clang/docs/AutomaticReferenceCounting.rst

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.. contents::
   :local:

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  <style>
    .arc-term { font-style: italic; font-weight: bold; }
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.. role:: arc-term
.. role:: revision
.. role:: when-revised

.. title:: Objective-C Automatic Reference Counting (ARC)

.. _arc.meta:

===================
About this document
===================

.. _arc.meta.purpose:

Purpose
=======

The first and primary purpose of this document is to serve as a complete
technical specification of Automatic Reference Counting.  Given a core
Objective-C compiler and runtime, it should be possible to write a compiler and
runtime which implements these new semantics.

The secondary purpose is to act as a rationale for why ARC was designed in this
way.  This should remain tightly focused on the technical design and should not
stray into marketing speculation.

.. _arc.meta.background:

Background
==========

This document assumes a basic familiarity with C.

:arc-term:`Blocks` are a C language extension for creating anonymous functions.
Users interact with and transfer block objects using :arc-term:`block
pointers`, which are represented like a normal pointer.  A block may capture
values from local variables; when this occurs, memory must be dynamically
allocated.  The initial allocation is done on the stack, but the runtime
provides a ``Block_copy`` function which, given a block pointer, either copies
the underlying block object to the heap, setting its reference count to 1 and
returning the new block pointer, or (if the block object is already on the
heap) increases its reference count by 1.  The paired function is
``Block_release``, which decreases the reference count by 1 and destroys the
object if the count reaches zero and is on the heap.

Objective-C is a set of language extensions, significant enough to be
considered a different language.  It is a strict superset of C.  The extensions
can also be imposed on C++, producing a language called Objective-C++.  The
primary feature is a single-inheritance object system; we briefly describe the
modern dialect.

Objective-C defines a new type kind, collectively called the :arc-term:`object
pointer types`.  This kind has two notable builtin members, ``id`` and
``Class``; ``id`` is the final supertype of all object pointers.  The validity
of conversions between object pointer types is not checked at runtime.  Users
may define :arc-term:`classes`; each class is a type, and the pointer to that
type is an object pointer type.  A class may have a superclass; its pointer
type is a subtype of its superclass's pointer type.  A class has a set of
:arc-term:`ivars`, fields which appear on all instances of that class.  For
every class *T* there's an associated metaclass; it has no fields, its
superclass is the metaclass of *T*'s superclass, and its metaclass is a global
class.  Every class has a global object whose class is the class's metaclass;
metaclasses have no associated type, so pointers to this object have type
``Class``.

A class declaration (``@interface``) declares a set of :arc-term:`methods`.  A
method has a return type, a list of argument types, and a :arc-term:`selector`:
a name like ``foo:bar:baz:``, where the number of colons corresponds to the
number of formal arguments.  A method may be an instance method, in which case
it can be invoked on objects of the class, or a class method, in which case it
can be invoked on objects of the metaclass.  A method may be invoked by
providing an object (called the :arc-term:`receiver`) and a list of formal
arguments interspersed with the selector, like so:

.. code-block:: objc

  [receiver foo: fooArg bar: barArg baz: bazArg]

This looks in the dynamic class of the receiver for a method with this name,
then in that class's superclass, etc., until it finds something it can execute.
The receiver "expression" may also be the name of a class, in which case the
actual receiver is the class object for that class, or (within method
definitions) it may be ``super``, in which case the lookup algorithm starts
with the static superclass instead of the dynamic class.  The actual methods
dynamically found in a class are not those declared in the ``@interface``, but
those defined in a separate ``@implementation`` declaration; however, when
compiling a call, typechecking is done based on the methods declared in the
``@interface``.

Method declarations may also be grouped into :arc-term:`protocols`, which are not
inherently associated with any class, but which classes may claim to follow.
Object pointer types may be qualified with additional protocols that the object
is known to support.

:arc-term:`Class extensions` are collections of ivars and methods, designed to
allow a class's ``@interface`` to be split across multiple files; however,
there is still a primary implementation file which must see the
``@interface``\ s of all class extensions.  :arc-term:`Categories` allow
methods (but not ivars) to be declared *post hoc* on an arbitrary class; the
methods in the category's ``@implementation`` will be dynamically added to that
class's method tables which the category is loaded at runtime, replacing those
methods in case of a collision.

In the standard environment, objects are allocated on the heap, and their
lifetime is manually managed using a reference count.  This is done using two
instance methods which all classes are expected to implement: ``retain``
increases the object's reference count by 1, whereas ``release`` decreases it
by 1 and calls the instance method ``dealloc`` if the count reaches 0.  To
simplify certain operations, there is also an :arc-term:`autorelease pool`, a
thread-local list of objects to call ``release`` on later; an object can be
added to this pool by calling ``autorelease`` on it.

Block pointers may be converted to type ``id``; block objects are laid out in a
way that makes them compatible with Objective-C objects.  There is a builtin
class that all block objects are considered to be objects of; this class
implements ``retain`` by adjusting the reference count, not by calling
``Block_copy``.

.. _arc.meta.evolution:

Evolution
=========

ARC is under continual evolution, and this document must be updated as the
language progresses.

If a change increases the expressiveness of the language, for example by
lifting a restriction or by adding new syntax, the change will be annotated
with a revision marker, like so:

  ARC applies to Objective-C pointer types, block pointer types, and
  :when-revised:`[beginning Apple 8.0, LLVM 3.8]` :revision:`BPTRs declared
  within` ``extern "BCPL"`` blocks.

For now, it is sensible to version this document by the releases of its sole
implementation (and its host project), clang.  "LLVM X.Y" refers to an
open-source release of clang from the LLVM project.  "Apple X.Y" refers to an
Apple-provided release of the Apple LLVM Compiler.  Other organizations that
prepare their own, separately-versioned clang releases and wish to maintain
similar information in this document should send requests to cfe-dev.

If a change decreases the expressiveness of the language, for example by
imposing a new restriction, this should be taken as an oversight in the
original specification and something to be avoided in all versions.  Such
changes are generally to be avoided.

.. _arc.general:

=======
General
=======

Automatic Reference Counting implements automatic memory management for
Objective-C objects and blocks, freeing the programmer from the need to
explicitly insert retains and releases.  It does not provide a cycle collector;
users must explicitly manage the lifetime of their objects, breaking cycles
manually or with weak or unsafe references.

ARC may be explicitly enabled with the compiler flag ``-fobjc-arc``.  It may
also be explicitly disabled with the compiler flag ``-fno-objc-arc``.  The last
of these two flags appearing on the compile line "wins".

If ARC is enabled, ``__has_feature(objc_arc)`` will expand to 1 in the
preprocessor.  For more information about ``__has_feature``, see the
:ref:`language extensions <langext-__has_feature-__has_extension>` document.

.. _arc.objects:

==========================
Retainable object pointers
==========================

This section describes retainable object pointers, their basic operations, and
the restrictions imposed on their use under ARC.  Note in particular that it
covers the rules for pointer *values* (patterns of bits indicating the location
of a pointed-to object), not pointer *objects* (locations in memory which store
pointer values).  The rules for objects are covered in the next section.

A :arc-term:`retainable object pointer` (or "retainable pointer") is a value of
a :arc-term:`retainable object pointer type` ("retainable type").  There are
three kinds of retainable object pointer types:

* block pointers (formed by applying the caret (``^``) declarator sigil to a
  function type)
* Objective-C object pointers (``id``, ``Class``, ``NSFoo*``, etc.)
* typedefs marked with ``__attribute__((NSObject))``

Other pointer types, such as ``int*`` and ``CFStringRef``, are not subject to
ARC's semantics and restrictions.

.. admonition:: Rationale

  We are not at liberty to require all code to be recompiled with ARC;
  therefore, ARC must interoperate with Objective-C code which manages retains
  and releases manually.  In general, there are three requirements in order for
  a compiler-supported reference-count system to provide reliable
  interoperation:

  * The type system must reliably identify which objects are to be managed.  An
    ``int*`` might be a pointer to a ``malloc``'ed array, or it might be an
    interior pointer to such an array, or it might point to some field or local
    variable.  In contrast, values of the retainable object pointer types are
    never interior.

  * The type system must reliably indicate how to manage objects of a type.
    This usually means that the type must imply a procedure for incrementing
    and decrementing retain counts.  Supporting single-ownership objects
    requires a lot more explicit mediation in the language.

  * There must be reliable conventions for whether and when "ownership" is
    passed between caller and callee, for both arguments and return values.
    Objective-C methods follow such a convention very reliably, at least for
    system libraries on Mac OS X, and functions always pass objects at +0.  The
    C-based APIs for Core Foundation objects, on the other hand, have much more
    varied transfer semantics.

The use of ``__attribute__((NSObject))`` typedefs is not recommended.  If it's
absolutely necessary to use this attribute, be very explicit about using the
typedef, and do not assume that it will be preserved by language features like
``__typeof`` and C++ template argument substitution.

