unable to access friend function in template class - c++

Question

The Pairwise class represents a pair with a key:value. I've made a template pair and was having errors trying to run with a key and value input to the class and printing it out.
Given my main:
#include "file_name.h"
int main (){
Pairwise<string, string> example = {{"key", "value"}};
cout << example << endl;
}
And my header file:
#pragma once
#include<iostream>
using std::ostream; using std::cout; using std::endl;
#include<string>
using std::string;
#include<utility>
using std::pair;
#include<sstream>
using std::ostringstream;
template<typename K, typename V>
struct Pairwise{
K first;
V second;
Pairwise() = default;
Pairwise(K, V);
//print out as a string in main
friend ostream& operator<<(ostream &out, const Pairwise &n) {
ostream oss;
string s;
oss << n.first + ":" + n.second; //possible error?
s = oss.str();
out << s;
return out;
}
};
My expected output after running main would be:
key:value
However, I am getting the error:
h:28:11: error: 'std::basic_ostream<_CharT, _Traits> is protected within..."

Answer 1

h:25:59: friend declaration delares a non template function.
You are missing to declare the function as a template that takes Pairwise<K, V>:
header.h:
#ifndef HEADER_H_INCLUDED /* or pragma once */
#define HEADER_H_INCLUDED /* if you like it */
#include <iostream> // or <ostream>
template<typename K, typename V>
class Pairwise { // made it a class so that the
K first; // friend actually makes sense.
V second;
public:
Pairwise() = default;
Pairwise(K first, V second)
: first{ first }, second{ second }
{}
template<typename K, typename V>
friend std::ostream& operator<<(std::ostream &out, Pairwise<K, V> const &p)
{
return out << p.first << ": " << p.second;
}
};
#endif /* HEADER_H_INCLUDED */
source file:
#include <iostream> // the user can't know a random header includes it
#include <string>
#include "header.h"
int main()
{
Pairwise<std::string, std::string> p{ "foo", "bar" };
std::cout << p << '\n';
}
Sidenote: You could also use
{
using Stringpair = Pairwise<std::string, std::string>;
// ...
Stringpair sp{ "foo", "bar" };
}
if you need that more often.
The other errors you got result from confusing std::ostringstream with std::ostream in operator<<().

Answer 2

As you write it, you define the operator as a member function, which is very likely not intended. Divide it like ...
template<typename K, typename V>
struct Pairwise{
K first;
V second;
Pairwise() = default;
Pairwise(K, V);
//print out as a string in main
friend ostream& operator<<(ostream &out, const Pairwise &n);
};
template<typename K, typename V>
ostream& operator<<(ostream &out, const Pairwise<K,V> &n) {
...
return out;
}
And it should work.
BTW: Note that in a struct all members are public by default; so you would be able to access them even in absence of the friend-declaration.

Answer 3

In c++, less is often more...
#pragma once
#include<iostream>
#include<string>
// never do this in a header file:
// using std::ostream;
template<typename K, typename V>
struct Pairwise{
K first;
V second;
Pairwise() = default;
Pairwise(K, V);
//print out as a string in main
friend std::ostream& operator<<(std::ostream &out, const Pairwise &n) {
return out << n.first << ':' << n.second;
}
};
int main (){
using std::cout; using std::endl; using std::string;
Pairwise<string, string> example = {"key", "value"};
cout << example << endl;
}
https://godbolt.org/z/ZUlLTu