How to overload the operator << inside a structure - c++

Question

I just want to print the two values of my structure, but can't compile my code - I get: no operator “<<” matches these operands.
#include <iostream>
using namespace std;
struct SCoor
{
int i, j;
bool operator == (const SCoor & tmp) const
{
return (i == tmp.i && j == tmp.j);
}
bool operator < (const SCoor & tmp) const
{
return (i < tmp.i || (i == tmp.i && j < tmp.j));
}
ostream& operator << (ostream &o) {
return o << i << " " << j;
}
};
int main()
{
SCoor tmp = { 3, 3 };
cout << tmp;
return 0;
}
How do I have to overload the operator "<<"?

Answer 1

You overload the << operator as a member function if you want your structure to be on the left hand side of the expression. So:
struct SCoor
{
// ...
SCoor& operator << (Stuff const& s) {
// ...
return *this;
}
};
// ...
Stuff stuff;
SCoor scoor;
scoor << s; // insert s into my SCoor
If you want to make your struct the right hand side of the << expression you have to define a standalone function:
std::ostream& operator<<(std::ostream& os, SCoor const& scoor)
{
return os << scoor.i << " " << scoor.j;
}
However it is quite common to make the external function a friend function and to define it in the struct definition:
struct SCoor
{
// ...
friend std::ostream& operator<<(std::ostream& os, SCoor const& scoor)
{
return os << scoor.i << " " << scoor.j;
}
};
// ...
SCoor scoor;
std::cout << scoor << '\n';
But it is not a member of your struct, it is just defined inside its definition for convenience.