.. admonition:: Rationale

  Any compiler operation which incidentally strips type "sugar" from a type
  will yield a type without the attribute, which may result in unexpected
  behavior.

.. _arc.objects.retains:

Retain count semantics
======================

A retainable object pointer is either a :arc-term:`null pointer` or a pointer
to a valid object.  Furthermore, if it has block pointer type and is not
``null`` then it must actually be a pointer to a block object, and if it has
``Class`` type (possibly protocol-qualified) then it must actually be a pointer
to a class object.  Otherwise ARC does not enforce the Objective-C type system
as long as the implementing methods follow the signature of the static type.
It is undefined behavior if ARC is exposed to an invalid pointer.

For ARC's purposes, a valid object is one with "well-behaved" retaining
operations.  Specifically, the object must be laid out such that the
Objective-C message send machinery can successfully send it the following
messages:

* ``retain``, taking no arguments and returning a pointer to the object.
* ``release``, taking no arguments and returning ``void``.
* ``autorelease``, taking no arguments and returning a pointer to the object.

The behavior of these methods is constrained in the following ways.  The term
:arc-term:`high-level semantics` is an intentionally vague term; the intent is
that programmers must implement these methods in a way such that the compiler,
modifying code in ways it deems safe according to these constraints, will not
violate their requirements.  For example, if the user puts logging statements
in ``retain``, they should not be surprised if those statements are executed
more or less often depending on optimization settings.  These constraints are
not exhaustive of the optimization opportunities: values held in local
variables are subject to additional restrictions, described later in this
document.

It is undefined behavior if a computation history featuring a send of
``retain`` followed by a send of ``release`` to the same object, with no
intervening ``release`` on that object, is not equivalent under the high-level
semantics to a computation history in which these sends are removed.  Note that
this implies that these methods may not raise exceptions.

It is undefined behavior if a computation history features any use whatsoever
of an object following the completion of a send of ``release`` that is not
preceded by a send of ``retain`` to the same object.

The behavior of ``autorelease`` must be equivalent to sending ``release`` when
one of the autorelease pools currently in scope is popped.  It may not throw an
exception.

When the semantics call for performing one of these operations on a retainable
object pointer, if that pointer is ``null`` then the effect is a no-op.

All of the semantics described in this document are subject to additional
:ref:`optimization rules <arc.optimization>` which permit the removal or
optimization of operations based on local knowledge of data flow.  The
semantics describe the high-level behaviors that the compiler implements, not
an exact sequence of operations that a program will be compiled into.

.. _arc.objects.operands:

Retainable object pointers as operands and arguments
====================================================

In general, ARC does not perform retain or release operations when simply using
a retainable object pointer as an operand within an expression.  This includes:

* loading a retainable pointer from an object with non-weak :ref:`ownership
  <arc.ownership>`,
* passing a retainable pointer as an argument to a function or method, and
* receiving a retainable pointer as the result of a function or method call.

.. admonition:: Rationale

  While this might seem uncontroversial, it is actually unsafe when multiple
  expressions are evaluated in "parallel", as with binary operators and calls,
  because (for example) one expression might load from an object while another
  writes to it.  However, C and C++ already call this undefined behavior
  because the evaluations are unsequenced, and ARC simply exploits that here to
  avoid needing to retain arguments across a large number of calls.

The remainder of this section describes exceptions to these rules, how those
exceptions are detected, and what those exceptions imply semantically.

.. _arc.objects.operands.consumed:

Consumed parameters
-------------------

A function or method parameter of retainable object pointer type may be marked
as :arc-term:`consumed`, signifying that the callee expects to take ownership
of a +1 retain count.  This is done by adding the ``ns_consumed`` attribute to
the parameter declaration, like so:

.. code-block:: objc

  void foo(__attribute((ns_consumed)) id x);
  - (void) foo: (id) __attribute((ns_consumed)) x;

This attribute is part of the type of the function or method, not the type of
the parameter.  It controls only how the argument is passed and received.

When passing such an argument, ARC retains the argument prior to making the
call.

When receiving such an argument, ARC releases the argument at the end of the
function, subject to the usual optimizations for local values.

.. admonition:: Rationale

  This formalizes direct transfers of ownership from a caller to a callee.  The
  most common scenario here is passing the ``self`` parameter to ``init``, but
  it is useful to generalize.  Typically, local optimization will remove any
  extra retains and releases: on the caller side the retain will be merged with
  a +1 source, and on the callee side the release will be rolled into the
  initialization of the parameter.

The implicit ``self`` parameter of a method may be marked as consumed by adding
``__attribute__((ns_consumes_self))`` to the method declaration.  Methods in
the ``init`` :ref:`family <arc.method-families>` are treated as if they were
implicitly marked with this attribute.

It is undefined behavior if an Objective-C message send to a method with
``ns_consumed`` parameters (other than self) is made with a null receiver.  It
is undefined behavior if the method to which an Objective-C message send
statically resolves to has a different set of ``ns_consumed`` parameters than
the method it dynamically resolves to.  It is undefined behavior if a block or
function call is made through a static type with a different set of
``ns_consumed`` parameters than the implementation of the called block or
function.

.. admonition:: Rationale

  Consumed parameters with null receiver are a guaranteed leak.  Mismatches
  with consumed parameters will cause over-retains or over-releases, depending
  on the direction.  The rule about function calls is really just an
  application of the existing C/C++ rule about calling functions through an
  incompatible function type, but it's useful to state it explicitly.

.. _arc.object.operands.retained-return-values:

Retained return values
----------------------

A function or method which returns a retainable object pointer type may be
marked as returning a retained value, signifying that the caller expects to take
ownership of a +1 retain count.  This is done by adding the
``ns_returns_retained`` attribute to the function or method declaration, like
so:

.. code-block:: objc

  id foo(void) __attribute((ns_returns_retained));
  - (id) foo __attribute((ns_returns_retained));

This attribute is part of the type of the function or method.

When returning from such a function or method, ARC retains the value at the
point of evaluation of the return statement, before leaving all local scopes.

When receiving a return result from such a function or method, ARC releases the
value at the end of the full-expression it is contained within, subject to the
usual optimizations for local values.

.. admonition:: Rationale

  This formalizes direct transfers of ownership from a callee to a caller.  The
  most common scenario this models is the retained return from ``init``,
  ``alloc``, ``new``, and ``copy`` methods, but there are other cases in the
  frameworks.  After optimization there are typically no extra retains and
  releases required.

Methods in the ``alloc``, ``copy``, ``init``, ``mutableCopy``, and ``new``
:ref:`families <arc.method-families>` are implicitly marked
``__attribute__((ns_returns_retained))``.  This may be suppressed by explicitly
marking the method ``__attribute__((ns_returns_not_retained))``.

It is undefined behavior if the method to which an Objective-C message send
statically resolves has different retain semantics on its result from the
method it dynamically resolves to.  It is undefined behavior if a block or
function call is made through a static type with different retain semantics on
its result from the implementation of the called block or function.

.. admonition:: Rationale

  Mismatches with returned results will cause over-retains or over-releases,
  depending on the direction.  Again, the rule about function calls is really
  just an application of the existing C/C++ rule about calling functions
  through an incompatible function type.

.. _arc.objects.operands.unretained-returns:

Unretained return values
------------------------

A method or function which returns a retainable object type but does not return
a retained value must ensure that the object is still valid across the return
boundary.

When returning from such a function or method, ARC retains the value at the
point of evaluation of the return statement, then leaves all local scopes, and
then balances out the retain while ensuring that the value lives across the
call boundary.  In the worst case, this may involve an ``autorelease``, but
callers must not assume that the value is actually in the autorelease pool.

ARC performs no extra mandatory work on the caller side, although it may elect
to do something to shorten the lifetime of the returned value.

.. admonition:: Rationale

  It is common in non-ARC code to not return an autoreleased value; therefore
  the convention does not force either path.  It is convenient to not be
  required to do unnecessary retains and autoreleases; this permits
  optimizations such as eliding retain/autoreleases when it can be shown that
  the original pointer will still be valid at the point of return.

A method or function may be marked with
``__attribute__((ns_returns_autoreleased))`` to indicate that it returns a
pointer which is guaranteed to be valid at least as long as the innermost
autorelease pool.  There are no additional semantics enforced in the definition
of such a method; it merely enables optimizations in callers.

.. _arc.objects.operands.casts:

Bridged casts
-------------

A :arc-term:`bridged cast` is a C-style cast annotated with one of three
keywords:

* ``(__bridge T) op`` casts the operand to the destination type ``T``.  If
  ``T`` is a retainable object pointer type, then ``op`` must have a
  non-retainable pointer type.  If ``T`` is a non-retainable pointer type,
  then ``op`` must have a retainable object pointer type.  Otherwise the cast
  is ill-formed.  There is no transfer of ownership, and ARC inserts no retain
  operations.
* ``(__bridge_retained T) op`` casts the operand, which must have retainable
  object pointer type, to the destination type, which must be a non-retainable
  pointer type.  ARC retains the value, subject to the usual optimizations on
  local values, and the recipient is responsible for balancing that +1.
* ``(__bridge_transfer T) op`` casts the operand, which must have
  non-retainable pointer type, to the destination type, which must be a
  retainable object pointer type.  ARC will release the value at the end of
  the enclosing full-expression, subject to the usual optimizations on local
  values.

