Overloading << and >> for STL map: "cannot bind lvalue" - c++

Question

I am trying to save an STL map containing custom objects to a file, using fstream. I am doing this using << and >> operator overloading.
#include <iostream>
#include <string>
#include <map>
#include <fstream>
#define DELIM '/'
struct Point{
public:
int x, y;
};
//operator overloads for point
std::istream& operator>>(std::istream& is, Point& p){
std::string input;
std::getline(is, input, DELIM);
p.x = std::stoi(input);
std::getline(is, input, DELIM);
p.y = std::stoi(input);
return is;
}
std::ostream& operator<<(std::ostream& os, Point& p){
os << p.x << DELIM << p.y << DELIM;
return os;
}
//operator overloads for map<string, point>
std::istream& operator>>(std::istream& is, std::map<std::string, Point>& m){
std::string input;
std::getline(is, input, DELIM);
int map_size = std::stoi(input);
for(int i = 0; i < map_size; i++){
std::getline(is, input, DELIM);
Point p; is >> p;
m[input] = p;
}
return is;
}
std::ostream& operator<<(std::ostream& os, std::map<std::string, Point>& m){
os << m.size() << DELIM;
for(const auto& pair : m){
os << pair.first << DELIM;
os << pair.second;
}
return os;
}
int main(){
Point p1;
p1.x = 1; p1.y = 2;
Point p2;
p2.x = 100; p2.y = 150;
std::map<std::string, Point> map;
map["p1"] = p1;
map["p2"] = p2;
return 0;
}
When I try to compile this file I get the following error:
test.cpp: In function 'std::ostream& operator<<(std::ostream&, std::map<std::basic_string<char>, Point>&)':
test.cpp:44:14: error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
os << pair.second;
Any idea how to fix this? I ran into this problem earlier with templates, however there are no templates used here. Other answers on SO also involved templates and did not help me. An explanation of why this is happening would be appreciated too!
The command I used to compile: (MinGW, gcc 4.8.1)
g++ test.cpp -o test -std=c++11 -Wall
The full error message:
test.cpp: In function 'std::ostream& operator<<(std::ostream&, std::map<std::basic_string<char>, Point>&)':
test.cpp:44:14: error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
os << pair.second;
^
In file included from c:\mingw\lib\gcc\x86_64-w64-mingw32\4.8.1\include\c++\iostream:39:0,
from test.cpp:1:
c:\mingw\lib\gcc\x86_64-w64-mingw32\4.8.1\include\c++\ostream:602:5: error: initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = Point]'
operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
^
Thank you!!

Answer 1

Here
for(const auto& pair : m){
os << pair.first << DELIM;
os << pair.second;
}
pair is const, so pair.second will be const Point too, which can't match the parameter type Point& of operator<<.
Change the parameter type of Point for operator<< to const reference:
std::ostream& operator<<(std::ostream& os, const Point& p){
~~~~~
os << p.x << DELIM << p.y << DELIM;
return os;
}
Mostly, the argument passed to operator<< is not expected to be changed, so it's good practice to declare the type to const& for both operator<<.
std::ostream& operator<<(std::ostream& os, const Point& p){
std::ostream& operator<<(std::ostream& os, const std::map<std::string, Point>& m){

Answer 2

The argument passed to the overloaded << operator should be a constant reference, not a mutable reference.