gcc/doc/objc.texi

@c Copyright (C) 1988-2022 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Objective-C
@comment  node-name,  next,  previous,  up

@chapter GNU Objective-C Features

This document is meant to describe some of the GNU Objective-C
features.  It is not intended to teach you Objective-C.  There are
several resources on the Internet that present the language.

@menu
* GNU Objective-C runtime API::
* Executing code before main::
* Type encoding::
* Garbage Collection::
* Constant string objects::
* compatibility_alias::
* Exceptions::
* Synchronization::
* Fast enumeration::
* Messaging with the GNU Objective-C runtime::
@end menu

@c =========================================================================
@node GNU Objective-C runtime API
@section GNU Objective-C Runtime API

This section is specific for the GNU Objective-C runtime.  If you are
using a different runtime, you can skip it.

The GNU Objective-C runtime provides an API that allows you to
interact with the Objective-C runtime system, querying the live
runtime structures and even manipulating them.  This allows you for
example to inspect and navigate classes, methods and protocols; to
define new classes or new methods, and even to modify existing classes
or protocols.

If you are using a ``Foundation'' library such as GNUstep-Base, this
library will provide you with a rich set of functionality to do most
of the inspection tasks, and you probably will only need direct access
to the GNU Objective-C runtime API to define new classes or methods.

@menu
* Modern GNU Objective-C runtime API::
* Traditional GNU Objective-C runtime API::
@end menu

@c =========================================================================
@node Modern GNU Objective-C runtime API
@subsection Modern GNU Objective-C Runtime API

The GNU Objective-C runtime provides an API which is similar to the
one provided by the ``Objective-C 2.0'' Apple/NeXT Objective-C
runtime.  The API is documented in the public header files of the GNU
Objective-C runtime:

@itemize @bullet

@item
@file{objc/objc.h}: this is the basic Objective-C header file,
defining the basic Objective-C types such as @code{id}, @code{Class}
and @code{BOOL}.  You have to include this header to do almost
anything with Objective-C.

@item
@file{objc/runtime.h}: this header declares most of the public runtime
API functions allowing you to inspect and manipulate the Objective-C
runtime data structures.  These functions are fairly standardized
across Objective-C runtimes and are almost identical to the Apple/NeXT
Objective-C runtime ones.  It does not declare functions in some
specialized areas (constructing and forwarding message invocations,
threading) which are in the other headers below.  You have to include
@file{objc/objc.h} and @file{objc/runtime.h} to use any of the
functions, such as @code{class_getName()}, declared in
@file{objc/runtime.h}.

@item
@file{objc/message.h}: this header declares public functions used to
construct, deconstruct and forward message invocations.  Because
messaging is done in quite a different way on different runtimes,
functions in this header are specific to the GNU Objective-C runtime
implementation.

@item
@file{objc/objc-exception.h}: this header declares some public
functions related to Objective-C exceptions.  For example functions in
this header allow you to throw an Objective-C exception from plain
C/C++ code.

@item
@file{objc/objc-sync.h}: this header declares some public functions
related to the Objective-C @code{@@synchronized()} syntax, allowing
you to emulate an Objective-C @code{@@synchronized()} block in plain
C/C++ code.

@item
@file{objc/thr.h}: this header declares a public runtime API threading
layer that is only provided by the GNU Objective-C runtime.  It
declares functions such as @code{objc_mutex_lock()}, which provide a
platform-independent set of threading functions.

@end itemize

The header files contain detailed documentation for each function in
the GNU Objective-C runtime API.

@c =========================================================================
@node Traditional GNU Objective-C runtime API
@subsection Traditional GNU Objective-C Runtime API

The GNU Objective-C runtime used to provide a different API, which we
call the ``traditional'' GNU Objective-C runtime API.  Functions
belonging to this API are easy to recognize because they use a
different naming convention, such as @code{class_get_super_class()}
(traditional API) instead of @code{class_getSuperclass()} (modern
API).  Software using this API includes the file
@file{objc/objc-api.h} where it is declared.

Starting with GCC 4.7.0, the traditional GNU runtime API is no longer
available.

@c =========================================================================
@node Executing code before main
@section @code{+load}: Executing Code before @code{main}

This section is specific for the GNU Objective-C runtime.  If you are
using a different runtime, you can skip it.

The GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the @code{main}
function.  The code is executed on a per-class and a per-category basis,
through a special class method @code{+load}.