These casts are required in order to transfer objects in and out of ARC
control; see the rationale in the section on :ref:`conversion of retainable
object pointers <arc.objects.restrictions.conversion>`.

Using a ``__bridge_retained`` or ``__bridge_transfer`` cast purely to convince
ARC to emit an unbalanced retain or release, respectively, is poor form.

.. _arc.objects.restrictions:

Restrictions
============

.. _arc.objects.restrictions.conversion:

Conversion of retainable object pointers
----------------------------------------

In general, a program which attempts to implicitly or explicitly convert a
value of retainable object pointer type to any non-retainable type, or
vice-versa, is ill-formed.  For example, an Objective-C object pointer shall
not be converted to ``void*``.  As an exception, cast to ``intptr_t`` is
allowed because such casts are not transferring ownership.  The :ref:`bridged
casts <arc.objects.operands.casts>` may be used to perform these conversions
where necessary.

.. admonition:: Rationale

  We cannot ensure the correct management of the lifetime of objects if they
  may be freely passed around as unmanaged types.  The bridged casts are
  provided so that the programmer may explicitly describe whether the cast
  transfers control into or out of ARC.

However, the following exceptions apply.

.. _arc.objects.restrictions.conversion.with.known.semantics:

Conversion to retainable object pointer type of expressions with known semantics
--------------------------------------------------------------------------------

:when-revised:`[beginning Apple 4.0, LLVM 3.1]`
:revision:`These exceptions have been greatly expanded; they previously applied
only to a much-reduced subset which is difficult to categorize but which
included null pointers, message sends (under the given rules), and the various
global constants.`

An unbridged conversion to a retainable object pointer type from a type other
than a retainable object pointer type is ill-formed, as discussed above, unless
the operand of the cast has a syntactic form which is known retained, known
unretained, or known retain-agnostic.

An expression is :arc-term:`known retain-agnostic` if it is:

* an Objective-C string literal,
* a load from a ``const`` system global variable of :ref:`C retainable pointer
  type <arc.misc.c-retainable>`, or
* a null pointer constant.

An expression is :arc-term:`known unretained` if it is an rvalue of :ref:`C
retainable pointer type <arc.misc.c-retainable>` and it is:

* a direct call to a function, and either that function has the
  ``cf_returns_not_retained`` attribute or it is an :ref:`audited
  <arc.misc.c-retainable.audit>` function that does not have the
  ``cf_returns_retained`` attribute and does not follow the create/copy naming
  convention,
* a message send, and the declared method either has the
  ``cf_returns_not_retained`` attribute or it has neither the
  ``cf_returns_retained`` attribute nor a :ref:`selector family
  <arc.method-families>` that implies a retained result.

An expression is :arc-term:`known retained` if it is an rvalue of :ref:`C
retainable pointer type <arc.misc.c-retainable>` and it is:

* a message send, and the declared method either has the
  ``cf_returns_retained`` attribute, or it does not have the
  ``cf_returns_not_retained`` attribute but it does have a :ref:`selector
  family <arc.method-families>` that implies a retained result.

Furthermore:

* a comma expression is classified according to its right-hand side,
* a statement expression is classified according to its result expression, if
  it has one,
* an lvalue-to-rvalue conversion applied to an Objective-C property lvalue is
  classified according to the underlying message send, and
* a conditional operator is classified according to its second and third
  operands, if they agree in classification, or else the other if one is known
  retain-agnostic.

If the cast operand is known retained, the conversion is treated as a
``__bridge_transfer`` cast.  If the cast operand is known unretained or known
retain-agnostic, the conversion is treated as a ``__bridge`` cast.

.. admonition:: Rationale

  Bridging casts are annoying.  Absent the ability to completely automate the
  management of CF objects, however, we are left with relatively poor attempts
  to reduce the need for a glut of explicit bridges.  Hence these rules.

  We've so far consciously refrained from implicitly turning retained CF
  results from function calls into ``__bridge_transfer`` casts.  The worry is
  that some code patterns  ---  for example, creating a CF value, assigning it
  to an ObjC-typed local, and then calling ``CFRelease`` when done  ---  are a
  bit too likely to be accidentally accepted, leading to mysterious behavior.

.. _arc.objects.restrictions.conversion-exception-contextual:

Conversion from retainable object pointer type in certain contexts
------------------------------------------------------------------

:when-revised:`[beginning Apple 4.0, LLVM 3.1]`

If an expression of retainable object pointer type is explicitly cast to a
:ref:`C retainable pointer type <arc.misc.c-retainable>`, the program is
ill-formed as discussed above unless the result is immediately used:

* to initialize a parameter in an Objective-C message send where the parameter
  is not marked with the ``cf_consumed`` attribute, or
* to initialize a parameter in a direct call to an
  :ref:`audited <arc.misc.c-retainable.audit>` function where the parameter is
  not marked with the ``cf_consumed`` attribute.

.. admonition:: Rationale

  Consumed parameters are left out because ARC would naturally balance them
  with a retain, which was judged too treacherous.  This is in part because
  several of the most common consuming functions are in the ``Release`` family,
  and it would be quite unfortunate for explicit releases to be silently
  balanced out in this way.

.. _arc.ownership:

=======================
Ownership qualification
=======================

This section describes the behavior of *objects* of retainable object pointer
type; that is, locations in memory which store retainable object pointers.

A type is a :arc-term:`retainable object owner type` if it is a retainable
object pointer type or an array type whose element type is a retainable object
owner type.

An :arc-term:`ownership qualifier` is a type qualifier which applies only to
retainable object owner types.  An array type is ownership-qualified according
to its element type, and adding an ownership qualifier to an array type so
qualifies its element type.

A program is ill-formed if it attempts to apply an ownership qualifier to a
type which is already ownership-qualified, even if it is the same qualifier.
There is a single exception to this rule: an ownership qualifier may be applied
to a substituted template type parameter, which overrides the ownership
qualifier provided by the template argument.

Except as described under the :ref:`inference rules <arc.ownership.inference>`,
a program is ill-formed if it attempts to form a pointer or reference type to a
retainable object owner type which lacks an ownership qualifier.

.. admonition:: Rationale

  These rules, together with the inference rules, ensure that all objects and
  lvalues of retainable object pointer type have an ownership qualifier.  The
  ability to override an ownership qualifier during template substitution is
  required to counteract the :ref:`inference of __strong for template type
  arguments <arc.ownership.inference.template.arguments>`.

There are four ownership qualifiers:

* ``__autoreleasing``
* ``__strong``
* ``__unsafe_unretained``
* ``__weak``

A type is :arc-term:`nontrivially ownership-qualified` if it is qualified with
``__autoreleasing``, ``__strong``, or ``__weak``.

.. _arc.ownership.spelling:

Spelling
========

The names of the ownership qualifiers are reserved for the implementation.  A
program may not assume that they are or are not implemented with macros, or
what those macros expand to.

An ownership qualifier may be written anywhere that any other type qualifier
may be written.

If an ownership qualifier appears in the *declaration-specifiers*, the
following rules apply:

* if the type specifier is a retainable object owner type, the qualifier
  applies to that type;
* if the outermost non-array part of the declarator is a pointer or block
  pointer, the qualifier applies to that type;
* otherwise the program is ill-formed.

If an ownership qualifier appears on the declarator name, or on the declared
object, it is applied to outermost pointer or block-pointer type.

If an ownership qualifier appears anywhere else in a declarator, it applies to
the type there.

.. _arc.ownership.spelling.property:

Property declarations
---------------------

A property of retainable object pointer type may have ownership.  If the
property's type is ownership-qualified, then the property has that ownership.
If the property has one of the following modifiers, then the property has the
corresponding ownership.  A property is ill-formed if it has conflicting
sources of ownership, or if it has redundant ownership modifiers, or if it has
``__autoreleasing`` ownership.

* ``assign`` implies ``__unsafe_unretained`` ownership.
* ``copy`` implies ``__strong`` ownership, as well as the usual behavior of
  copy semantics on the setter.
* ``retain`` implies ``__strong`` ownership.
* ``strong`` implies ``__strong`` ownership.
* ``unsafe_unretained`` implies ``__unsafe_unretained`` ownership.
* ``weak`` implies ``__weak`` ownership.

With the exception of ``weak``, these modifiers are available in non-ARC
modes.

A property's specified ownership is preserved in its metadata, but otherwise
the meaning is purely conventional unless the property is synthesized.  If a
property is synthesized, then the :arc-term:`associated instance variable` is
the instance variable which is named, possibly implicitly, by the
``@synthesize`` declaration.  If the associated instance variable already
exists, then its ownership qualification must equal the ownership of the
property; otherwise, the instance variable is created with that ownership
qualification.

