reference to ostream object

[Kreadeth]

Hey there.

I'm told, in overloading functions topic, if, for example, I had a class called "Number" and an object in that class called "ten", and intended to print out the value using the following syntax,

std::cout << ten;

I should define a friend function and implement it as follows:

using namespace std;

ostream & operator<< (ostream & os, Number & num)
{
os << num.value;
return os;
}

I'm just curious, why should the function return type would be a refernce? Why should I pass a reference to an ostream object as the argument? Why the otherwise won't work?

Thanks in advance.

[LB]

C++ streams are not copyable (for good reason), so you have to pass them around by reference. The reason you return the reference is because the << and >> operators need to be able to be chained.