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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:
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.
@throw an exception
from Objective-C and catch it in C++, or vice versa
(i.e., throw … @catch).
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