A property of retainable object pointer type which is synthesized without a
source of ownership has the ownership of its associated instance variable, if it
already exists; otherwise, :when-revised:`[beginning Apple 3.1, LLVM 3.1]`
:revision:`its ownership is implicitly` ``strong``.  Prior to this revision, it
was ill-formed to synthesize such a property.

.. admonition:: Rationale

  Using ``strong`` by default is safe and consistent with the generic ARC rule
  about :ref:`inferring ownership <arc.ownership.inference.variables>`.  It is,
  unfortunately, inconsistent with the non-ARC rule which states that such
  properties are implicitly ``assign``.  However, that rule is clearly
  untenable in ARC, since it leads to default-unsafe code.  The main merit to
  banning the properties is to avoid confusion with non-ARC practice, which did
  not ultimately strike us as sufficient to justify requiring extra syntax and
  (more importantly) forcing novices to understand ownership rules just to
  declare a property when the default is so reasonable.  Changing the rule away
  from non-ARC practice was acceptable because we had conservatively banned the
  synthesis in order to give ourselves exactly this leeway.

Applying ``__attribute__((NSObject))`` to a property not of retainable object
pointer type has the same behavior it does outside of ARC: it requires the
property type to be some sort of pointer and permits the use of modifiers other
than ``assign``.  These modifiers only affect the synthesized getter and
setter; direct accesses to the ivar (even if synthesized) still have primitive
semantics, and the value in the ivar will not be automatically released during
deallocation.

.. _arc.ownership.semantics:

Semantics
=========

There are five :arc-term:`managed operations` which may be performed on an
object of retainable object pointer type.  Each qualifier specifies different
semantics for each of these operations.  It is still undefined behavior to
access an object outside of its lifetime.

A load or store with "primitive semantics" has the same semantics as the
respective operation would have on an ``void*`` lvalue with the same alignment
and non-ownership qualification.

:arc-term:`Reading` occurs when performing a lvalue-to-rvalue conversion on an
object lvalue.

* For ``__weak`` objects, the current pointee is retained and then released at
  the end of the current full-expression.  This must execute atomically with
  respect to assignments and to the final release of the pointee.
* For all other objects, the lvalue is loaded with primitive semantics.

:arc-term:`Assignment` occurs when evaluating an assignment operator.  The
semantics vary based on the qualification:

* For ``__strong`` objects, the new pointee is first retained; second, the
  lvalue is loaded with primitive semantics; third, the new pointee is stored
  into the lvalue with primitive semantics; and finally, the old pointee is
  released.  This is not performed atomically; external synchronization must be
  used to make this safe in the face of concurrent loads and stores.
* For ``__weak`` objects, the lvalue is updated to point to the new pointee,
  unless the new pointee is an object currently undergoing deallocation, in
  which case the lvalue is updated to a null pointer.  This must execute
  atomically with respect to other assignments to the object, to reads from the
  object, and to the final release of the new pointee.
* For ``__unsafe_unretained`` objects, the new pointee is stored into the
  lvalue using primitive semantics.
* For ``__autoreleasing`` objects, the new pointee is retained, autoreleased,
  and stored into the lvalue using primitive semantics.

:arc-term:`Initialization` occurs when an object's lifetime begins, which
depends on its storage duration.  Initialization proceeds in two stages:

#. First, a null pointer is stored into the lvalue using primitive semantics.
   This step is skipped if the object is ``__unsafe_unretained``.
#. Second, if the object has an initializer, that expression is evaluated and
   then assigned into the object using the usual assignment semantics.

:arc-term:`Destruction` occurs when an object's lifetime ends.  In all cases it
is semantically equivalent to assigning a null pointer to the object, with the
proviso that of course the object cannot be legally read after the object's
lifetime ends.

:arc-term:`Moving` occurs in specific situations where an lvalue is "moved
from", meaning that its current pointee will be used but the object may be left
in a different (but still valid) state.  This arises with ``__block`` variables
and rvalue references in C++.  For ``__strong`` lvalues, moving is equivalent
to loading the lvalue with primitive semantics, writing a null pointer to it
with primitive semantics, and then releasing the result of the load at the end
of the current full-expression.  For all other lvalues, moving is equivalent to
reading the object.

.. _arc.ownership.restrictions:

Restrictions
============

.. _arc.ownership.restrictions.weak:

Weak-unavailable types
----------------------

It is explicitly permitted for Objective-C classes to not support ``__weak``
references.  It is undefined behavior to perform an operation with weak
assignment semantics with a pointer to an Objective-C object whose class does
not support ``__weak`` references.

.. admonition:: Rationale

  Historically, it has been possible for a class to provide its own
  reference-count implementation by overriding ``retain``, ``release``, etc.
  However, weak references to an object require coordination with its class's
  reference-count implementation because, among other things, weak loads and
  stores must be atomic with respect to the final release.  Therefore, existing
  custom reference-count implementations will generally not support weak
  references without additional effort.  This is unavoidable without breaking
  binary compatibility.

A class may indicate that it does not support weak references by providing the
``objc_arc_weak_unavailable`` attribute on the class's interface declaration.  A
retainable object pointer type is **weak-unavailable** if
is a pointer to an (optionally protocol-qualified) Objective-C class ``T`` where
``T`` or one of its superclasses has the ``objc_arc_weak_unavailable``
attribute.  A program is ill-formed if it applies the ``__weak`` ownership
qualifier to a weak-unavailable type or if the value operand of a weak
assignment operation has a weak-unavailable type.

.. _arc.ownership.restrictions.autoreleasing:

Storage duration of ``__autoreleasing`` objects
-----------------------------------------------

A program is ill-formed if it declares an ``__autoreleasing`` object of
non-automatic storage duration.  A program is ill-formed if it captures an
``__autoreleasing`` object in a block or, unless by reference, in a C++11
lambda.

.. admonition:: Rationale

  Autorelease pools are tied to the current thread and scope by their nature.
  While it is possible to have temporary objects whose instance variables are
  filled with autoreleased objects, there is no way that ARC can provide any
  sort of safety guarantee there.

It is undefined behavior if a non-null pointer is assigned to an
``__autoreleasing`` object while an autorelease pool is in scope and then that
object is read after the autorelease pool's scope is left.

.. _arc.ownership.restrictions.conversion.indirect:

Conversion of pointers to ownership-qualified types
---------------------------------------------------

A program is ill-formed if an expression of type ``T*`` is converted,
explicitly or implicitly, to the type ``U*``, where ``T`` and ``U`` have
different ownership qualification, unless:

* ``T`` is qualified with ``__strong``, ``__autoreleasing``, or
  ``__unsafe_unretained``, and ``U`` is qualified with both ``const`` and
  ``__unsafe_unretained``; or
* either ``T`` or ``U`` is ``cv void``, where ``cv`` is an optional sequence
  of non-ownership qualifiers; or
* the conversion is requested with a ``reinterpret_cast`` in Objective-C++; or
* the conversion is a well-formed :ref:`pass-by-writeback
  <arc.ownership.restrictions.pass_by_writeback>`.

The analogous rule applies to ``T&`` and ``U&`` in Objective-C++.

.. admonition:: Rationale

  These rules provide a reasonable level of type-safety for indirect pointers,
  as long as the underlying memory is not deallocated.  The conversion to
  ``const __unsafe_unretained`` is permitted because the semantics of reads are
  equivalent across all these ownership semantics, and that's a very useful and
  common pattern.  The interconversion with ``void*`` is useful for allocating
  memory or otherwise escaping the type system, but use it carefully.
  ``reinterpret_cast`` is considered to be an obvious enough sign of taking
  responsibility for any problems.

It is undefined behavior to access an ownership-qualified object through an
lvalue of a differently-qualified type, except that any non-``__weak`` object
may be read through an ``__unsafe_unretained`` lvalue.

It is undefined behavior if a managed operation is performed on a ``__strong``
or ``__weak`` object without a guarantee that it contains a primitive zero
bit-pattern, or if the storage for such an object is freed or reused without the
object being first assigned a null pointer.

.. admonition:: Rationale

  ARC cannot differentiate between an assignment operator which is intended to
  "initialize" dynamic memory and one which is intended to potentially replace
  a value.  Therefore the object's pointer must be valid before letting ARC at
  it.  Similarly, C and Objective-C do not provide any language hooks for
  destroying objects held in dynamic memory, so it is the programmer's
  responsibility to avoid leaks (``__strong`` objects) and consistency errors
  (``__weak`` objects).

These requirements are followed automatically in Objective-C++ when creating
objects of retainable object owner type with ``new`` or ``new[]`` and destroying
them with ``delete``, ``delete[]``, or a pseudo-destructor expression.  Note
that arrays of nontrivially-ownership-qualified type are not ABI compatible with
non-ARC code because the element type is non-POD: such arrays that are
``new[]``'d in ARC translation units cannot be ``delete[]``'d in non-ARC
translation units and vice-versa.

