The unwind_protect Statement

Octave supports a limited form of exception handling modelled after the unwind-protect form of Lisp.

The general form of an unwind_protect block looks like this:

     unwind_protect
       body
     unwind_protect_cleanup
       cleanup
     end_unwind_protect
     

Where body and cleanup are both optional and may contain any Octave expressions or commands. The statements in cleanup are guaranteed to be executed regardless of how control exits body.

This is useful to protect temporary changes to global variables from possible errors. For example the following code will always restore the original value of the built-in variable warn_fortran_indexing even if an error occurs while performing the indexing operation.

     save_warn_fortran_indexing = warn_fortran_indexing;
     unwind_protect
       warn_fortran_indexing = 1;
       elt = a (idx)
     unwind_protect_cleanup
       warn_fortran_indexing = save_warn_fortran_indexing;
     end_unwind_protect
     

Without unwind_protect the value of warn_fortran_indexing would not be restored if an error occurs while performing the indexing operation because evaluation would stop at the point of the error and the statement to restore the value would not be executed.