This facility is very useful if you want to initialize global variables
which can be accessed by the program directly, without sending a message
to the class first.  The usual way to initialize global variables, in the
@code{+initialize} method, might not be useful because
@code{+initialize} is only called when the first message is sent to a
class object, which in some cases could be too late.

Suppose for example you have a @code{FileStream} class that declares
@code{Stdin}, @code{Stdout} and @code{Stderr} as global variables, like
below:

@smallexample

FileStream *Stdin = nil;
FileStream *Stdout = nil;
FileStream *Stderr = nil;

@@implementation FileStream

+ (void)initialize
@{
    Stdin = [[FileStream new] initWithFd:0];
    Stdout = [[FileStream new] initWithFd:1];
    Stderr = [[FileStream new] initWithFd:2];
@}

/* @r{Other methods here} */
@@end

@end smallexample

In this example, the initialization of @code{Stdin}, @code{Stdout} and
@code{Stderr} in @code{+initialize} occurs too late.  The programmer can
send a message to one of these objects before the variables are actually
initialized, thus sending messages to the @code{nil} object.  The
@code{+initialize} method which actually initializes the global
variables is not invoked until the first message is sent to the class
object.  The solution would require these variables to be initialized
just before entering @code{main}.

The correct solution of the above problem is to use the @code{+load}
method instead of @code{+initialize}:

@smallexample

@@implementation FileStream

+ (void)load
@{
    Stdin = [[FileStream new] initWithFd:0];
    Stdout = [[FileStream new] initWithFd:1];
    Stderr = [[FileStream new] initWithFd:2];
@}

/* @r{Other methods here} */
@@end

@end smallexample

The @code{+load} is a method that is not overridden by categories.  If a
class and a category of it both implement @code{+load}, both methods are
invoked.  This allows some additional initializations to be performed in
a category.

This mechanism is not intended to be a replacement for @code{+initialize}.
You should be aware of its limitations when you decide to use it
instead of @code{+initialize}.

@menu
* What you can and what you cannot do in +load::
@end menu


@node What you can and what you cannot do in +load
@subsection What You Can and Cannot Do in @code{+load}

@code{+load} is to be used only as a last resort.  Because it is
executed very early, most of the Objective-C runtime machinery will
not be ready when @code{+load} is executed; hence @code{+load} works
best for executing C code that is independent on the Objective-C
runtime.

The @code{+load} implementation in the GNU runtime guarantees you the
following things:

@itemize @bullet

@item
you can write whatever C code you like;

@item
you can allocate and send messages to objects whose class is implemented
in the same file;

@item
the @code{+load} implementation of all super classes of a class are
executed before the @code{+load} of that class is executed;

@item
the @code{+load} implementation of a class is executed before the
@code{+load} implementation of any category.

@end itemize

In particular, the following things, even if they can work in a
particular case, are not guaranteed:

@itemize @bullet

@item
allocation of or sending messages to arbitrary objects;

@item
allocation of or sending messages to objects whose classes have a
category implemented in the same file;

@item
sending messages to Objective-C constant strings (@code{@@"this is a
constant string"});

@end itemize

You should make no assumptions about receiving @code{+load} in sibling
classes when you write @code{+load} of a class.  The order in which
sibling classes receive @code{+load} is not guaranteed.

The order in which @code{+load} and @code{+initialize} are called could
be problematic if this matters.  If you don't allocate objects inside
@code{+load}, it is guaranteed that @code{+load} is called before
@code{+initialize}.  If you create an object inside @code{+load} the
@code{+initialize} method of object's class is invoked even if
@code{+load} was not invoked.  Note if you explicitly call @code{+load}
on a class, @code{+initialize} will be called first.  To avoid possible
problems try to implement only one of these methods.

The @code{+load} method is also invoked when a bundle is dynamically
loaded into your running program.  This happens automatically without any
intervening operation from you.  When you write bundles and you need to
write @code{+load} you can safely create and send messages to objects whose
classes already exist in the running program.  The same restrictions as
above apply to classes defined in bundle.



@node Type encoding
@section Type Encoding

This is an advanced section.  Type encodings are used extensively by
the compiler and by the runtime, but you generally do not need to know
about them to use Objective-C.

The Objective-C compiler generates type encodings for all the types.
These type encodings are used at runtime to find out information about
selectors and methods and about objects and classes.

