A: Provide a friend operator<< which calls a protected virtual function

class Base {

public:

friend std::ostream& operator<< (std::ostream& o, const Base& b);

... protected:

virtual void printOn(std::ostream& o) const;

};

inline std::ostream& operator<< (std::ostream& o, const Base& b)

{ b.printOn(o); return o;

}

class Derived : public Base {

protected:

virtual void printOn(std::ostream& o) const;

};

The ending result is that operator<< acts as if it were bound dynamically, even though it's a friend function. It is called the Virtual Friend Function Idiom.

Note down that derived classes override printOn(std::ostream&) const. Particularly, they do not provide their own operator<<.

Of course if Base is an ABC, Base::printOn(std::ostream&) const may be declared pure virtual using the "= 0" syntax.

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