Validating Input

The pointer returned by astReadastRead (ยง15.4) could identify any class of ObjectObject—this is determined entirely by the external data being read. If it is necessary to test for a particular class (say a FrameFrame), this may be done as follows using the appropriate member of the astIsA1#1Class2#2astIsA1#1Class2#2 family of functions:


232#232

Note, however, that this will accept any Frame, so would be equally happy with a basic Frame or a SkyFrameSkyFrame. An alternative validation strategy would be to obtain the value of the Object's ClassClass attribute and then test this character string, as follows:


233#233

This would only accept a basic Frame and would reject a SkyFrame.