As chris already pointed out in comments very quickly (as usual), you have a temporary created in here:
cout << a + b << endl;
You cannot bind that to a non-const reference. You will need to change the signature of your operator overloading by adding the const keyword to the reference.
This code works for me with a dummy BigInt implementation (as you have not shared yours):
#include <iostream>
using namespace std;
class BigInt
{
public:
bool getSign() const { return true; }
int getNumber() const { return 0; }
const BigInt operator+(const BigInt &other) const {}
};
inline std::ostream& operator << (ostream &stream, const BigInt &B){
// ^^^^^
if (!B.getSign()){
stream << '-';
}
stream << B.getNumber();
return stream;
}
int main()
{
BigInt a, b, c;
c = a + b;
cout << c << endl;
cout << a + b << endl;
return 0;
}
But yeah, I agree that the error message is not self-explanatory in this particular case.