contravariant argument types: wanna?

[grauzone]

Steven Schveighoffer wrote:
> The solution we ended up using is that *all* streams defined the seek 
> function, even if they didn't support seeking, and if you called it on 
> such objects, they just throw an exception.

So, if you want to see if an object supports seeking, you must call the 
method + catch the exception? How is this better than a dynamic cast?