Re: [Boost-users] what should be used if shared_polymorphic_cast isdeprecated?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-16 16:16:23

Bernard Chen wrote:
> Hello,
>
> I was looking at the casting functions and the one that had the
> behavior I wanted most was the shared_polymorphic_cast. I do not want
> an empty shared_ptr<T> to be created if the cast cannot be performed.
> I would prefer to have an exception thrown in debug mode and release
> mode. The problem is that while going through the shared_ptr.hpp, I
> found a comment on line number 388 just above all the cast conversion
> functions "// shared_*_cast names are deprecated. Use *_pointer_cast
> instead." I have not found a *_pointer_cast function that does what I
> want. The dynamic_pointer_cast does not throw an exception. Is there
> a good alternative?

shared_polymorphic_cast is not part of the Library Technical Report:

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1745.pdf

so, even though it's not likely to go away, you should probably not use it 
if you are going to migrate to std::tr1::shared_ptr when it becomes 
available for your compiler/STL.

One alternative is to write a simple wrapper over dynamic_pointer_cast and 
use that instead. Of course in situations where you don't need a shared_ptr 
you can use dynamic_cast<T&>(*p).

> Should I enforce a standard on my team of using dynamic_pointer_cast
> along with a run-time check of seeing if an empty shared_ptr<T> was
> created?

Not a bad idea, because you can throw an exception that better describes the 
situation instead of the generic bad_cast. The idiomatic way to use 
dynamic_pointer_cast is

if( shared_ptr<T> pt = dynamic_pointer_cast<T>( pu ) )
{
// use pt
}
else
{
// handle failure
}