Using C++ exceptions instead of error_state

[John W. Eaton]

I've been experimenting with having the error function throw a C++ 
exception instead of setting error_state.  The current draft of my patch 
is attached.  All the tests that were passing before the change appear 
to still be passing, so I think it is mostly functional.

With these changes, you can simply write

  if (condition)
    error ("lookout!");

  ... more code that should only execute if there is no error ...

and the error message and stack trace will be generated and execution 
will jump to the top level magically.  There's no longer any need to 
check the value of error_state after operations that might generate 
error messages.  This change will make error handling in Octave's C++ 
code much more like what is done in .m files.

The error_state variable remains because a lot of code still uses it. 
There are more than 1600 locations where it is used in core Octave that 
I have not yet fixed.

Even though it will require quite a bit more work to finish this change 
(those 1600+ uses of error_state, for one thing) I think it would be a 
big step forward.

Comments?  Objections?

jwe

exception-error-diffs.txt
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