iOS5 classes on iOS4

Question

I'd like to use some classes introduced in iOS 5 in an app that also runs on iOS 4. I understand how to use respondsToSelector: to selectively invoke new methods on the newer OS versions. What is the equivalent to determine, at runtime, whether the class exists.

Answer 1

if (NSClassFromString(@"ClassName") != nil)
{
    // use class
}
else
{
    // not available, deal with it.
}