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<title>Using the GNU Compiler Collection (GCC): Objective-C and Objective-C++ Dialect Options</title>
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<a name="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options"></a>
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<div class="header">
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<p>
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Next: <a href="Diagnostic-Message-Formatting-Options.html#Diagnostic-Message-Formatting-Options" accesskey="n" rel="next">Diagnostic Message Formatting Options</a>, Previous: <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" accesskey="p" rel="prev">C++ Dialect Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</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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</div>
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<hr>
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<a name="Options-Controlling-Objective_002dC-and-Objective_002dC_002b_002b-Dialects"></a>
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<h3 class="section">3.6 Options Controlling Objective-C and Objective-C++ Dialects</h3>
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<a name="index-compiler-options_002c-Objective_002dC-and-Objective_002dC_002b_002b"></a>
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<a name="index-Objective_002dC-and-Objective_002dC_002b_002b-options_002c-command_002dline"></a>
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<a name="index-options_002c-Objective_002dC-and-Objective_002dC_002b_002b"></a>
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<p>(NOTE: This manual does not describe the Objective-C and Objective-C++
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languages themselves. See <a href="Standards.html#Standards">Language Standards
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Supported by GCC</a>, for references.)
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</p>
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<p>This section describes the command-line options that are only meaningful
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for Objective-C and Objective-C++ programs. You can also use most of
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the language-independent GNU compiler options.
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For example, you might compile a file <samp>some_class.m</samp> like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">gcc -g -fgnu-runtime -O -c some_class.m
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</pre></div>
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<p>In this example, <samp>-fgnu-runtime</samp> is an option meant only for
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Objective-C and Objective-C++ programs; you can use the other options with
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any language supported by GCC.
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</p>
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<p>Note that since Objective-C is an extension of the C language, Objective-C
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compilations may also use options specific to the C front-end (e.g.,
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<samp>-Wtraditional</samp>). Similarly, Objective-C++ compilations may use
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C++-specific options (e.g., <samp>-Wabi</samp>).
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</p>
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<p>Here is a list of options that are <em>only</em> for compiling Objective-C
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and Objective-C++ programs:
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</p>
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<dl compact="compact">
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<dt><code>-fconstant-string-class=<var>class-name</var></code></dt>
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<dd><a name="index-fconstant_002dstring_002dclass"></a>
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<p>Use <var>class-name</var> as the name of the class to instantiate for each
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literal string specified with the syntax <code>@"…"</code>. The default
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class name is <code>NXConstantString</code> if the GNU runtime is being used, and
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<code>NSConstantString</code> if the NeXT runtime is being used (see below). The
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<samp>-fconstant-cfstrings</samp> option, if also present, overrides the
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<samp>-fconstant-string-class</samp> setting and cause <code>@"…"</code> literals
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to be laid out as constant CoreFoundation strings.
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</p>
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</dd>
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<dt><code>-fgnu-runtime</code></dt>
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<dd><a name="index-fgnu_002druntime"></a>
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<p>Generate object code compatible with the standard GNU Objective-C
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runtime. This is the default for most types of systems.
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</p>
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</dd>
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<dt><code>-fnext-runtime</code></dt>
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<dd><a name="index-fnext_002druntime"></a>
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<p>Generate output compatible with the NeXT runtime. This is the default
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for NeXT-based systems, including Darwin and Mac OS X. The macro
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<code>__NEXT_RUNTIME__</code> is predefined if (and only if) this option is
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used.
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</p>
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</dd>
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<dt><code>-fno-nil-receivers</code></dt>
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<dd><a name="index-fno_002dnil_002dreceivers"></a>
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<p>Assume that all Objective-C message dispatches (<code>[receiver
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message:arg]</code>) in this translation unit ensure that the receiver is
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not <code>nil</code>. This allows for more efficient entry points in the
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runtime to be used. This option is only available in conjunction with
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the NeXT runtime and ABI version 0 or 1.
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</p>
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</dd>
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<dt><code>-fobjc-abi-version=<var>n</var></code></dt>
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<dd><a name="index-fobjc_002dabi_002dversion"></a>
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<p>Use version <var>n</var> of the Objective-C ABI for the selected runtime.
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This option is currently supported only for the NeXT runtime. In that
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case, Version 0 is the traditional (32-bit) ABI without support for
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properties and other Objective-C 2.0 additions. Version 1 is the
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traditional (32-bit) ABI with support for properties and other
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Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If
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nothing is specified, the default is Version 0 on 32-bit target
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machines, and Version 2 on 64-bit target machines.
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</p>
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</dd>
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<dt><code>-fobjc-call-cxx-cdtors</code></dt>
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<dd><a name="index-fobjc_002dcall_002dcxx_002dcdtors"></a>
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<p>For each Objective-C class, check if any of its instance variables is a
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C++ object with a non-trivial default constructor. If so, synthesize a
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special <code>- (id) .cxx_construct</code> instance method which runs
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non-trivial default constructors on any such instance variables, in order,
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and then return <code>self</code>. Similarly, check if any instance variable
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is a C++ object with a non-trivial destructor, and if so, synthesize a
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special <code>- (void) .cxx_destruct</code> method which runs
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all such default destructors, in reverse order.
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</p>
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<p>The <code>- (id) .cxx_construct</code> and <code>- (void) .cxx_destruct</code>
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methods thusly generated only operate on instance variables
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declared in the current Objective-C class, and not those inherited
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from superclasses. It is the responsibility of the Objective-C
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runtime to invoke all such methods in an object’s inheritance
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hierarchy. The <code>- (id) .cxx_construct</code> methods are invoked
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by the runtime immediately after a new object instance is allocated;
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the <code>- (void) .cxx_destruct</code> methods are invoked immediately
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before the runtime deallocates an object instance.
