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4.6. Options Controlling Objective-C Dialect

(NOTE: This manual does not describe the Objective-C language itself. See http://gcc.gnu.org/readings.html for references.)

This section describes the command-line options that are only meaningful for Objective-C programs, but you can also use most of the GNU compiler options regardless of what language your program is in. 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 programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling 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). The -fconstant-cfstrings option, if also present, will override the -fconstant-string-class setting and cause @"…" literals to be laid out as constant CoreFoundation strings.

-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. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.

-fno-nil-receivers

Assume that all Objective-C message dispatches (e.g., [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. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fobjc-exceptions

Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

  @try {
    …
       @throw expr;
    …
  }
  @catch (AnObjCClass *exc) {
    …
      @throw expr;
    …
      @throw;
    …
  }
  @catch (AnotherClass *exc) {
    …
  }
  @catch (id allOthers) {
    …
  }
  @finally {
    …
      @throw expr;
    …
  }

The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

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 @catch clause capable of handling objects of that type, analogously to how catch blocks work in C++ and Java. A @catch(id …) clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

The @finally clause, if present, will be executed upon exit from the immediately preceding @try … @catch section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the @try … @catch section, analogously to the behavior of the finally clause in Java.

There are several caveats to using the new exception mechanism:

  • Although currently designed to be binary compatible with NS_HANDLER-style idioms provided by the NSException 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.

  • 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 @throw an exception from Objective-C and catch it in C++, or vice versa (i.e., throw … @catch).

The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:

  @synchronized (ObjCClass *guard) {
    …
  }

Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding guard object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once guard becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making guard available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

-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.

-gen-decls

Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.

-Wno-protocol

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.

-Wselector

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.

-Wundeclared-selector

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.

 
 
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