How to properly overload the << operator for an ostream?

I am writing a small matrix library in C++ for matrix operations. However my compiler complains, where before it did not. This code was left on a shelf for 6 months and in between I upgraded my computer from debian etch to lenny (g++ (Debian 4.3.2-1.1) 4.3.2 ) however I have the same problem on a Ubuntu system with the same g++.

Here is the relevant part of my matrix class:

namespace Math
{
    class Matrix
    {
    public:

        [...]

        friend std::ostream& operator<< (std::ostream& stream, const Matrix& matrix);
    }
}

And the "implementation":

using namespace Math;

std::ostream& Matrix::operator <<(std::ostream& stream, const Matrix& matrix) {

    [...]

}

This is the error given by the compiler:

matrix.cpp:459: error: 'std::ostream& Math::Matrix::operator<<(std::ostream&, const Math::Matrix&)' must take exactly one argument

I'm a bit confused by this error, but then again my C++ has gotten a bit rusty after doing lots of Java those 6 months. :-)

Just telling you about one other possibility: I like using friend definitions for that:

namespace Math
{
    class Matrix
    {
    public:

        [...]

        friend std::ostream& operator<< (std::ostream& stream, const Matrix& matrix) {
            [...]
        }
    };
}

The function will be automatically targeted into the surrounding namespace Math (even though its definition appears within the scope of that class) but will not be visible unless you call operator<< with a Matrix object which will make argument dependent lookup find that operator definition. That can sometimes help with ambiguous calls, since it's invisible for argument types other than Matrix. When writing its definition, you can also refer directly to names defined in Matrix and to Matrix itself, without qualifying the name with some possibly long prefix and providing template parameters like Math::Matrix<TypeA, N>.

You have declared your function as friend. It's not a member of the class. You should remove Matrix:: from the implementation. friend means that the specified function (which is not a member of the class) can access private member variables. The way you implemented the function is like an instance method for Matrix class which is wrong.