input/output operator overloading in C++

1) operator<< and operator>> should not be member functions, otherwise you would need to use something like s << cout or s >> cin instead of cout << s and cin >> s

2) You will need to make them "friends" since they need to access private members.

3) Their return type needs to be Objects of type ostream& and istream&. To see the reason, think about the following case:

cout<< s << "is the sparse matrix!" << endl;

Since the value of cout << s is cout (which is of type ostream), both "is the sparse matrix!" and endl can be subsequently inserted into cout.

Thus there is the signature:
class SparseMatrix
{
public:
...
friend ostream& operator<< (ostream& out, const SparseMatrix& s);
...

private:
...
};