The types are encoded in the following way:

@c @sp 1

@multitable @columnfractions .25 .75
@item @code{_Bool}
@tab @code{B}
@item @code{char}
@tab @code{c}
@item @code{unsigned char}
@tab @code{C}
@item @code{short}
@tab @code{s}
@item @code{unsigned short}
@tab @code{S}
@item @code{int}
@tab @code{i}
@item @code{unsigned int}
@tab @code{I}
@item @code{long}
@tab @code{l}
@item @code{unsigned long}
@tab @code{L}
@item @code{long long}
@tab @code{q}
@item @code{unsigned long long}
@tab @code{Q}
@item @code{float}
@tab @code{f}
@item @code{double}
@tab @code{d}
@item @code{long double}
@tab @code{D}
@item @code{void}
@tab @code{v}
@item @code{id}
@tab @code{@@}
@item @code{Class}
@tab @code{#}
@item @code{SEL}
@tab @code{:}
@item @code{char*}
@tab @code{*}
@item @code{enum}
@tab an @code{enum} is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration
values.  Often the compiler users @code{unsigned int}, which is then encoded as @code{I}.
@item unknown type
@tab @code{?}
@item Complex types
@tab @code{j} followed by the inner type.  For example @code{_Complex double} is encoded as "jd".
@item bit-fields
@tab @code{b} followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
@end multitable

@c @sp 1

The encoding of bit-fields has changed to allow bit-fields to be
properly handled by the runtime functions that compute sizes and
alignments of types that contain bit-fields.  The previous encoding
contained only the size of the bit-field.  Using only this information
it is not possible to reliably compute the size occupied by the
bit-field.  This is very important in the presence of the Boehm's
garbage collector because the objects are allocated using the typed
memory facility available in this collector.  The typed memory
allocation requires information about where the pointers are located
inside the object.

The position in the bit-field is the position, counting in bits, of the
bit closest to the beginning of the structure.

The non-atomic types are encoded as follows:

@c @sp 1

@multitable @columnfractions .2 .8
@item pointers
@tab @samp{^} followed by the pointed type.
@item arrays
@tab @samp{[} followed by the number of elements in the array followed by the type of the elements followed by @samp{]}
@item structures
@tab @samp{@{} followed by the name of the structure (or @samp{?} if the structure is unnamed), the @samp{=} sign, the type of the members and by @samp{@}}
@item unions
@tab @samp{(} followed by the name of the structure (or @samp{?} if the union is unnamed), the @samp{=} sign, the type of the members followed by @samp{)}
@item vectors
@tab @samp{![} followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by @samp{]}
@end multitable

Here are some types and their encodings, as they are generated by the
compiler on an i386 machine:

@sp 1

@multitable @columnfractions .60 .40
@headitem Objective-C type
@tab Compiler encoding
@item
@smallexample
int a[10];
@end smallexample
@tab @code{[10i]}
@item
@smallexample
struct @{
  int i;
  float f[3];
  int a:3;
  int b:2;
  char c;
@}
@end smallexample
@tab @code{@{?=i[3f]b128i3b131i2c@}}
@item
@smallexample
int a __attribute__ ((vector_size (16)));
@end smallexample
@tab @code{![16,16i]} (alignment depends on the machine)
@end multitable

@sp 1

In addition to the types the compiler also encodes the type
specifiers.  The table below describes the encoding of the current
Objective-C type specifiers:

@sp 1

@multitable @columnfractions .25 .75
@headitem Specifier
@tab Encoding
@item @code{const}
@tab @code{r}
@item @code{in}
@tab @code{n}
@item @code{inout}
@tab @code{N}
@item @code{out}
@tab @code{o}
@item @code{bycopy}
@tab @code{O}
@item @code{byref}
@tab @code{R}
@item @code{oneway}
@tab @code{V}
@end multitable

@sp 1

The type specifiers are encoded just before the type.  Unlike types
however, the type specifiers are only encoded when they appear in method
argument types.

Note how @code{const} interacts with pointers:

@sp 1

@multitable @columnfractions .25 .75
@headitem Objective-C type
@tab Compiler encoding
@item
@smallexample
const int
@end smallexample
@tab @code{ri}
@item
@smallexample
const int*
@end smallexample
@tab @code{^ri}
@item
@smallexample
int *const
@end smallexample
@tab @code{r^i}
@end multitable

@sp 1

@code{const int*} is a pointer to a @code{const int}, and so is
encoded as @code{^ri}.  @code{int* const}, instead, is a @code{const}
pointer to an @code{int}, and so is encoded as @code{r^i}.

