Assuming that we're talking about overloading operator << for all classes derived from std::ostream to handle the Matrix class (and not overloading << for Matrix class), it makes more sense to declare the overload function outside the Math namespace in the header.
Use a friend function only if the functionality cannot be achieved via the public interfaces.
Matrix.h
namespace Math {
class Matrix {
//...
};
}
std::ostream& operator<<(std::ostream&, const Math::Matrix&);
Note that the operator overload is declared outside the namespace.
Matrix.cpp
using namespace Math;
using namespace std;
ostream& operator<< (ostream& os, const Matrix& obj) {
os << obj.getXYZ() << obj.getABC() << '\n';
return os;
}
On the other hand, if your overload function does need to be made a friend i.e. needs access to private and protected members.
Math.h
namespace Math {
class Matrix {
public:
friend std::ostream& operator<<(std::ostream&, const Matrix&);
};
}
You need to enclose the function definition with a namespace block instead of just using namespace Math;.
Matrix.cpp
using namespace Math;
using namespace std;
namespace Math {
ostream& operator<<(ostream& os, const Matrix& obj) {
os << obj.XYZ << obj.ABC << '\n';
return os;
}
}