The purpose of Output_rep is to provide a way to control output formatting that works independently of the object's stream output operator.

If you dont specialize Output_rep for T, T's stream output operator is called from within Output_rep, by default. If you want another behaviour for your type T, you have to provide a specialization for that type. Furthermore, you can provide specializations with a second template parameter (a formatting tag). The second template parameter defaults to Null_tag and means DEFAULT BEHAVIOUR.

Specializations of Output_rep should provide the following features:

template< class F >
struct Output_rep< Some_type, F > {
    Output_rep( const Some_type& t );
    std::ostream& operator()( std::ostream& out ) const;

You can also specialize for a formatting tag F.