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 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).