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. :-)

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.