C++ stream insertion overloading error - c++

Question

I'm having trouble with a basic stream insertion overload. The code is fairly self-explanatory:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <map>
#include <string>
class dictionary
{
public:
using map_type = std::map<std::string, std::string>;
using ostream_iterator_type = std::ostream_iterator<map_type::value_type>;
private:
map_type inner;
public:
dictionary() : inner()
{
inner["foo"] = "lorem";
inner["baz"] = "ipsum";
}
void
write(std::ostream& os)
{
os << "{\n";
std::copy(std::begin(inner), std::end(inner), ostream_iterator_type(os, "\n"));
os << "}";
}
};
dictionary::ostream_iterator_type::ostream_type&
operator<<(dictionary::ostream_iterator_type::ostream_type& os,
dictionary::map_type::value_type const& p)
{
os << p.first << " => " << p.second;
return os;
}
This code throws some huge template errors, claiming that there's no appropriate overload for operator<< with the two arguments that I've clearly defined. What obvious problem am I missing?
Thanks!

Answer 1

The first thing I've noticed is the lack of <ostream> and <algorithm> includes. Also you seem to define operator << only after it is required. But the real problem is that you are trying to define operator << taking ::std::pair as second parameter outside of std namespace. You can fix it appropriately redefining it like this:
#include <iterator>
#include <map>
#include <string>
#include <ostream>
#include <algorithm>
class dictionary
{
public:
using map_type = std::map<std::string, std::string>;
private:
map_type inner;
public:
dictionary() : inner()
{
inner["foo"] = "lorem";
inner["baz"] = "ipsum";
}
void
write(std::ostream& os);
};
namespace std
{
std::ostream &
operator <<(std::ostream & os, dictionary::map_type::const_reference & p)
{
os << p.first << " => " << p.second;
return os;
}
} // namespace std
void dictionary::
write(std::ostream& os)
{
using ostream_iterator_type = std::ostream_iterator<map_type::value_type>;
os << "{\n";
std::copy(std::begin(inner), std::end(inner), ostream_iterator_type(os, "\n"));
os << "}";
}