Making operator<< virtual?

Question

I need to use a virtual << operator. However, when I try to write:

virtual friend ostream & operator<<(ostream& os,const Advertising& add);

I get the compiler error

Error 1 error C2575: 'operator <<' : only member functions and bases can be virtual

How can I turn this operator virtual?

Answer 1

You define your operator << to call a virtual print method:

class Base
{
    protected:
        virtual void print(std::ostream& str) const = 0;
    public:
        friend std::ostream& operator<<(std::ostream& str, Base const& data)
        {
            data.print(str);
            return str;
        }
}

Answer 2

The problem with this setup is that the operator<< you defined above is a free function, which can't be virtual (it has no receiver object). In order to make the function virtual, it must be defined as a member of some class, which is problematic here because if you define operator<< as a member of a class then the operands will be in the wrong order:

class MyClass {
public:
    virtual ostream& operator<< (ostream& out) const;
};

means that

MyClass myObject;
cout << myObject;

will not compile, but

MyClass myObject;
myObject << cout;

will be legal.

To fix this, you can apply the Fundamental Theorem of Software Engineering - any problem can be solved by adding another layer of indirection. Rather than making operator<< virtual, consider adding a new virtual function to the class that looks like this:

class MyClass {
public:
    virtual void print(ostream& where) const;
};

Then, define operator<< as

ostream& operator<< (ostream& out, const MyClass& mc) {
    mc.print(out);
    return out;
}

This way, the operator<< free function has the right parameter order, but the behavior of operator<< can be customized in subclasses.

Hope this helps!

Answer 3

It looks like you really want to provide output functionality for a hierarchy of classes, and if so, you can provide a friend operator << that calls a virtual function.

class Parent
{
public:
    friend std::ostream& operator<< (std::ostream& os, const Parent& p);
    // ... other class stuff
protected:
    virtual void printMyself(std::ostream& os) const
    {
        // do something if you must, or make this a pure virtual
    }
};

std::ostream& operator<< (std::ostream& os, const Parent& p)
{
    p.printMyself(os);
    return os;
}

class Child : public Parent
{
    // other class stuff...
protected:
    virtual void printMyself(std::ostream os) const
    {
        // whatever you need to do
    }
};

Also detailed in the C++ FAQ