Finally, there is a complication when encoding @code{const char *}
versus @code{char * const}.  Because @code{char *} is encoded as
@code{*} and not as @code{^c}, there is no way to express the fact
that @code{r} applies to the pointer or to the pointee.

Hence, it is assumed as a convention that @code{r*} means @code{const
char *} (since it is what is most often meant), and there is no way to
encode @code{char *const}.  @code{char *const} would simply be encoded
as @code{*}, and the @code{const} is lost.

@menu
* Legacy type encoding::
* @@encode::
* Method signatures::
@end menu

@node Legacy type encoding
@subsection Legacy Type Encoding

Unfortunately, historically GCC used to have a number of bugs in its
encoding code.  The NeXT runtime expects GCC to emit type encodings in
this historical format (compatible with GCC-3.3), so when using the
NeXT runtime, GCC will introduce on purpose a number of incorrect
encodings:

@itemize @bullet

@item
the read-only qualifier of the pointee gets emitted before the '^'.
The read-only qualifier of the pointer itself gets ignored, unless it
is a typedef.  Also, the 'r' is only emitted for the outermost type.

@item
32-bit longs are encoded as 'l' or 'L', but not always.  For typedefs,
the compiler uses 'i' or 'I' instead if encoding a struct field or a
pointer.

@item
@code{enum}s are always encoded as 'i' (int) even if they are actually
unsigned or long.

@end itemize

In addition to that, the NeXT runtime uses a different encoding for
bitfields.  It encodes them as @code{b} followed by the size, without
a bit offset or the underlying field type.

@node @@encode
@subsection @code{@@encode}

GNU Objective-C supports the @code{@@encode} syntax that allows you to
create a type encoding from a C/Objective-C type.  For example,
@code{@@encode(int)} is compiled by the compiler into @code{"i"}.

@code{@@encode} does not support type qualifiers other than
@code{const}.  For example, @code{@@encode(const char*)} is valid and
is compiled into @code{"r*"}, while @code{@@encode(bycopy char *)} is
invalid and will cause a compilation error.

@node Method signatures
@subsection Method Signatures

This section documents the encoding of method types, which is rarely
needed to use Objective-C.  You should skip it at a first reading; the
runtime provides functions that will work on methods and can walk
through the list of parameters and interpret them for you.  These
functions are part of the public ``API'' and are the preferred way to
interact with method signatures from user code.

But if you need to debug a problem with method signatures and need to
know how they are implemented (i.e., the ``ABI''), read on.

Methods have their ``signature'' encoded and made available to the
runtime.  The ``signature'' encodes all the information required to
dynamically build invocations of the method at runtime: return type
and arguments.

The ``signature'' is a null-terminated string, composed of the following:

@itemize @bullet

@item
The return type, including type qualifiers.  For example, a method
returning @code{int} would have @code{i} here.

@item
The total size (in bytes) required to pass all the parameters.  This
includes the two hidden parameters (the object @code{self} and the
method selector @code{_cmd}).

@item
Each argument, with the type encoding, followed by the offset (in
bytes) of the argument in the list of parameters.

@end itemize

For example, a method with no arguments and returning @code{int} would
have the signature @code{i8@@0:4} if the size of a pointer is 4.  The
signature is interpreted as follows: the @code{i} is the return type
(an @code{int}), the @code{8} is the total size of the parameters in
bytes (two pointers each of size 4), the @code{@@0} is the first
parameter (an object at byte offset @code{0}) and @code{:4} is the
second parameter (a @code{SEL} at byte offset @code{4}).

You can easily find more examples by running the ``strings'' program
on an Objective-C object file compiled by GCC.  You'll see a lot of
strings that look very much like @code{i8@@0:4}.  They are signatures
of Objective-C methods.


@node Garbage Collection
@section Garbage Collection

This section is specific for the GNU Objective-C runtime.  If you are
using a different runtime, you can skip it.

Support for garbage collection with the GNU runtime has been added by
using a powerful conservative garbage collector, known as the
Boehm-Demers-Weiser conservative garbage collector.

To enable the support for it you have to configure the compiler using
an additional argument, @w{@option{--enable-objc-gc}}.  This will
build the boehm-gc library, and build an additional runtime library
which has several enhancements to support the garbage collector.  The
new library has a new name, @file{libobjc_gc.a} to not conflict with
the non-garbage-collected library.