.. _arc.ownership.restrictions.pass_by_writeback:

Passing to an out parameter by writeback
----------------------------------------

If the argument passed to a parameter of type ``T __autoreleasing *`` has type
``U oq *``, where ``oq`` is an ownership qualifier, then the argument is a
candidate for :arc-term:`pass-by-writeback`` if:

* ``oq`` is ``__strong`` or ``__weak``, and
* it would be legal to initialize a ``T __strong *`` with a ``U __strong *``.

For purposes of overload resolution, an implicit conversion sequence requiring
a pass-by-writeback is always worse than an implicit conversion sequence not
requiring a pass-by-writeback.

The pass-by-writeback is ill-formed if the argument expression does not have a
legal form:

* ``&var``, where ``var`` is a scalar variable of automatic storage duration
  with retainable object pointer type
* a conditional expression where the second and third operands are both legal
  forms
* a cast whose operand is a legal form
* a null pointer constant

.. admonition:: Rationale

  The restriction in the form of the argument serves two purposes.  First, it
  makes it impossible to pass the address of an array to the argument, which
  serves to protect against an otherwise serious risk of mis-inferring an
  "array" argument as an out-parameter.  Second, it makes it much less likely
  that the user will see confusing aliasing problems due to the implementation,
  below, where their store to the writeback temporary is not immediately seen
  in the original argument variable.

A pass-by-writeback is evaluated as follows:

#. The argument is evaluated to yield a pointer ``p`` of type ``U oq *``.
#. If ``p`` is a null pointer, then a null pointer is passed as the argument,
   and no further work is required for the pass-by-writeback.
#. Otherwise, a temporary of type ``T __autoreleasing`` is created and
   initialized to a null pointer.
#. If the parameter is not an Objective-C method parameter marked ``out``,
   then ``*p`` is read, and the result is written into the temporary with
   primitive semantics.
#. The address of the temporary is passed as the argument to the actual call.
#. After the call completes, the temporary is loaded with primitive
   semantics, and that value is assigned into ``*p``.

.. admonition:: Rationale

  This is all admittedly convoluted.  In an ideal world, we would see that a
  local variable is being passed to an out-parameter and retroactively modify
  its type to be ``__autoreleasing`` rather than ``__strong``.  This would be
  remarkably difficult and not always well-founded under the C type system.
  However, it was judged unacceptably invasive to require programmers to write
  ``__autoreleasing`` on all the variables they intend to use for
  out-parameters.  This was the least bad solution.

.. _arc.ownership.restrictions.records:

Ownership-qualified fields of structs and unions
------------------------------------------------

A program is ill-formed if it declares a member of a C struct or union to have
a nontrivially ownership-qualified type.

.. admonition:: Rationale

  The resulting type would be non-POD in the C++ sense, but C does not give us
  very good language tools for managing the lifetime of aggregates, so it is
  more convenient to simply forbid them.  It is still possible to manage this
  with a ``void*`` or an ``__unsafe_unretained`` object.

This restriction does not apply in Objective-C++.  However, nontrivally
ownership-qualified types are considered non-POD: in C++11 terms, they are not
trivially default constructible, copy constructible, move constructible, copy
assignable, move assignable, or destructible.  It is a violation of C++'s One
Definition Rule to use a class outside of ARC that, under ARC, would have a
nontrivially ownership-qualified member.

.. admonition:: Rationale

  Unlike in C, we can express all the necessary ARC semantics for
  ownership-qualified subobjects as suboperations of the (default) special
  member functions for the class.  These functions then become non-trivial.
  This has the non-obvious result that the class will have a non-trivial copy
  constructor and non-trivial destructor; if this would not normally be true
  outside of ARC, objects of the type will be passed and returned in an
  ABI-incompatible manner.

.. _arc.ownership.inference:

Ownership inference
===================

.. _arc.ownership.inference.variables:

Objects
-------

If an object is declared with retainable object owner type, but without an
explicit ownership qualifier, its type is implicitly adjusted to have
``__strong`` qualification.

As a special case, if the object's base type is ``Class`` (possibly
protocol-qualified), the type is adjusted to have ``__unsafe_unretained``
qualification instead.

.. _arc.ownership.inference.indirect_parameters:

Indirect parameters
-------------------

If a function or method parameter has type ``T*``, where ``T`` is an
ownership-unqualified retainable object pointer type, then:

* if ``T`` is ``const``-qualified or ``Class``, then it is implicitly
  qualified with ``__unsafe_unretained``;
* otherwise, it is implicitly qualified with ``__autoreleasing``.

.. admonition:: Rationale

  ``__autoreleasing`` exists mostly for this case, the Cocoa convention for
  out-parameters.  Since a pointer to ``const`` is obviously not an
  out-parameter, we instead use a type more useful for passing arrays.  If the
  user instead intends to pass in a *mutable* array, inferring
  ``__autoreleasing`` is the wrong thing to do; this directs some of the
  caution in the following rules about writeback.

Such a type written anywhere else would be ill-formed by the general rule
requiring ownership qualifiers.

This rule does not apply in Objective-C++ if a parameter's type is dependent in
a template pattern and is only *instantiated* to a type which would be a
pointer to an unqualified retainable object pointer type.  Such code is still
ill-formed.

.. admonition:: Rationale

  The convention is very unlikely to be intentional in template code.

.. _arc.ownership.inference.template.arguments:

Template arguments
------------------

If a template argument for a template type parameter is an retainable object
owner type that does not have an explicit ownership qualifier, it is adjusted
to have ``__strong`` qualification.  This adjustment occurs regardless of
whether the template argument was deduced or explicitly specified.

.. admonition:: Rationale

  ``__strong`` is a useful default for containers (e.g., ``std::vector<id>``),
  which would otherwise require explicit qualification.  Moreover, unqualified
  retainable object pointer types are unlikely to be useful within templates,
  since they generally need to have a qualifier applied to the before being
  used.

.. _arc.method-families:

===============
Method families
===============

An Objective-C method may fall into a :arc-term:`method family`, which is a
conventional set of behaviors ascribed to it by the Cocoa conventions.

A method is in a certain method family if:

* it has a ``objc_method_family`` attribute placing it in that family; or if
  not that,
* it does not have an ``objc_method_family`` attribute placing it in a
  different or no family, and
* its selector falls into the corresponding selector family, and
* its signature obeys the added restrictions of the method family.

A selector is in a certain selector family if, ignoring any leading
underscores, the first component of the selector either consists entirely of
the name of the method family or it begins with that name followed by a
character other than a lowercase letter.  For example, ``_perform:with:`` and
``performWith:`` would fall into the ``perform`` family (if we recognized one),
but ``performing:with`` would not.

The families and their added restrictions are:

* ``alloc`` methods must return a retainable object pointer type.
* ``copy`` methods must return a retainable object pointer type.
* ``mutableCopy`` methods must return a retainable object pointer type.
* ``new`` methods must return a retainable object pointer type.
* ``init`` methods must be instance methods and must return an Objective-C
  pointer type.  Additionally, a program is ill-formed if it declares or
  contains a call to an ``init`` method whose return type is neither ``id`` nor
  a pointer to a super-class or sub-class of the declaring class (if the method
  was declared on a class) or the static receiver type of the call (if it was
  declared on a protocol).

  .. admonition:: Rationale

    There are a fair number of existing methods with ``init``-like selectors
    which nonetheless don't follow the ``init`` conventions.  Typically these
    are either accidental naming collisions or helper methods called during
    initialization.  Because of the peculiar retain/release behavior of
    ``init`` methods, it's very important not to treat these methods as
    ``init`` methods if they aren't meant to be.  It was felt that implicitly
    defining these methods out of the family based on the exact relationship
    between the return type and the declaring class would be much too subtle
    and fragile.  Therefore we identify a small number of legitimate-seeming
    return types and call everything else an error.  This serves the secondary
    purpose of encouraging programmers not to accidentally give methods names
    in the ``init`` family.

    Note that a method with an ``init``-family selector which returns a
    non-Objective-C type (e.g. ``void``) is perfectly well-formed; it simply
    isn't in the ``init`` family.

A program is ill-formed if a method's declarations, implementations, and
overrides do not all have the same method family.

.. _arc.family.attribute:

Explicit method family control
==============================

A method may be annotated with the ``objc_method_family`` attribute to
precisely control which method family it belongs to.  If a method in an
``@implementation`` does not have this attribute, but there is a method
declared in the corresponding ``@interface`` that does, then the attribute is
copied to the declaration in the ``@implementation``.  The attribute is
available outside of ARC, and may be tested for with the preprocessor query
``__has_attribute(objc_method_family)``.

The attribute is spelled
``__attribute__((objc_method_family(`` *family* ``)))``.  If *family* is
``none``, the method has no family, even if it would otherwise be considered to
have one based on its selector and type.  Otherwise, *family* must be one of
``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``, in which case the
method is considered to belong to the corresponding family regardless of its
selector.  It is an error if a method that is explicitly added to a family in
this way does not meet the requirements of the family other than the selector
naming convention.

