objc.texi

理解和实践操作系统的一本好书

@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
@c 1999, 2000, 2001, 2002, 2003, 2004, 2005 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];
@}

/* @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,  , 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{_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{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 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.