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

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!