Exceptions#
GNU Objective-C provides exception support built into the language, as in the following example:
@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
-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
-fexceptions
and-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 theNSException
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 andcatch
it in C++, or vice versa (i.e.,throw ... @catch
).