gcc/doc/objc.texi

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@c 1999, 2000, 2001, 2002, 2003, 2004 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 runtime features

This document is meant to describe some of the GNU Objective-C runtime
features.  It is not intended to teach you Objective-C, there are several
resources on the Internet that present the language.  Questions and
comments about this document to Ovidiu Predescu
@email{ovidiu@@cup.hp.com}.

@menu
* Executing code before main::
* Type encoding::
* Garbage Collection::
* Constant string objects::
* compatibility_alias::
@end menu

@node Executing code before main, Type encoding, Objective-C, Objective-C
@section @code{+load}: Executing code before main

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];
@}

/* 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];
@}

/* 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,  , Executing code before main, Executing code before main
@subsection What you can and what you cannot do in @code{+load}

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

@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;

@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, Garbage Collection, Executing code before main, Objective-C
@section Type encoding

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{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{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 unknown type
@tab @code{?}
@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{)}
@end multitable

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

@sp 1

@multitable @columnfractions .25 .75
@item 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@}}
@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
@item 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{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.


@node Garbage Collection, Constant string objects, Type encoding, Objective-C
@section Garbage Collection

Support for a new memory management policy has been added by using a
powerful conservative garbage collector, known as the
Boehm-Demers-Weiser conservative garbage collector.  It is available from
@w{@uref{http://www.hpl.hp.com/personal/Hans_Boehm/gc/}}.

To enable the support for it you have to configure the compiler using an
additional argument, @w{@option{--enable-objc-gc}}.  You need to have
garbage collector installed before building the compiler.  This will
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
@{
  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 compatibility_alias

This is a feature of the Objective-C compiler rather than of the
runtime, anyway since it is documented nowhere and its existence was
forgotten, we are documenting it here.

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