.. admonition:: Rationale

  The rules codified in this document describe the standard conventions of
  Objective-C.  However, as these conventions have not heretofore been enforced
  by an unforgiving mechanical system, they are only imperfectly kept,
  especially as they haven't always even been precisely defined.  While it is
  possible to define low-level ownership semantics with attributes like
  ``ns_returns_retained``, this attribute allows the user to communicate
  semantic intent, which is of use both to ARC (which, e.g., treats calls to
  ``init`` specially) and the static analyzer.

.. _arc.family.semantics:

Semantics of method families
============================

A method's membership in a method family may imply non-standard semantics for
its parameters and return type.

Methods in the ``alloc``, ``copy``, ``mutableCopy``, and ``new`` families ---
that is, methods in all the currently-defined families except ``init`` ---
implicitly :ref:`return a retained object
<arc.object.operands.retained-return-values>` as if they were annotated with
the ``ns_returns_retained`` attribute.  This can be overridden by annotating
the method with either of the ``ns_returns_autoreleased`` or
``ns_returns_not_retained`` attributes.

Properties also follow same naming rules as methods.  This means that those in
the ``alloc``, ``copy``, ``mutableCopy``, and ``new`` families provide access
to :ref:`retained objects <arc.object.operands.retained-return-values>`.  This
can be overridden by annotating the property with ``ns_returns_not_retained``
attribute.

.. _arc.family.semantics.init:

Semantics of ``init``
---------------------

Methods in the ``init`` family implicitly :ref:`consume
<arc.objects.operands.consumed>` their ``self`` parameter and :ref:`return a
retained object <arc.object.operands.retained-return-values>`.  Neither of
these properties can be altered through attributes.

A call to an ``init`` method with a receiver that is either ``self`` (possibly
parenthesized or casted) or ``super`` is called a :arc-term:`delegate init
call`.  It is an error for a delegate init call to be made except from an
``init`` method, and excluding blocks within such methods.

As an exception to the :ref:`usual rule <arc.misc.self>`, the variable ``self``
is mutable in an ``init`` method and has the usual semantics for a ``__strong``
variable.  However, it is undefined behavior and the program is ill-formed, no
diagnostic required, if an ``init`` method attempts to use the previous value
of ``self`` after the completion of a delegate init call.  It is conventional,
but not required, for an ``init`` method to return ``self``.

It is undefined behavior for a program to cause two or more calls to ``init``
methods on the same object, except that each ``init`` method invocation may
perform at most one delegate init call.

.. _arc.family.semantics.result_type:

Related result types
--------------------

Certain methods are candidates to have :arc-term:`related result types`:

* class methods in the ``alloc`` and ``new`` method families
* instance methods in the ``init`` family
* the instance method ``self``
* outside of ARC, the instance methods ``retain`` and ``autorelease``

If the formal result type of such a method is ``id`` or protocol-qualified
``id``, or a type equal to the declaring class or a superclass, then it is said
to have a related result type.  In this case, when invoked in an explicit
message send, it is assumed to return a type related to the type of the
receiver:

* if it is a class method, and the receiver is a class name ``T``, the message
  send expression has type ``T*``; otherwise
* if it is an instance method, and the receiver has type ``T``, the message
  send expression has type ``T``; otherwise
* the message send expression has the normal result type of the method.

This is a new rule of the Objective-C language and applies outside of ARC.

.. admonition:: Rationale

  ARC's automatic code emission is more prone than most code to signature
  errors, i.e. errors where a call was emitted against one method signature,
  but the implementing method has an incompatible signature.  Having more
  precise type information helps drastically lower this risk, as well as
  catching a number of latent bugs.

.. _arc.optimization:

============
Optimization
============

ARC applies aggressive rules for the optimization of local behavior.  These
rules are based around a core assumption of :arc-term:`local balancing`: that
other code will perform retains and releases as necessary (and only as
necessary) for its own safety, and so the optimizer does not need to consider
global properties of the retain and release sequence.  For example, if a retain
and release immediately bracket a call, the optimizer can delete the retain and
release on the assumption that the called function will not do a constant
number of unmotivated releases followed by a constant number of "balancing"
retains, such that the local retain/release pair is the only thing preventing
the called function from ending up with a dangling reference.

The optimizer assumes that when a new value enters local control, e.g. from a
load of a non-local object or as the result of a function call, it is
instaneously valid.  Subsequently, a retain and release of a value are
necessary on a computation path only if there is a use of that value before the
release and after any operation which might cause a release of the value
(including indirectly or non-locally), and only if the value is not
demonstrably already retained.

The complete optimization rules are quite complicated, but it would still be
useful to document them here.

.. _arc.optimization.precise:

Precise lifetime semantics
==========================

In general, ARC maintains an invariant that a retainable object pointer held in
a ``__strong`` object will be retained for the full formal lifetime of the
object.  Objects subject to this invariant have :arc-term:`precise lifetime
semantics`.

By default, local variables of automatic storage duration do not have precise
lifetime semantics.  Such objects are simply strong references which hold
values of retainable object pointer type, and these values are still fully
subject to the optimizations on values under local control.

.. admonition:: Rationale

  Applying these precise-lifetime semantics strictly would be prohibitive.
  Many useful optimizations that might theoretically decrease the lifetime of
  an object would be rendered impossible.  Essentially, it promises too much.

A local variable of retainable object owner type and automatic storage duration
may be annotated with the ``objc_precise_lifetime`` attribute to indicate that
it should be considered to be an object with precise lifetime semantics.

.. admonition:: Rationale

  Nonetheless, it is sometimes useful to be able to force an object to be
  released at a precise time, even if that object does not appear to be used.
  This is likely to be uncommon enough that the syntactic weight of explicitly
  requesting these semantics will not be burdensome, and may even make the code
  clearer.

.. _arc.misc:

=============
Miscellaneous
=============

.. _arc.misc.special_methods:

Special methods
===============

.. _arc.misc.special_methods.retain:

Memory management methods
-------------------------

A program is ill-formed if it contains a method definition, message send, or
``@selector`` expression for any of the following selectors:

* ``autorelease``
* ``release``
* ``retain``
* ``retainCount``

.. admonition:: Rationale

  ``retainCount`` is banned because ARC robs it of consistent semantics.  The
  others were banned after weighing three options for how to deal with message
  sends:

  **Honoring** them would work out very poorly if a programmer naively or
  accidentally tried to incorporate code written for manual retain/release code
  into an ARC program.  At best, such code would do twice as much work as
  necessary; quite frequently, however, ARC and the explicit code would both
  try to balance the same retain, leading to crashes.  The cost is losing the
  ability to perform "unrooted" retains, i.e. retains not logically
  corresponding to a strong reference in the object graph.

  **Ignoring** them would badly violate user expectations about their code.
  While it *would* make it easier to develop code simultaneously for ARC and
  non-ARC, there is very little reason to do so except for certain library
  developers.  ARC and non-ARC translation units share an execution model and
  can seamlessly interoperate.  Within a translation unit, a developer who
  faithfully maintains their code in non-ARC mode is suffering all the
  restrictions of ARC for zero benefit, while a developer who isn't testing the
  non-ARC mode is likely to be unpleasantly surprised if they try to go back to
  it.

  **Banning** them has the disadvantage of making it very awkward to migrate
  existing code to ARC.  The best answer to that, given a number of other
  changes and restrictions in ARC, is to provide a specialized tool to assist
  users in that migration.

  Implementing these methods was banned because they are too integral to the
  semantics of ARC; many tricks which worked tolerably under manual reference
  counting will misbehave if ARC performs an ephemeral extra retain or two.  If
  absolutely required, it is still possible to implement them in non-ARC code,
  for example in a category; the implementations must obey the :ref:`semantics
  <arc.objects.retains>` laid out elsewhere in this document.

.. _arc.misc.special_methods.dealloc:

``dealloc``
-----------

A program is ill-formed if it contains a message send or ``@selector``
expression for the selector ``dealloc``.

.. admonition:: Rationale

  There are no legitimate reasons to call ``dealloc`` directly.

A class may provide a method definition for an instance method named
``dealloc``.  This method will be called after the final ``release`` of the
object but before it is deallocated or any of its instance variables are
destroyed.  The superclass's implementation of ``dealloc`` will be called
automatically when the method returns.

.. admonition:: Rationale

  Even though ARC destroys instance variables automatically, there are still
  legitimate reasons to write a ``dealloc`` method, such as freeing
  non-retainable resources.  Failing to call ``[super dealloc]`` in such a
  method is nearly always a bug.  Sometimes, the object is simply trying to
  prevent itself from being destroyed, but ``dealloc`` is really far too late
  for the object to be raising such objections.  Somewhat more legitimately, an
  object may have been pool-allocated and should not be deallocated with
  ``free``; for now, this can only be supported with a ``dealloc``
  implementation outside of ARC.  Such an implementation must be very careful
  to do all the other work that ``NSObject``'s ``dealloc`` would, which is
  outside the scope of this document to describe.