When the garbage collector is used, the objects are allocated using the
so-called typed memory allocation mechanism available in the
Boehm-Demers-Weiser collector.  This mode requires precise information on
where pointers are located inside objects.  This information is computed
once per class, immediately after the class has been initialized.

There is a new runtime function @code{class_ivar_set_gcinvisible()}
which can be used to declare a so-called @dfn{weak pointer}
reference.  Such a pointer is basically hidden for the garbage collector;
this can be useful in certain situations, especially when you want to
keep track of the allocated objects, yet allow them to be
collected.  This kind of pointers can only be members of objects, you
cannot declare a global pointer as a weak reference.  Every type which is
a pointer type can be declared a weak pointer, including @code{id},
@code{Class} and @code{SEL}.

Here is an example of how to use this feature.  Suppose you want to
implement a class whose instances hold a weak pointer reference; the
following class does this:

@smallexample

@@interface WeakPointer : Object
@{
    const void* weakPointer;
@}

- initWithPointer:(const void*)p;
- (const void*)weakPointer;
@@end


@@implementation WeakPointer

+ (void)initialize
@{
  if (self == objc_lookUpClass ("WeakPointer"))
    class_ivar_set_gcinvisible (self, "weakPointer", YES);
@}

- initWithPointer:(const void*)p
@{
  weakPointer = p;
  return self;
@}

- (const void*)weakPointer
@{
  return weakPointer;
@}

@@end

@end smallexample

Weak pointers are supported through a new type character specifier
represented by the @samp{!} character.  The
@code{class_ivar_set_gcinvisible()} function adds or removes this
specifier to the string type description of the instance variable named
as argument.

@c =========================================================================
@node Constant string objects
@section Constant String Objects

GNU Objective-C provides constant string objects that are generated
directly by the compiler.  You declare a constant string object by
prefixing a C constant string with the character @samp{@@}:

@smallexample
  id myString = @@"this is a constant string object";
@end smallexample

The constant string objects are by default instances of the
@code{NXConstantString} class which is provided by the GNU Objective-C
runtime.  To get the definition of this class you must include the
@file{objc/NXConstStr.h} header file.

User defined libraries may want to implement their own constant string
class.  To be able to support them, the GNU Objective-C compiler provides
a new command line options @option{-fconstant-string-class=@var{class-name}}.
The provided class should adhere to a strict structure, the same
as @code{NXConstantString}'s structure:

@smallexample

@@interface MyConstantStringClass
@{
  Class isa;
  char *c_string;
  unsigned int len;
@}
@@end

@end smallexample

@code{NXConstantString} inherits from @code{Object}; user class
libraries may choose to inherit the customized constant string class
from a different class than @code{Object}.  There is no requirement in
the methods the constant string class has to implement, but the final
ivar layout of the class must be the compatible with the given
structure.

When the compiler creates the statically allocated constant string
object, the @code{c_string} field will be filled by the compiler with
the string; the @code{length} field will be filled by the compiler with
the string length; the @code{isa} pointer will be filled with
@code{NULL} by the compiler, and it will later be fixed up automatically
at runtime by the GNU Objective-C runtime library to point to the class
which was set by the @option{-fconstant-string-class} option when the
object file is loaded (if you wonder how it works behind the scenes, the
name of the class to use, and the list of static objects to fixup, are
stored by the compiler in the object file in a place where the GNU
runtime library will find them at runtime).

As a result, when a file is compiled with the
@option{-fconstant-string-class} option, all the constant string objects
will be instances of the class specified as argument to this option.  It
is possible to have multiple compilation units referring to different
constant string classes, neither the compiler nor the linker impose any
restrictions in doing this.

@c =========================================================================
@node compatibility_alias
@section @code{compatibility_alias}

The keyword @code{@@compatibility_alias} allows you to define a class name
as equivalent to another class name.  For example:

@smallexample
@@compatibility_alias WOApplication GSWApplication;
@end smallexample

tells the compiler that each time it encounters @code{WOApplication} as
a class name, it should replace it with @code{GSWApplication} (that is,
@code{WOApplication} is just an alias for @code{GSWApplication}).

There are some constraints on how this can be used---

@itemize @bullet

@item @code{WOApplication} (the alias) must not be an existing class;

@item @code{GSWApplication} (the real class) must be an existing class.

