How can I use void printMatrix( ostream& os = cout ) const?

Question

I am trying to learn C++, and I've got a task, to do some printing with this function, and I don't understand how to use the ostream. Can anyone help me please?

    void Matrix::printMatrix( ostream& os = cout ) const{
    for(int i=0; i<n; i++)
      for(int j=0; i<m; j++)
        os<<elements[i][j]<<"\n";
    }

I've tried to do this, but it threw me some errors, and I don't know how to handle this. The errors:

Matrix.cpp:47:48: error: default argument given for parameter 1 of 'void Matrix::printMatrix(std::ostream&) const' [-fpermissive] In file included from Matrix.cpp:8:0: Matrix.h:25:10: error: after previous specification in 'void Matrix::printMatrix(std::ostream&) const' [-fpermissive]

Answer 1

You should not specify the default argument of a function both in a declaration and in a definition:

class Matrix
{
    // ...

    // Default argument specified in the declaration...
    void printMatrix( ostream& os = cout ) const;

    // ...
};

// ...so you shouldn't (cannot) specify it also in the definition,
// even though you specify the exact same value.
void Matrix::printMatrix( ostream& os /* = cout */ ) const{
//                                    ^^^^^^^^^^^^
//                                    Remove this


    ...
}

Alternatively, you can keep the default argument specification in the definition and omit it in the declaration. What's important is that you don't have it in both.

Answer 2

The function has an output stream as parameter, and has the standard output ( std::cout ) as default (albeit incorrectly specified in the function definition, not in the declaration as it should be). You can do this:

// use default parameter std::cout
Matrix m + ...;
m.printMatrix();

// explicitly use std::cout
m.printMatrix(std::cout);

// write to a file
std::ofstream outfile("matrix.txt");
m.printMatrix(outfile);