Question

Considering the following code:

namespace myNS {
class MyClass {

//.....
};
} //end of namespace myNS


using namespace myNS;

//overloading of '<<' operator
std::ostream &myNS::operator<<(std::ostream &os, MyClass &c){  /*....*/ } 

In the last line, why is &myNS:: needed?

Answer 1

& is needed because, conventionally, streaming operators return a reference to the stream to allow them to be chained:

stream << something << something_else;

with something_else being passed to the stream reference returned by stream << something.

myNS:: is needed if this operator is supposed to be scoped inside the namespace, in which case there must also be a previous declaration inside the namespace. If you want the operator to be in the current namespace (presumably the global namespace in this example), then it’s not needed.