@end itemize

@c =========================================================================
@node Exceptions
@section Exceptions

GNU Objective-C provides exception support built into the language, as
in the following example:

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

The @code{@@throw} statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @code{@@catch} block, the
@code{@@throw} may appear without an argument (as shown above), in
which case the object caught by the @code{@@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 @code{@@catch} clause capable of handling objects of
that type, analogously to how @code{catch} blocks work in C++ and
Java.  A @code{@@catch(id @dots{})} clause (as shown above) may also
be provided to catch any and all Objective-C exceptions not caught by
previous @code{@@catch} clauses (if any).

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

There are several caveats to using the new exception mechanism:

@itemize @bullet
@item
The @option{-fobjc-exceptions} command line option must be used when
compiling Objective-C files that use exceptions.

@item
With the GNU runtime, exceptions are always implemented as ``native''
exceptions and it is recommended that the @option{-fexceptions} and
@option{-shared-libgcc} options are used when linking.

@item
With the NeXT runtime, although currently designed to be binary
compatible with @code{NS_HANDLER}-style idioms provided by the
@code{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.

@item
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 @code{@@throw} an exception
from Objective-C and @code{catch} it in C++, or vice versa
(i.e., @code{throw @dots{} @@catch}).
@end itemize

@c =========================================================================
@node Synchronization
@section Synchronization

GNU Objective-C provides support for synchronized blocks:

@smallexample
  @@synchronized (ObjCClass *guard) @{
    @dots{}
  @}
@end smallexample

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

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

Because of the interactions between synchronization and exception
handling, you can only use @code{@@synchronized} when compiling with
exceptions enabled, that is with the command line option
@option{-fobjc-exceptions}.


@c =========================================================================
@node Fast enumeration
@section Fast Enumeration

@menu
* Using fast enumeration::
* c99-like fast enumeration syntax::
* Fast enumeration details::
* Fast enumeration protocol::
@end menu

@c ================================
@node Using fast enumeration
@subsection Using Fast Enumeration

GNU Objective-C provides support for the fast enumeration syntax:

@smallexample
  id array = @dots{};
  id object;

  for (object in array)
  @{
    /* Do something with 'object' */
  @}
@end smallexample

@code{array} needs to be an Objective-C object (usually a collection
object, for example an array, a dictionary or a set) which implements
the ``Fast Enumeration Protocol'' (see below).  If you are using a
Foundation library such as GNUstep Base or Apple Cocoa Foundation, all
collection objects in the library implement this protocol and can be
used in this way.

The code above would iterate over all objects in @code{array}.  For
each of them, it assigns it to @code{object}, then executes the
@code{Do something with 'object'} statements.

Here is a fully worked-out example using a Foundation library (which
provides the implementation of @code{NSArray}, @code{NSString} and
@code{NSLog}):

@smallexample
  NSArray *array = [NSArray arrayWithObjects: @@"1", @@"2", @@"3", nil];
  NSString *object;

  for (object in array)
    NSLog (@@"Iterating over %@@", object);
@end smallexample


@c ================================
@node c99-like fast enumeration syntax
@subsection C99-Like Fast Enumeration Syntax

A c99-like declaration syntax is also allowed:

@smallexample
  id array = @dots{};

  for (id object in array)
  @{
    /* Do something with 'object'  */
  @}
@end smallexample

this is completely equivalent to:

@smallexample
  id array = @dots{};

  @{
    id object;
    for (object in array)
    @{
      /* Do something with 'object'  */
    @}
  @}
@end smallexample

but can save some typing.

Note that the option @option{-std=c99} is not required to allow this
syntax in Objective-C.

@c ================================
@node Fast enumeration details
@subsection Fast Enumeration Details

Here is a more technical description with the gory details.  Consider the code

@smallexample
  for (@var{object expression} in @var{collection expression})
  @{
    @var{statements}
  @}
@end smallexample

here is what happens when you run it:

@itemize @bullet
@item
@code{@var{collection expression}} is evaluated exactly once and the
result is used as the collection object to iterate over.  This means
it is safe to write code such as @code{for (object in [NSDictionary
keyEnumerator]) @dots{}}.

@item
the iteration is implemented by the compiler by repeatedly getting
batches of objects from the collection object using the fast
enumeration protocol (see below), then iterating over all objects in
the batch.  This is faster than a normal enumeration where objects are
retrieved one by one (hence the name ``fast enumeration'').

@item
if there are no objects in the collection, then
@code{@var{object expression}} is set to @code{nil} and the loop
immediately terminates.

