NameLookupAndInterfacePrinciple2

//*** Example 5 (a) -- nonvirtual streaming 

class X
{
	/*...ostream is never mentioned here...*/
};

ostream& operator<<( ostream& o, const X& x )
{
	/* code to output an X to a stream */
	return o;
}

//Here's the second:

//*** Example 5 (b) -- virtual streaming 

class X
{
	/*...*/
public:
	virtual ostream& print( ostream& ) const;
};
ostream& X::print( ostream& o ) const
{
	/* code to output an X to a stream */
	return o;
}

ostream& operator<<( ostream& o, const X& x )
{
	return x.print( o );
}