Non-const Ostream Object Overloading <<

[informrefer]

Why can't os be const when overloading the << operator? You don't want to change os, right?

  ostream& operator<<(ostream& os, const Date& dt)
{
    os << dt.mo << '/' << dt.da << '/' << dt.yr;
    return os;
}

instead of

ostream& operator<<(const ostream& os, const Date& dt)
{
    os << dt.mo << '/' << dt.da << '/' << dt.yr;
    return os;
}

[Peter87]

You change the state of os when you write to it.

[shadowCODE]

Peter87 said it all