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<h3 class="section">3.6 Options Controlling Objective-C and Objective-C++ Dialects</h3>

<p><a name="index-compiler-options_002c-Objective_002dC-and-Objective_002dC_002b_002b-194"></a><a name="index-Objective_002dC-and-Objective_002dC_002b_002b-options_002c-command-line-195"></a><a name="index-options_002c-Objective_002dC-and-Objective_002dC_002b_002b-196"></a>(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.  See <a href="Standards.html#Standards">Language Standards Supported by GCC</a>, for references.)

 <p>This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs, but you can also use most of
the language-independent GNU compiler options.
For example, you might compile a file <code>some_class.m</code> like this:

<pre class="smallexample">     gcc -g -fgnu-runtime -O -c some_class.m
</pre>
 <p class="noindent">In this example, <samp><span class="option">-fgnu-runtime</span></samp> is an option meant only for
Objective-C and Objective-C++ programs; you can use the other options with
any language supported by GCC.

 <p>Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
<samp><span class="option">-Wtraditional</span></samp>).  Similarly, Objective-C++ compilations may use
C++-specific options (e.g., <samp><span class="option">-Wabi</span></samp>).

 <p>Here is a list of options that are <em>only</em> for compiling Objective-C
and Objective-C++ programs:

     <dl>
<dt><code>-fconstant-string-class=</code><var>class-name</var><dd><a name="index-fconstant_002dstring_002dclass-197"></a>Use <var>class-name</var> as the name of the class to instantiate for each
literal string specified with the syntax <code>@"..."</code>.  The default
class name is <code>NXConstantString</code> if the GNU runtime is being used, and
<code>NSConstantString</code> if the NeXT runtime is being used (see below).  The
<samp><span class="option">-fconstant-cfstrings</span></samp> option, if also present, will override the
<samp><span class="option">-fconstant-string-class</span></samp> setting and cause <code>@"..."</code> literals
to be laid out as constant CoreFoundation strings.

     <br><dt><code>-fgnu-runtime</code><dd><a name="index-fgnu_002druntime-198"></a>Generate object code compatible with the standard GNU Objective-C
runtime.  This is the default for most types of systems.

     <br><dt><code>-fnext-runtime</code><dd><a name="index-fnext_002druntime-199"></a>Generate output compatible with the NeXT runtime.  This is the default
for NeXT-based systems, including Darwin and Mac OS X.  The macro
<code>__NEXT_RUNTIME__</code> is predefined if (and only if) this option is
used.

     <br><dt><code>-fno-nil-receivers</code><dd><a name="index-fno_002dnil_002dreceivers-200"></a>Assume that all Objective-C message dispatches (<code>[receiver
message:arg]</code>) in this translation unit ensure that the receiver is
not <code>nil</code>.  This allows for more efficient entry points in the
runtime to be used.  This option is only available in conjunction with
the NeXT runtime and ABI version 0 or 1.

     <br><dt><code>-fobjc-abi-version=</code><var>n</var><dd><a name="index-fobjc_002dabi_002dversion-201"></a>Use version <var>n</var> of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime.  In that
case, Version 0 is the traditional (32-bit) ABI without support for
properties and other Objective-C 2.0 additions.  Version 1 is the
traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions.  Version 2 is the modern (64-bit) ABI.  If
nothing is specified, the default is Version 0 on 32-bit target
machines, and Version 2 on 64-bit target machines.

     <br><dt><code>-fobjc-call-cxx-cdtors</code><dd><a name="index-fobjc_002dcall_002dcxx_002dcdtors-202"></a>For each Objective-C class, check if any of its instance variables is a
C++ object with a non-trivial default constructor.  If so, synthesize a
special <code>- (id) .cxx_construct</code> instance method that will run
non-trivial default constructors on any such instance variables, in order,
and then return <code>self</code>.  Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special <code>- (void) .cxx_destruct</code> method that will run
all such default destructors, in reverse order.

     <p>The <code>- (id) .cxx_construct</code> and <code>- (void) .cxx_destruct</code>
methods thusly generated will only operate on instance variables
declared in the current Objective-C class, and not those inherited
from superclasses.  It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy.  The <code>- (id) .cxx_construct</code> methods will be invoked
by the runtime immediately after a new object instance is allocated;
the <code>- (void) .cxx_destruct</code> methods will be invoked immediately
before the runtime deallocates an object instance.

     <p>As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the <code>- (id) .cxx_construct</code> and
<code>- (void) .cxx_destruct</code> methods.

     <br><dt><code>-fobjc-direct-dispatch</code><dd><a name="index-fobjc_002ddirect_002ddispatch-203"></a>Allow fast jumps to the message dispatcher.  On Darwin this is
accomplished via the comm page.

     <br><dt><code>-fobjc-exceptions</code><dd><a name="index-fobjc_002dexceptions-204"></a>Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java.  This option
is required to use the Objective-C keywords <code>@try</code>,
<code>@throw</code>, <code>@catch</code>, <code>@finally</code> and
<code>@synchronized</code>.  This option is available with both the GNU
runtime and the NeXT runtime (but not available in conjunction with
the NeXT runtime on Mac OS X 10.2 and earlier).

