objc.texi

理解和实践操作系统的一本好书

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