@item
if there are objects in the collection, then for each object in the
collection (in the order they are returned) @code{@var{object expression}}
is set to the object, then @code{@var{statements}} are executed.

@item
@code{@var{statements}} can contain @code{break} and @code{continue}
commands, which will abort the iteration or skip to the next loop
iteration as expected.

@item
when the iteration ends because there are no more objects to iterate
over, @code{@var{object expression}} is set to @code{nil}.  This allows
you to determine whether the iteration finished because a @code{break}
command was used (in which case @code{@var{object expression}} will remain
set to the last object that was iterated over) or because it iterated
over all the objects (in which case @code{@var{object expression}} will be
set to @code{nil}).

@item
@code{@var{statements}} must not make any changes to the collection
object; if they do, it is a hard error and the fast enumeration
terminates by invoking @code{objc_enumerationMutation}, a runtime
function that normally aborts the program but which can be customized
by Foundation libraries via @code{objc_set_mutation_handler} to do
something different, such as raising an exception.

@end itemize

@c ================================
@node Fast enumeration protocol
@subsection Fast Enumeration Protocol

If you want your own collection object to be usable with fast
enumeration, you need to have it implement the method

@smallexample
- (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state
                                      objects: (id *)objects
                                        count: (unsigned long)len;
@end smallexample

where @code{NSFastEnumerationState} must be defined in your code as follows:

@smallexample
typedef struct
@{
  unsigned long state;
  id            *itemsPtr;
  unsigned long *mutationsPtr;
  unsigned long extra[5];
@} NSFastEnumerationState;
@end smallexample

If no @code{NSFastEnumerationState} is defined in your code, the
compiler will automatically replace @code{NSFastEnumerationState *}
with @code{struct __objcFastEnumerationState *}, where that type is
silently defined by the compiler in an identical way.  This can be
confusing and we recommend that you define
@code{NSFastEnumerationState} (as shown above) instead.

The method is called repeatedly during a fast enumeration to retrieve
batches of objects.  Each invocation of the method should retrieve the
next batch of objects.

The return value of the method is the number of objects in the current
batch; this should not exceed @code{len}, which is the maximum size of
a batch as requested by the caller.  The batch itself is returned in
the @code{itemsPtr} field of the @code{NSFastEnumerationState} struct.

To help with returning the objects, the @code{objects} array is a C
array preallocated by the caller (on the stack) of size @code{len}.
In many cases you can put the objects you want to return in that
@code{objects} array, then do @code{itemsPtr = objects}.  But you
don't have to; if your collection already has the objects to return in
some form of C array, it could return them from there instead.

The @code{state} and @code{extra} fields of the
@code{NSFastEnumerationState} structure allows your collection object
to keep track of the state of the enumeration.  In a simple array
implementation, @code{state} may keep track of the index of the last
object that was returned, and @code{extra} may be unused.

The @code{mutationsPtr} field of the @code{NSFastEnumerationState} is
used to keep track of mutations.  It should point to a number; before
working on each object, the fast enumeration loop will check that this
number has not changed.  If it has, a mutation has happened and the
fast enumeration will abort.  So, @code{mutationsPtr} could be set to
point to some sort of version number of your collection, which is
increased by one every time there is a change (for example when an
object is added or removed).  Or, if you are content with less strict
mutation checks, it could point to the number of objects in your
collection or some other value that can be checked to perform an
approximate check that the collection has not been mutated.

Finally, note how we declared the @code{len} argument and the return
value to be of type @code{unsigned long}.  They could also be declared
to be of type @code{unsigned int} and everything would still work.

@c =========================================================================
@node Messaging with the GNU Objective-C runtime
@section Messaging with the GNU Objective-C Runtime

This section is specific for the GNU Objective-C runtime.  If you are
using a different runtime, you can skip it.

The implementation of messaging in the GNU Objective-C runtime is
designed to be portable, and so is based on standard C.

Sending a message in the GNU Objective-C runtime is composed of two
separate steps.  First, there is a call to the lookup function,
@code{objc_msg_lookup ()} (or, in the case of messages to super,
@code{objc_msg_lookup_super ()}).  This runtime function takes as
argument the receiver and the selector of the method to be called; it
returns the @code{IMP}, that is a pointer to the function implementing
the method.  The second step of method invocation consists of casting
this pointer function to the appropriate function pointer type, and
calling the function pointed to it with the right arguments.

