Calling const-members

Hi again! I am still learning through examples, and still I am
struggling with matrix classes! :)

I have a probably naive question: how do I "force" C++ to use a const
member? I mean, I have learned (through your suggestions) about const
members, for example my at() member in the matrix class:

class matrix
{
public:
    matrix(unsigned int rows, unsigned int cols);
    matrix(matrix const& src);

    virtual ~matrix();

    virtual double& at(unsigned int r, unsigned int c);
    virtual double at(unsigned int r, unsigned int c) const;
    // ...
}

Now, when I print my matrix using std::cout, the non-const member is
called, while I was expecting otherwise! My logging member is nothing
but a simple double loop:

void matrix::log()
{
    // ...
    for (i = 0; i < getRows(); i++)
    {
        for (j = 0; j < getColumns(); j++)
            cout << showpos << scientific << at(i, j) << " ";

        cout << endl;
    }
}

What am I missing here?

Thanks!