The instance variables for an ARC-compiled class will be destroyed at some
point after control enters the ``dealloc`` method for the root class of the
class.  The ordering of the destruction of instance variables is unspecified,
both within a single class and between subclasses and superclasses.

.. admonition:: Rationale

  The traditional, non-ARC pattern for destroying instance variables is to
  destroy them immediately before calling ``[super dealloc]``.  Unfortunately,
  message sends from the superclass are quite capable of reaching methods in
  the subclass, and those methods may well read or write to those instance
  variables.  Making such message sends from dealloc is generally discouraged,
  since the subclass may well rely on other invariants that were broken during
  ``dealloc``, but it's not so inescapably dangerous that we felt comfortable
  calling it undefined behavior.  Therefore we chose to delay destroying the
  instance variables to a point at which message sends are clearly disallowed:
  the point at which the root class's deallocation routines take over.

  In most code, the difference is not observable.  It can, however, be observed
  if an instance variable holds a strong reference to an object whose
  deallocation will trigger a side-effect which must be carefully ordered with
  respect to the destruction of the super class.  Such code violates the design
  principle that semantically important behavior should be explicit.  A simple
  fix is to clear the instance variable manually during ``dealloc``; a more
  holistic solution is to move semantically important side-effects out of
  ``dealloc`` and into a separate teardown phase which can rely on working with
  well-formed objects.

.. _arc.misc.autoreleasepool:

``@autoreleasepool``
====================

To simplify the use of autorelease pools, and to bring them under the control
of the compiler, a new kind of statement is available in Objective-C.  It is
written ``@autoreleasepool`` followed by a *compound-statement*, i.e.  by a new
scope delimited by curly braces.  Upon entry to this block, the current state
of the autorelease pool is captured.  When the block is exited normally,
whether by fallthrough or directed control flow (such as ``return`` or
``break``), the autorelease pool is restored to the saved state, releasing all
the objects in it.  When the block is exited with an exception, the pool is not
drained.

``@autoreleasepool`` may be used in non-ARC translation units, with equivalent
semantics.

A program is ill-formed if it refers to the ``NSAutoreleasePool`` class.

.. admonition:: Rationale

  Autorelease pools are clearly important for the compiler to reason about, but
  it is far too much to expect the compiler to accurately reason about control
  dependencies between two calls.  It is also very easy to accidentally forget
  to drain an autorelease pool when using the manual API, and this can
  significantly inflate the process's high-water-mark.  The introduction of a
  new scope is unfortunate but basically required for sane interaction with the
  rest of the language.  Not draining the pool during an unwind is apparently
  required by the Objective-C exceptions implementation.

.. _arc.misc.self:

``self``
========

The ``self`` parameter variable of an Objective-C method is never actually
retained by the implementation.  It is undefined behavior, or at least
dangerous, to cause an object to be deallocated during a message send to that
object.

To make this safe, for Objective-C instance methods ``self`` is implicitly
``const`` unless the method is in the :ref:`init family
<arc.family.semantics.init>`.  Further, ``self`` is **always** implicitly
``const`` within a class method.

.. admonition:: Rationale

  The cost of retaining ``self`` in all methods was found to be prohibitive, as
  it tends to be live across calls, preventing the optimizer from proving that
  the retain and release are unnecessary --- for good reason, as it's quite
  possible in theory to cause an object to be deallocated during its execution
  without this retain and release.  Since it's extremely uncommon to actually
  do so, even unintentionally, and since there's no natural way for the
  programmer to remove this retain/release pair otherwise (as there is for
  other parameters by, say, making the variable ``__unsafe_unretained``), we
  chose to make this optimizing assumption and shift some amount of risk to the
  user.

.. _arc.misc.enumeration:

Fast enumeration iteration variables
====================================

If a variable is declared in the condition of an Objective-C fast enumeration
loop, and the variable has no explicit ownership qualifier, then it is
qualified with ``const __strong`` and objects encountered during the
enumeration are not actually retained.

.. admonition:: Rationale

  This is an optimization made possible because fast enumeration loops promise
  to keep the objects retained during enumeration, and the collection itself
  cannot be synchronously modified.  It can be overridden by explicitly
  qualifying the variable with ``__strong``, which will make the variable
  mutable again and cause the loop to retain the objects it encounters.

.. _arc.misc.blocks:

Blocks
======

The implicit ``const`` capture variables created when evaluating a block
literal expression have the same ownership semantics as the local variables
they capture.  The capture is performed by reading from the captured variable
and initializing the capture variable with that value; the capture variable is
destroyed when the block literal is, i.e. at the end of the enclosing scope.

The :ref:`inference <arc.ownership.inference>` rules apply equally to
``__block`` variables, which is a shift in semantics from non-ARC, where
``__block`` variables did not implicitly retain during capture.

``__block`` variables of retainable object owner type are moved off the stack
by initializing the heap copy with the result of moving from the stack copy.

With the exception of retains done as part of initializing a ``__strong``
parameter variable or reading a ``__weak`` variable, whenever these semantics
call for retaining a value of block-pointer type, it has the effect of a
``Block_copy``.  The optimizer may remove such copies when it sees that the
result is used only as an argument to a call.

.. _arc.misc.exceptions:

Exceptions
==========

By default in Objective C, ARC is not exception-safe for normal releases:

* It does not end the lifetime of ``__strong`` variables when their scopes are
  abnormally terminated by an exception.
* It does not perform releases which would occur at the end of a
  full-expression if that full-expression throws an exception.

A program may be compiled with the option ``-fobjc-arc-exceptions`` in order to
enable these, or with the option ``-fno-objc-arc-exceptions`` to explicitly
disable them, with the last such argument "winning".

.. admonition:: Rationale

  The standard Cocoa convention is that exceptions signal programmer error and
  are not intended to be recovered from.  Making code exceptions-safe by
  default would impose severe runtime and code size penalties on code that
  typically does not actually care about exceptions safety.  Therefore,
  ARC-generated code leaks by default on exceptions, which is just fine if the
  process is going to be immediately terminated anyway.  Programs which do care
  about recovering from exceptions should enable the option.

In Objective-C++, ``-fobjc-arc-exceptions`` is enabled by default.

.. admonition:: Rationale

  C++ already introduces pervasive exceptions-cleanup code of the sort that ARC
  introduces.  C++ programmers who have not already disabled exceptions are
  much more likely to actual require exception-safety.

ARC does end the lifetimes of ``__weak`` objects when an exception terminates
their scope unless exceptions are disabled in the compiler.

.. admonition:: Rationale

  The consequence of a local ``__weak`` object not being destroyed is very
  likely to be corruption of the Objective-C runtime, so we want to be safer
  here.  Of course, potentially massive leaks are about as likely to take down
  the process as this corruption is if the program does try to recover from
  exceptions.

.. _arc.misc.interior:

Interior pointers
=================

An Objective-C method returning a non-retainable pointer may be annotated with
the ``objc_returns_inner_pointer`` attribute to indicate that it returns a
handle to the internal data of an object, and that this reference will be
invalidated if the object is destroyed.  When such a message is sent to an
object, the object's lifetime will be extended until at least the earliest of:

* the last use of the returned pointer, or any pointer derived from it, in the
  calling function or
* the autorelease pool is restored to a previous state.

.. admonition:: Rationale

  Rationale: not all memory and resources are managed with reference counts; it
  is common for objects to manage private resources in their own, private way.
  Typically these resources are completely encapsulated within the object, but
  some classes offer their users direct access for efficiency.  If ARC is not
  aware of methods that return such "interior" pointers, its optimizations can
  cause the owning object to be reclaimed too soon.  This attribute informs ARC
  that it must tread lightly.

  The extension rules are somewhat intentionally vague.  The autorelease pool
  limit is there to permit a simple implementation to simply retain and
  autorelease the receiver.  The other limit permits some amount of
  optimization.  The phrase "derived from" is intended to encompass the results
  both of pointer transformations, such as casts and arithmetic, and of loading
  from such derived pointers; furthermore, it applies whether or not such
  derivations are applied directly in the calling code or by other utility code
  (for example, the C library routine ``strchr``).  However, the implementation
  never need account for uses after a return from the code which calls the
  method returning an interior pointer.

As an exception, no extension is required if the receiver is loaded directly
from a ``__strong`` object with :ref:`precise lifetime semantics
<arc.optimization.precise>`.

.. admonition:: Rationale

  Implicit autoreleases carry the risk of significantly inflating memory use,
  so it's important to provide users a way of avoiding these autoreleases.
  Tying this to precise lifetime semantics is ideal, as for local variables
  this requires a very explicit annotation, which allows ARC to trust the user
  with good cheer.

.. _arc.misc.c-retainable:

C retainable pointer types
==========================

A type is a :arc-term:`C retainable pointer type`` if it is a pointer to
(possibly qualified) ``void`` or a pointer to a (possibly qualifier) ``struct``
or ``class`` type.

