Programmatically control iPhone OS versions to enable functions for both OS 3.x and 4 - MFMessageComposeViewController problem

Question

In order to support iPhone OS 3.x and 4.0 I programmatically control MFMessageComposeViewController functionality like this (use it only if the OS version is 4.0 or above):

Answer 1

What about if you have functions that you only want to include conditionally.

- (void)messageComposeViewController:(MFMessageComposeViewController *)controller   
                 didFinishWithResult:(MessageComposeResult)result {
    [self dismissModalViewControllerAnimated:YES];
}

For example, if the function definition contained MFMessageViewController, how do you make this part conditional (that is, not cause a problem with OS 3.x). Or is it not a problem at all?