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| <h3 class="section">8.7 Exceptions</h3> |
| |
| <p>GNU Objective-C provides exception support built into the language, as |
| in the following example: |
| |
| <pre class="smallexample"> @try { |
| ... |
| @throw expr; |
| ... |
| } |
| @catch (AnObjCClass *exc) { |
| ... |
| @throw expr; |
| ... |
| @throw; |
| ... |
| } |
| @catch (AnotherClass *exc) { |
| ... |
| } |
| @catch (id allOthers) { |
| ... |
| } |
| @finally { |
| ... |
| @throw expr; |
| ... |
| } |
| </pre> |
| <p>The <code>@throw</code> statement may appear anywhere in an Objective-C or |
| Objective-C++ program; when used inside of a <code>@catch</code> block, the |
| <code>@throw</code> may appear without an argument (as shown above), in |
| which case the object caught by the <code>@catch</code> will be rethrown. |
| |
| <p>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 <code>@catch</code> clause capable of handling objects of |
| that type, analogously to how <code>catch</code> blocks work in C++ and |
| Java. A <code>@catch(id ...)</code> clause (as shown above) may also |
| be provided to catch any and all Objective-C exceptions not caught by |
| previous <code>@catch</code> clauses (if any). |
| |
| <p>The <code>@finally</code> clause, if present, will be executed upon exit |
| from the immediately preceding <code>@try ... @catch</code> section. |
| This will happen regardless of whether any exceptions are thrown, |
| caught or rethrown inside the <code>@try ... @catch</code> section, |
| analogously to the behavior of the <code>finally</code> clause in Java. |
| |
| <p>There are several caveats to using the new exception mechanism: |
| |
| <ul> |
| <li>The <samp><span class="option">-fobjc-exceptions</span></samp> command line option must be used when |
| compiling Objective-C files that use exceptions. |
| |
| <li>With the GNU runtime, exceptions are always implemented as “native” |
| exceptions and it is recommended that the <samp><span class="option">-fexceptions</span></samp> and |
| <samp><span class="option">-shared-libgcc</span></samp> options are used when linking. |
| |
| <li>With the NeXT runtime, although currently designed to be binary |
| compatible with <code>NS_HANDLER</code>-style idioms provided by the |
| <code>NSException</code> 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. |
| |
| <li>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 <code>@throw</code> an exception |
| from Objective-C and <code>catch</code> it in C++, or vice versa |
| (i.e., <code>throw ... @catch</code>). |
| </ul> |
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