Re: Why RTTI is considered as a bad design?

Usually i see dynamic_cast<>() more used for cross-casting rather than down=
-casting. I bring an example. A specific Door passed to SomeDoer::open(Door=
*) may have Lockable interface or not. To have all Doors Lockable by defaul=
t would deviate from sane design since most doors can not be locked. Succes=
s of unlocking may depend of availability of Keys or LockPicks or whatever =
for this SomeDoer so open() forwards these problems to SomeDoer::unlock(Lo=
ckable*):

 bool SomeDoer::open( Door* door ) const
 {
   // we might need to unlock first on some cases
   Lockable* lock = dynamic_cast<Lockable*>(door);
   if ( lock != NULL && lock->isLocked() && !unlock( lock ) )
   {
      return false;
   }

   return door->open();
 }

Can you tell how such a situation can be resolved as elegantly without RTTI=
(or some self-made substitution of RTTI)?