(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See Language Standards Supported by GCC, for references.)
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.
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., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name ¶Use class-name as the name of the class to instantiate for each
literal string specified with the syntax @"…". The default
class name is NXConstantString if the GNU runtime is being used, and
NSConstantString if the NeXT runtime is being used (see below). On
Darwin / macOS platforms, the -fconstant-cfstrings option, if
also present, overrides the -fconstant-string-class setting and cause
@"…" literals to be laid out as constant CoreFoundation strings.
Note that -fconstant-cfstrings is an alias for the target-specific
-mconstant-cfstrings equivalent.
-fgnu-runtime ¶Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.
-fnext-runtime ¶Generate output compatible with the NeXT runtime. This is the default
for NeXT-based systems, including Darwin / macOS. The macro
__NEXT_RUNTIME__ is predefined if (and only if) this option is
used.
-fno-nil-receivers ¶Assume that all Objective-C message dispatches ([receiver
message:arg]) in this translation unit ensure that the receiver is
not nil. 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.
-fobjc-abi-version=n ¶Use version n 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.
-fobjc-call-cxx-cdtors ¶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 - (id) .cxx_construct instance method which runs
non-trivial default constructors on any such instance variables, in order,
and then return self. Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special - (void) .cxx_destruct method which runs
all such default destructors, in reverse order.
The - (id) .cxx_construct and - (void) .cxx_destruct
methods thusly generated 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 - (id) .cxx_construct methods are invoked
by the runtime immediately after a new object instance is allocated;
the - (void) .cxx_destruct methods are invoked immediately
before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the - (id) .cxx_construct and
- (void) .cxx_destruct methods.
-fobjc-direct-dispatch ¶Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions ¶Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++. This option
is required to use the Objective-C keywords @try,
@throw, @catch, @finally and
@synchronized. 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).
-fobjc-gc ¶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.
-fobjc-nilcheck ¶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 -fno-objc-nilcheck. 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.
-fobjc-std=objc1 ¶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.
-freplace-objc-classes ¶Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) 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.
-fzero-link ¶When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to objc_getClass("…") (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 -fzero-link flag
suppresses this behavior and causes calls to objc_getClass("…")
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 objc_get_class("…")
regardless of command-line options.
-fno-local-ivars ¶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 -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues.
-fivar-visibility=[public|protected|private|package] ¶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.
-gen-decls ¶Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only) ¶Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-property-assign-default (Objective-C and Objective-C++ only) ¶Do not warn if a property for an Objective-C object has no assign semantics specified.
-Wno-protocol (Objective-C and Objective-C++ only) ¶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 -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
-Wobjc-root-class (Objective-C and Objective-C++ only) ¶Warn if a class interface lacks a superclass. Most classes will inherit
from NSObject (or Object) for example. When declaring
classes intended to be root classes, the warning can be suppressed by
marking their interfaces with __attribute__((objc_root_class)).
-Wselector (Objective-C and Objective-C++ only) ¶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 @selector(…)
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 -fsyntax-only option is
being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only) ¶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 id or Class. 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.
-Wundeclared-selector (Objective-C and Objective-C++ only) ¶Warn if a @selector(…) expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
@selector(…) expression, either explicitly in an
@interface or @protocol declaration, or implicitly in
an @implementation section. This option always performs its
checks as soon as a @selector(…) expression is found,
while -Wselector 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.
-print-objc-runtime-info ¶Generate C header describing the largest structure that is passed by value, if any.