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</p>
|
<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
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<code>- (void) .cxx_destruct</code> methods.
|
</p>
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</dd>
|
<dt><code>-fobjc-direct-dispatch</code></dt>
|
<dd><a name="index-fobjc_002ddirect_002ddispatch"></a>
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<p>Allow fast jumps to the message dispatcher. On Darwin this is
|
accomplished via the comm page.
|
</p>
|
</dd>
|
<dt><code>-fobjc-exceptions</code></dt>
|
<dd><a name="index-fobjc_002dexceptions"></a>
|
<p>Enable syntactic support for structured exception handling in
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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).
|
</p>
|
</dd>
|
<dt><code>-fobjc-gc</code></dt>
|
<dd><a name="index-fobjc_002dgc"></a>
|
<p>Enable garbage collection (GC) in Objective-C and Objective-C++
|
programs. This option is only available with the NeXT runtime; the
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GNU runtime has a different garbage collection implementation that
|
does not require special compiler flags.
|
</p>
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</dd>
|
<dt><code>-fobjc-nilcheck</code></dt>
|
<dd><a name="index-fobjc_002dnilcheck"></a>
|
<p>For the NeXT runtime with version 2 of the ABI, check for a nil
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receiver in method invocations before doing the actual method call.
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This is the default and can be disabled using
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<samp>-fno-objc-nilcheck</samp>. Class methods and super calls are never
|
checked for nil in this way no matter what this flag is set to.
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Currently this flag does nothing when the GNU runtime, or an older
|
version of the NeXT runtime ABI, is used.
|
</p>
|
</dd>
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<dt><code>-fobjc-std=objc1</code></dt>
|
<dd><a name="index-fobjc_002dstd"></a>
|
<p>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.
|
</p>
|
</dd>
|
<dt><code>-freplace-objc-classes</code></dt>
|
<dd><a name="index-freplace_002dobjc_002dclasses"></a>
|
<p>Emit a special marker instructing <code>ld(1)</code> not to statically link in
|
the resulting object file, and allow <code>dyld(1)</code> 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.
|
</p>
|
</dd>
|
<dt><code>-fzero-link</code></dt>
|
<dd><a name="index-fzero_002dlink"></a>
|
<p>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>-fzero-link</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.
|
</p>
|
</dd>
|
<dt><code>-fno-local-ivars</code></dt>
|
<dd><a name="index-fno_002dlocal_002divars"></a>
|
<a name="index-flocal_002divars"></a>
|
<p>By default instance variables in Objective-C can be accessed as if
|
they were local variables from within the methods of the class they’re
|
declared in. This can lead to shadowing between instance variables
|
and other variables declared either locally inside a class method or
|
globally with the same name. Specifying the <samp>-fno-local-ivars</samp>
|
flag disables this behavior thus avoiding variable shadowing issues.
|
</p>
|
</dd>
|
<dt><code>-fivar-visibility=<span class="roman">[</span>public<span class="roman">|</span>protected<span class="roman">|</span>private<span class="roman">|</span>package<span class="roman">]</span></code></dt>
|
<dd><a name="index-fivar_002dvisibility"></a>
|
<p>Set the default instance variable visibility to the specified option
|
so that instance variables declared outside the scope of any access
|
modifier directives default to the specified visibility.
|
</p>
|
</dd>
|
<dt><code>-gen-decls</code></dt>
|
<dd><a name="index-gen_002ddecls"></a>
|
<p>Dump interface declarations for all classes seen in the source file to a
|
file named <samp><var>sourcename</var>.decl</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wassign-intercept <span class="roman">(Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wassign_002dintercept"></a>
|
<a name="index-Wno_002dassign_002dintercept"></a>
|
<p>Warn whenever an Objective-C assignment is being intercepted by the
|
garbage collector.
|
</p>
|
</dd>
|
<dt><code>-Wno-protocol <span class="roman">(Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wno_002dprotocol"></a>
|
<a name="index-Wprotocol"></a>
|
<p>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>-Wno-protocol</samp> option, then
|
methods inherited from the superclass are considered to be implemented,
|
and no warning is issued for them.
|
</p>
|
</dd>
|
<dt><code>-Wselector <span class="roman">(Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wselector"></a>
|
<a name="index-Wno_002dselector"></a>
|
<p>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>-fsyntax-only</samp> option is
|
being used.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-selector-match <span class="roman">(Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wstrict_002dselector_002dmatch"></a>
|
<a name="index-Wno_002dstrict_002dselector_002dmatch"></a>
|
<p>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 omits such warnings
|
if any differences found are confined to types that share the same size
|
and alignment.
|
</p>
|
</dd>
|
<dt><code>-Wundeclared-selector <span class="roman">(Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wundeclared_002dselector"></a>
|
<a name="index-Wno_002dundeclared_002dselector"></a>
|
<p>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>-Wselector</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.
|
</p>
|
</dd>
|
<dt><code>-print-objc-runtime-info</code></dt>
|
<dd><a name="index-print_002dobjc_002druntime_002dinfo"></a>
|
<p>Generate C header describing the largest structure that is passed by
|
value, if any.
|
</p>
|
</dd>
|
</dl>
|
|
<hr>
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<div class="header">
|
<p>
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Next: <a href="Diagnostic-Message-Formatting-Options.html#Diagnostic-Message-Formatting-Options" accesskey="n" rel="next">Diagnostic Message Formatting Options</a>, Previous: <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" accesskey="p" rel="prev">C++ Dialect Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</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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