Exceptions
**********

GNU Objective-C provides exception support built into the language, as
in the following example:

.. code-block:: c++

    try {
      ...
         throw expr;
      ...
    }
    catch (AnObjCClass *exc) {
      ...
        throw expr;
      ...
        throw;
      ...
    }
    catch (AnotherClass *exc) {
      ...
    }
    catch (id allOthers) {
      ...
    }
    finally {
      ...
        throw expr;
      ...
    }

The ``throw`` statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a ``catch`` block, the
``throw`` may appear without an argument (as shown above), in
which case the object caught by the ``catch`` will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme.  When an object is thrown, it will be caught
by the nearest ``catch`` clause capable of handling objects of
that type, analogously to how ``catch`` blocks work in C++ and
Java.  A ``catch(id ...)`` clause (as shown above) may also
be provided to catch any and all Objective-C exceptions not caught by
previous ``catch`` clauses (if any).

The ``finally`` clause, if present, will be executed upon exit
from the immediately preceding ``try ... catch`` section.
This will happen regardless of whether any exceptions are thrown,
caught or rethrown inside the ``try ... catch`` section,
analogously to the behavior of the ``finally`` clause in Java.

There are several caveats to using the new exception mechanism:

* The :option:`-fobjc-exceptions` command line option must be used when
  compiling Objective-C files that use exceptions.

  * With the GNU runtime, exceptions are always implemented as native
  exceptions and it is recommended that the :option:`-fexceptions` and
  :option:`-shared-libgcc` options are used when linking.

  * With the NeXT runtime, although currently designed to be binary
  compatible with ``NS_HANDLER``-style idioms provided by the
  ``NSException`` class, the new exceptions can only be used on Mac
  OS X 10.3 (Panther) and later systems, due to additional functionality
  needed in the NeXT Objective-C runtime.

  * As mentioned above, the new exceptions do not support handling
  types other than Objective-C objects.   Furthermore, when used from
  Objective-C++, the Objective-C exception model does not interoperate with C++
  exceptions at this time.  This means you cannot ``throw`` an exception
  from Objective-C and ``catch`` it in C++, or vice versa
  (i.e., ``throw ... catch``).

.. =========================================================================