For example, when the compiler encounters a method invocation such as
@code{[object init]}, it compiles it into a call to
@code{objc_msg_lookup (object, @@selector(init))} followed by a cast
of the returned value to the appropriate function pointer type, and
then it calls it.

@menu
* Dynamically registering methods::
* Forwarding hook::
@end menu

@c =========================================================================
@node Dynamically registering methods
@subsection Dynamically Registering Methods

If @code{objc_msg_lookup()} does not find a suitable method
implementation, because the receiver does not implement the required
method, it tries to see if the class can dynamically register the
method.

To do so, the runtime checks if the class of the receiver implements
the method

@smallexample
+ (BOOL) resolveInstanceMethod: (SEL)selector;
@end smallexample

in the case of an instance method, or

@smallexample
+ (BOOL) resolveClassMethod: (SEL)selector;
@end smallexample

in the case of a class method.  If the class implements it, the
runtime invokes it, passing as argument the selector of the original
method, and if it returns @code{YES}, the runtime tries the lookup
again, which could now succeed if a matching method was added
dynamically by @code{+resolveInstanceMethod:} or
@code{+resolveClassMethod:}.

This allows classes to dynamically register methods (by adding them to
the class using @code{class_addMethod}) when they are first called.
To do so, a class should implement @code{+resolveInstanceMethod:} (or,
depending on the case, @code{+resolveClassMethod:}) and have it
recognize the selectors of methods that can be registered dynamically
at runtime, register them, and return @code{YES}.  It should return
@code{NO} for methods that it does not dynamically registered at
runtime.

If @code{+resolveInstanceMethod:} (or @code{+resolveClassMethod:}) is
not implemented or returns @code{NO}, the runtime then tries the
forwarding hook.

Support for @code{+resolveInstanceMethod:} and
@code{resolveClassMethod:} was added to the GNU Objective-C runtime in
GCC version 4.6.

@c =========================================================================
@node Forwarding hook
@subsection Forwarding Hook

The GNU Objective-C runtime provides a hook, called
@code{__objc_msg_forward2}, which is called by
@code{objc_msg_lookup()} when it cannot find a method implementation in
the runtime tables and after calling @code{+resolveInstanceMethod:}
and @code{+resolveClassMethod:} has been attempted and did not succeed
in dynamically registering the method.

To configure the hook, you set the global variable
@code{__objc_msg_forward2} to a function with the same argument and
return types of @code{objc_msg_lookup()}.  When
@code{objc_msg_lookup()} cannot find a method implementation, it
invokes the hook function you provided to get a method implementation
to return.  So, in practice @code{__objc_msg_forward2} allows you to
extend @code{objc_msg_lookup()} by adding some custom code that is
called to do a further lookup when no standard method implementation
can be found using the normal lookup.

This hook is generally reserved for ``Foundation'' libraries such as
GNUstep Base, which use it to implement their high-level method
forwarding API, typically based around the @code{forwardInvocation:}
method.  So, unless you are implementing your own ``Foundation''
library, you should not set this hook.

In a typical forwarding implementation, the @code{__objc_msg_forward2}
hook function determines the argument and return type of the method
that is being looked up, and then creates a function that takes these
arguments and has that return type, and returns it to the caller.
Creating this function is non-trivial and is typically performed using
a dedicated library such as @code{libffi}.

The forwarding method implementation thus created is returned by
@code{objc_msg_lookup()} and is executed as if it was a normal method
implementation.  When the forwarding method implementation is called,
it is usually expected to pack all arguments into some sort of object
(typically, an @code{NSInvocation} in a ``Foundation'' library), and
hand it over to the programmer (@code{forwardInvocation:}) who is then
allowed to manipulate the method invocation using a high-level API
provided by the ``Foundation'' library.  For example, the programmer
may want to examine the method invocation arguments and name and
potentially change them before forwarding the method invocation to one
or more local objects (@code{performInvocation:}) or even to remote
objects (by using Distributed Objects or some other mechanism).  When
all this completes, the return value is passed back and must be
returned correctly to the original caller.

Note that the GNU Objective-C runtime currently provides no support
for method forwarding or method invocations other than the
@code{__objc_msg_forward2} hook.

If the forwarding hook does not exist or returns @code{NULL}, the
runtime currently attempts forwarding using an older, deprecated API,
and if that fails, it aborts the program.  In future versions of the
GNU Objective-C runtime, the runtime will immediately abort.