The << operator, when applied to streams, is called the insertion operator. (Likewise, the extraction operator is >>). If we overload it for a class (as non-member), we can output instances of that class. That’s very handy.
Note that in order for the overloaded operator<<() method to have access to private members of the class, we need to declare it as friend.
Take the very basic example of 2D coordinates:
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#include <iostream>
class Coords {
uint16_t _x;
uint16_t _y;
public:
Coords(uint16_t x, uint16_t y): _x(x), _y(y)
{
}
friend std::ostream& operator<<(std::ostream& out, const Coords& coords)
{
out << "(" << coords._x << ", " << coords._y << ")";
return out;
}
};
int main()
{
Coords c(0, 1);
std::cout << c << std::endl;
}
This produces, as expected:
1
(0, 1)
The problem with operator<<() (through my Python-tinted glasses, of course) is that it cannot unfortunately be used directly to create a custom exception description 
We must use a stringstream instead:
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#include <exception>
#include <sstream>
class Coords {
// ...
void move_left()
{
if (_x == 0) {
std::stringstream ss;
ss << "Coords: cannot move left from " << *this;
throw std::invalid_argument(ss.str());
}
}
};
This produces:
1
Coords: cannot move left from (0, 1)
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