std::experimental::ostream_joiner and std::pair

Question

In c++17/g++7, there's finally the long missed ostream_joiner. It enables proper output to ostreams, separating collection elements with infix delimiters.

#include <algorithm>
#include <experimental/iterator>
#include <iostream>
#include <iterator>
#include <vector>
#include <string>

using string = std::string;
#if 1
struct pair {
    string first;
    string second;
};
#else
using pair = std::pair<string,string>;
#endif


std::ostream& operator<<(std::ostream& lhs, const pair &p) {
    return lhs << p.first << "=" << p.second;
}

int main()
{
    std::vector<pair> pairs = {{"foo", "bar"}, {"baz", "42"}};
    std::copy(std::begin(pairs),
          std::end(pairs),
          std::experimental::make_ostream_joiner(std::cout, ", "));
}

Whilst the code piece succesfully compiles and outputs ...

foo=bar, baz=42

... changing the #if 1 to a #if 0 in the snippet makes the compiler complaining about missing the proper shift operator:

main.cpp:29:70:   required from here
/usr/local/include/c++/7.2.0/experimental/iterator:88:10: error: no match for 
'operator<<' (operand types are 
'std::experimental::fundamentals_v2::ostream_joiner<const char*, char, 
std::char_traits<char> >::ostream_type {aka std::basic_ostream<char>}' and 
'const std::pair<std::__cxx11::basic_string<char>, 
std::__cxx11::basic_string<char> >')
  *_M_out << __value;

Does somebody have a clue why?

update

Barry has given the right answer to the question. It however does not solve the problem, and running a manual loop is not in the sense of reusing existing stl code, so the question gets extended to:

Is it possible to make the stream operator work without polluting the std namespace?

Answer 1

Somewhere inside of the implementation of ostream_joiner, there will be an attempt to something like:

os << value;

where os is a std::basic_ostream and value is your pair type. In order to determine what to do for that operator<< call, we lookup all the overloads operator<<() visible at the point of definition of this template as well as as the overloads in the associated namespaces of the arguments (this is known as argument-dependent lookup).

When you use your struct pair, the associated namespace of pair is ::, so ADL will find your ::operator<<(std::ostream&, pair const&). This overload works, is chosen, everything is happy.

When you use std::pair, the associated namespace of pair is std and there is no operator<<() that can be found that takes a std::pair. Hence the error.

---

You could instead create your own type in your own namespace for which you can add an overloaded operator<<, this can be fully your own type (the way it is in the question) or you could inherit the one in std:

struct pair : std::pair<string,string> {
    using std::pair<string,string>::pair;
};
std::ostream& operator<<(std::ostream&, my_pair const& ) {...}

Alternatively, you could just not use make_ostream_joiner. Could replace this:

std::copy(std::begin(pairs),
      std::end(pairs),
      std::experimental::make_ostream_joiner(std::cout, ", "));

with this:

const char* delim = "";
for (auto const& pair : pairs) {
    std::cout << delim << pair; // now, our point of definition does include
                                // our operator<<() declaration, we don't need ADL
    delim = ", ";
}