     <br><dt><code>-fobjc-gc</code><dd><a name="index-fobjc_002dgc-205"></a>Enable garbage collection (GC) in Objective-C and Objective-C++
programs.  This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.

     <br><dt><code>-fobjc-nilcheck</code><dd><a name="index-fobjc_002dnilcheck-206"></a>For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using
<samp><span class="option">-fno-objc-nilcheck</span></samp>.  Class methods and super calls are never
checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an older
version of the NeXT runtime ABI, is used.

     <br><dt><code>-fobjc-std=objc1</code><dd><a name="index-fobjc_002dstd-207"></a>Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0.  This only affects the Objective-C additions to
the C/C++ language; it does not affect conformance to C/C++ standards,
which is controlled by the separate C/C++ dialect option flags.  When
this option is used with the Objective-C or Objective-C++ compiler,
any Objective-C syntax that is not recognized by GCC 4.0 is rejected.
This is useful if you need to make sure that your Objective-C code can
be compiled with older versions of GCC.

     <br><dt><code>-freplace-objc-classes</code><dd><a name="index-freplace_002dobjc_002dclasses-208"></a>Emit a special marker instructing <samp><span class="command">ld(1)</span></samp> not to statically link in
the resulting object file, and allow <samp><span class="command">dyld(1)</span></samp> to load it in at
run time instead.  This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself.  Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.

     <br><dt><code>-fzero-link</code><dd><a name="index-fzero_002dlink-209"></a>When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to <code>objc_getClass("...")</code> (when the name of the class is known at
compile time) with static class references that get initialized at load time,
which improves run-time performance.  Specifying the <samp><span class="option">-fzero-link</span></samp> flag
suppresses this behavior and causes calls to <code>objc_getClass("...")</code>
to be retained.  This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.
The GNU runtime currently always retains calls to <code>objc_get_class("...")</code>
regardless of command line options.

     <br><dt><code>-gen-decls</code><dd><a name="index-gen_002ddecls-210"></a>Dump interface declarations for all classes seen in the source file to a
file named <samp><var>sourcename</var><span class="file">.decl</span></samp>.

     <br><dt><code>-Wassign-intercept </code><span class="roman">(Objective-C and Objective-C++ only)</span><dd><a name="index-Wassign_002dintercept-211"></a><a name="index-Wno_002dassign_002dintercept-212"></a>Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.

     <br><dt><code>-Wno-protocol </code><span class="roman">(Objective-C and Objective-C++ only)</span><dd><a name="index-Wno_002dprotocol-213"></a><a name="index-Wprotocol-214"></a>If a class is declared to implement a protocol, a warning is issued for
every method in the protocol that is not implemented by the class.  The
default behavior is to issue a warning for every method not explicitly
implemented in the class, even if a method implementation is inherited
from the superclass.  If you use the <samp><span class="option">-Wno-protocol</span></samp> option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.

     <br><dt><code>-Wselector </code><span class="roman">(Objective-C and Objective-C++ only)</span><dd><a name="index-Wselector-215"></a><a name="index-Wno_002dselector-216"></a>Warn if multiple methods of different types for the same selector are
found during compilation.  The check is performed on the list of methods
in the final stage of compilation.  Additionally, a check is performed
for each selector appearing in a <code>@selector(...)</code>
expression, and a corresponding method for that selector has been found
during compilation.  Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the <samp><span class="option">-fsyntax-only</span></samp> option is
being used.

     <br><dt><code>-Wstrict-selector-match </code><span class="roman">(Objective-C and Objective-C++ only)</span><dd><a name="index-Wstrict_002dselector_002dmatch-217"></a><a name="index-Wno_002dstrict_002dselector_002dmatch-218"></a>Warn if multiple methods with differing argument and/or return types are
found for a given selector when attempting to send a message using this
selector to a receiver of type <code>id</code> or <code>Class</code>.  When this flag
is off (which is the default behavior), the compiler will omit such warnings
if any differences found are confined to types which share the same size
and alignment.

     <br><dt><code>-Wundeclared-selector </code><span class="roman">(Objective-C and Objective-C++ only)</span><dd><a name="index-Wundeclared_002dselector-219"></a><a name="index-Wno_002dundeclared_002dselector-220"></a>Warn if a <code>@selector(...)</code> expression referring to an
undeclared selector is found.  A selector is considered undeclared if no
method with that name has been declared before the
<code>@selector(...)</code> expression, either explicitly in an
<code>@interface</code> or <code>@protocol</code> declaration, or implicitly in
an <code>@implementation</code> section.  This option always performs its
checks as soon as a <code>@selector(...)</code> expression is found,
while <samp><span class="option">-Wselector</span></samp> only performs its checks in the final stage of
compilation.  This also enforces the coding style convention
that methods and selectors must be declared before being used.

     <br><dt><code>-print-objc-runtime-info</code><dd><a name="index-print_002dobjc_002druntime_002dinfo-221"></a>Generate C header describing the largest structure that is passed by
value, if any.

 </dl>

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