template <class T> void sc_signal<T>
{
protected:
    T                  m_cur_val;
    T                  m_new_val;

public: 
 // read the current value
    virtual const T& read() const
        { return m_cur_val; }


    sc_signal<T>& operator = ( const T& a )
        { write( a ); return *this; }


  void write( const T& value_ )
  {
      m_new_val = value_;
      if( !( m_new_val == m_cur_val ) ) {
	  request_update();
      }
  }

  ...
}