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8.7 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: