Re: Polymorphism and ostream &operator<<()

James Kanze <>
Wed, 17 Jun 2009 01:23:31 -0700 (PDT)
On Jun 17, 3:55 am, Jeff Dege <> wrote:

The canonical method for making a class work with ostreams is
to declare a function similar to this:

class NameValuePair;
std::ostream &operator<<(std::ostream &o, const NameValuePair &nvp)
   return o << << "\t" << nvp.value;

Where operator<<() might be declared a friend to the class, if
it needed to access an object's internals, or it might not.
But in neither case is it a member function of the class.

That may be the most wide-spread solution, but it's far from the
only one. I generally do something like:

    class NameValuePair
        : public IOStreamOperators< NameValuePair >
        void print( std::ostream& dest ) const ;
        // ...
    } ;

The IOStreamOperators class template provides the << operator
(calling print), using the Barton and Nackman trick: basically:

    template< typename T >
    class IOStreamOperators
        friend std::ostream&operator<<(
            std::ostream& dest,
            T const& object )
            object.print( dest ) ;
            return dest ;
        friend std::istream&operator>>(
            std::istream& source,
            T& object )
            object.scan( source ) ;
            return source ;

        ~IOStreamOperators() {}
    } ;

Which means that it's not polymorphic.

Make print virtual, in the above, and it's polymorphic.


One obvious way to make this work is to provide some sort of
output member function within the classes, marked virtual so
that we get the polymorphism we desire, then to implement an
ostream &operator<<() on the parent class that calls that
member function:

class Parent;
std::ostream &operator<<(std::ostream &o, const Parent &parent)
   return o << Parent.to_string();

But I can envision situations where this might not be an ideal
solution. If the internal representation of an object might
result in a very large string, this approach could have
significant performance penalties.

Why should the function return a string? Why shouldn't it just
do the output, as above. (I've not thought about it, but one
could easily add a "asString" function to IOStreamOperators,
    asString() const
        std::ostringstream s ;
        static_cast< T const* >( this )->print( s ) ;
        return s.str() ;

In general, it is more natural in C++ to implement output to a
stream first, and asString in terms of it.)

James Kanze (GABI Software)
Conseils en informatique orient=E9e objet/
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