type ;
     map_type numbers ;
     numbers["one"] = 1 ;
     numbers["two"] = 2 ;
     numbers["three"] = 3 ;

     std::copy(numbers.begin(), numbers.end(),
         std::ostream_iterator<map_type::value_type>(std::cout, "\n")) ;

This will not compile in g++ 4.1, nor with Comeau's online compiler. On
a strange hunch I put the operator<< definition in namespace std, and it
compiled fine (on both compilers). Does anybody have any insight into
what is going on there?

Alan Johnson

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