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<title>Using the GNU Compiler Collection (GCC): Exceptions</title>
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<a name="Exceptions"></a>
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<p>
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Next: <a href="Synchronization.html#Synchronization" accesskey="n" rel="next">Synchronization</a>, Previous: <a href="compatibility_005falias.html#compatibility_005falias" accesskey="p" rel="prev">compatibility_alias</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Exceptions-1"></a>
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<h3 class="section">8.7 Exceptions</h3>
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<p>GNU Objective-C provides exception support built into the language, as
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in the following example:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> @try {
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…
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@throw expr;
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…
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}
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@catch (AnObjCClass *exc) {
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…
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@throw expr;
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…
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@throw;
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…
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}
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@catch (AnotherClass *exc) {
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…
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}
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@catch (id allOthers) {
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…
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}
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@finally {
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…
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@throw expr;
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…
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}
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</pre></div>
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<p>The <code>@throw</code> statement may appear anywhere in an Objective-C or
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Objective-C++ program; when used inside of a <code>@catch</code> block, the
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<code>@throw</code> may appear without an argument (as shown above), in
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which case the object caught by the <code>@catch</code> will be rethrown.
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</p>
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<p>Note that only (pointers to) Objective-C objects may be thrown and
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caught using this scheme. When an object is thrown, it will be caught
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by the nearest <code>@catch</code> clause capable of handling objects of
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that type, analogously to how <code>catch</code> blocks work in C++ and
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Java. A <code>@catch(id …)</code> clause (as shown above) may also
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be provided to catch any and all Objective-C exceptions not caught by
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previous <code>@catch</code> clauses (if any).
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</p>
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<p>The <code>@finally</code> clause, if present, will be executed upon exit
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from the immediately preceding <code>@try … @catch</code> section.
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This will happen regardless of whether any exceptions are thrown,
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caught or rethrown inside the <code>@try … @catch</code> section,
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analogously to the behavior of the <code>finally</code> clause in Java.
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</p>
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<p>There are several caveats to using the new exception mechanism:
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</p>
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<ul>
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<li> The <samp>-fobjc-exceptions</samp> command line option must be used when
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compiling Objective-C files that use exceptions.
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</li><li> With the GNU runtime, exceptions are always implemented as “native”
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exceptions and it is recommended that the <samp>-fexceptions</samp> and
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<samp>-shared-libgcc</samp> options are used when linking.
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</li><li> With the NeXT runtime, although currently designed to be binary
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compatible with <code>NS_HANDLER</code>-style idioms provided by the
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<code>NSException</code> class, the new exceptions can only be used on Mac
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OS X 10.3 (Panther) and later systems, due to additional functionality
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needed in the NeXT Objective-C runtime.
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</li><li> As mentioned above, the new exceptions do not support handling
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types other than Objective-C objects. Furthermore, when used from
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Objective-C++, the Objective-C exception model does not interoperate with C++
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exceptions at this time. This means you cannot <code>@throw</code> an exception
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from Objective-C and <code>catch</code> it in C++, or vice versa
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(i.e., <code>throw … @catch</code>).
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</li></ul>
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<p>
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Next: <a href="Synchronization.html#Synchronization" accesskey="n" rel="next">Synchronization</a>, Previous: <a href="compatibility_005falias.html#compatibility_005falias" accesskey="p" rel="prev">compatibility_alias</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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