What's the point of IsA() in C++?

Question

I\'m trying to figure out why some code bases use IsA() to determine object polymorphism if in C++ you can already safely upcast and down cast (using dynamic_cast) ?

So

Answer 1

Sometimes you dont have RTTI ( maybe you're working on a memory/cpu constrained system and disabling RTTI is the mandated solution) In that case you don't have dynamic_cast available to you. If you want to use something similar to RTTI you usually end up with solutions that are IsA() with static_cast.

Another possible use for an IsA() function is when you don't know all your classes at compile-time (maybe they've been loaded from shared library), or you dont want to list all the types explicitly. This lets you write things like

handleInstance( Instance * i )
{
  //libs has been filled through loading dynamic libraries.
  for( auto it = libs.begin() ; it!=libs.end() ; ++it )
  {
     if( i->IsA( it->type_key ) )
     {
        it->process( i );
     }
  }
}

Though in this case I might turn the test condition inside out like

if( it->can_process( i ) )

and can_process would be free to use dynamic_cast.