Boost Exception

namespace
boost
    {
    template <class E, class Tag, class T>
    E const & operator<<( E const & x, error_info<Tag,T> const & v );
    }

Requirements:

E must be boost::exception, or a type that derives (indirectly) from boost::exception.

Postcondition:

A copy of v is stored into x. If x already contains data of type error_info<Tag,T>, that data is overwritten. Basic exception safety guarantee.

Returns:

x.