.. admonition:: Rationale

  ARC does not manage pointers of CoreFoundation type (or any of the related
  families of retainable C pointers which interoperate with Objective-C for
  retain/release operation).  In fact, ARC does not even know how to
  distinguish these types from arbitrary C pointer types.  The intent of this
  concept is to filter out some obviously non-object types while leaving a hook
  for later tightening if a means of exhaustively marking CF types is made
  available.

.. _arc.misc.c-retainable.audit:

Auditing of C retainable pointer interfaces
-------------------------------------------

:when-revised:`[beginning Apple 4.0, LLVM 3.1]`

A C function may be marked with the ``cf_audited_transfer`` attribute to
express that, except as otherwise marked with attributes, it obeys the
parameter (consuming vs. non-consuming) and return (retained vs. non-retained)
conventions for a C function of its name, namely:

* A parameter of C retainable pointer type is assumed to not be consumed
  unless it is marked with the ``cf_consumed`` attribute, and
* A result of C retainable pointer type is assumed to not be returned retained
  unless the function is either marked ``cf_returns_retained`` or it follows
  the create/copy naming convention and is not marked
  ``cf_returns_not_retained``.

A function obeys the :arc-term:`create/copy` naming convention if its name
contains as a substring:

* either "Create" or "Copy" not followed by a lowercase letter, or
* either "create" or "copy" not followed by a lowercase letter and
  not preceded by any letter, whether uppercase or lowercase.

A second attribute, ``cf_unknown_transfer``, signifies that a function's
transfer semantics cannot be accurately captured using any of these
annotations.  A program is ill-formed if it annotates the same function with
both ``cf_audited_transfer`` and ``cf_unknown_transfer``.

A pragma is provided to facilitate the mass annotation of interfaces:

.. code-block:: objc

  #pragma clang arc_cf_code_audited begin
  ...
  #pragma clang arc_cf_code_audited end

All C functions declared within the extent of this pragma are treated as if
annotated with the ``cf_audited_transfer`` attribute unless they otherwise have
the ``cf_unknown_transfer`` attribute.  The pragma is accepted in all language
modes.  A program is ill-formed if it attempts to change files, whether by
including a file or ending the current file, within the extent of this pragma.

It is possible to test for all the features in this section with
``__has_feature(arc_cf_code_audited)``.

.. admonition:: Rationale

  A significant inconvenience in ARC programming is the necessity of
  interacting with APIs based around C retainable pointers.  These features are
  designed to make it relatively easy for API authors to quickly review and
  annotate their interfaces, in turn improving the fidelity of tools such as
  the static analyzer and ARC.  The single-file restriction on the pragma is
  designed to eliminate the risk of accidentally annotating some other header's
  interfaces.

.. _arc.runtime:

===============
Runtime support
===============

This section describes the interaction between the ARC runtime and the code
generated by the ARC compiler.  This is not part of the ARC language
specification; instead, it is effectively a language-specific ABI supplement,
akin to the "Itanium" generic ABI for C++.

Ownership qualification does not alter the storage requirements for objects,
except that it is undefined behavior if a ``__weak`` object is inadequately
aligned for an object of type ``id``.  The other qualifiers may be used on
explicitly under-aligned memory.

The runtime tracks ``__weak`` objects which holds non-null values.  It is
undefined behavior to direct modify a ``__weak`` object which is being tracked
by the runtime except through an
:ref:`objc_storeWeak <arc.runtime.objc_storeWeak>`,
:ref:`objc_destroyWeak <arc.runtime.objc_destroyWeak>`, or
:ref:`objc_moveWeak <arc.runtime.objc_moveWeak>` call.

The runtime must provide a number of new entrypoints which the compiler may
emit, which are described in the remainder of this section.

.. admonition:: Rationale

  Several of these functions are semantically equivalent to a message send; we
  emit calls to C functions instead because:

  * the machine code to do so is significantly smaller,
  * it is much easier to recognize the C functions in the ARC optimizer, and
  * a sufficient sophisticated runtime may be able to avoid the message send in
    common cases.

  Several other of these functions are "fused" operations which can be
  described entirely in terms of other operations.  We use the fused operations
  primarily as a code-size optimization, although in some cases there is also a
  real potential for avoiding redundant operations in the runtime.

.. _arc.runtime.objc_autorelease:

``id objc_autorelease(id value);``
==================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it adds the object
to the innermost autorelease pool exactly as if the object had been sent the
``autorelease`` message.

Always returns ``value``.

.. _arc.runtime.objc_autoreleasePoolPop:

``void objc_autoreleasePoolPop(void *pool);``
=============================================

*Precondition:* ``pool`` is the result of a previous call to
:ref:`objc_autoreleasePoolPush <arc.runtime.objc_autoreleasePoolPush>` on the
current thread, where neither ``pool`` nor any enclosing pool have previously
been popped.

Releases all the objects added to the given autorelease pool and any
autorelease pools it encloses, then sets the current autorelease pool to the
pool directly enclosing ``pool``.

.. _arc.runtime.objc_autoreleasePoolPush:

``void *objc_autoreleasePoolPush(void);``
=========================================

Creates a new autorelease pool that is enclosed by the current pool, makes that
the current pool, and returns an opaque "handle" to it.

.. admonition:: Rationale

  While the interface is described as an explicit hierarchy of pools, the rules
  allow the implementation to just keep a stack of objects, using the stack
  depth as the opaque pool handle.

.. _arc.runtime.objc_autoreleaseReturnValue:

``id objc_autoreleaseReturnValue(id value);``
=============================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it makes a best
effort to hand off ownership of a retain count on the object to a call to
:ref:`objc_retainAutoreleasedReturnValue
<arc.runtime.objc_retainAutoreleasedReturnValue>` for the same object in an
enclosing call frame.  If this is not possible, the object is autoreleased as
above.

Always returns ``value``.

.. _arc.runtime.objc_copyWeak:

``void objc_copyWeak(id *dest, id *src);``
==========================================

*Precondition:* ``src`` is a valid pointer which either contains a null pointer
or has been registered as a ``__weak`` object.  ``dest`` is a valid pointer
which has not been registered as a ``__weak`` object.

``dest`` is initialized to be equivalent to ``src``, potentially registering it
with the runtime.  Equivalent to the following code:

.. code-block:: objc

  void objc_copyWeak(id *dest, id *src) {
    objc_release(objc_initWeak(dest, objc_loadWeakRetained(src)));
  }

Must be atomic with respect to calls to ``objc_storeWeak`` on ``src``.

.. _arc.runtime.objc_destroyWeak:

``void objc_destroyWeak(id *object);``
======================================

*Precondition:* ``object`` is a valid pointer which either contains a null
pointer or has been registered as a ``__weak`` object.

``object`` is unregistered as a weak object, if it ever was.  The current value
of ``object`` is left unspecified; otherwise, equivalent to the following code:

.. code-block:: objc

  void objc_destroyWeak(id *object) {
    objc_storeWeak(object, nil);
  }

Does not need to be atomic with respect to calls to ``objc_storeWeak`` on
``object``.

.. _arc.runtime.objc_initWeak:

``id objc_initWeak(id *object, id value);``
===========================================

*Precondition:* ``object`` is a valid pointer which has not been registered as
a ``__weak`` object.  ``value`` is null or a pointer to a valid object.

If ``value`` is a null pointer or the object to which it points has begun
deallocation, ``object`` is zero-initialized.  Otherwise, ``object`` is
registered as a ``__weak`` object pointing to ``value``.  Equivalent to the
following code:

.. code-block:: objc

  id objc_initWeak(id *object, id value) {
    *object = nil;
    return objc_storeWeak(object, value);
  }

Returns the value of ``object`` after the call.

Does not need to be atomic with respect to calls to ``objc_storeWeak`` on
``object``.

.. _arc.runtime.objc_loadWeak:

``id objc_loadWeak(id *object);``
=================================

*Precondition:* ``object`` is a valid pointer which either contains a null
pointer or has been registered as a ``__weak`` object.

If ``object`` is registered as a ``__weak`` object, and the last value stored
into ``object`` has not yet been deallocated or begun deallocation, retains and
autoreleases that value and returns it.  Otherwise returns null.  Equivalent to
the following code:

.. code-block:: objc

  id objc_loadWeak(id *object) {
    return objc_autorelease(objc_loadWeakRetained(object));
  }

Must be atomic with respect to calls to ``objc_storeWeak`` on ``object``.

.. admonition:: Rationale

  Loading weak references would be inherently prone to race conditions without
  the retain.

.. _arc.runtime.objc_loadWeakRetained:

``id objc_loadWeakRetained(id *object);``
=========================================

*Precondition:* ``object`` is a valid pointer which either contains a null
pointer or has been registered as a ``__weak`` object.

If ``object`` is registered as a ``__weak`` object, and the last value stored
into ``object`` has not yet been deallocated or begun deallocation, retains
that value and returns it.  Otherwise returns null.

Must be atomic with respect to calls to ``objc_storeWeak`` on ``object``.

.. _arc.runtime.objc_moveWeak:

``void objc_moveWeak(id *dest, id *src);``
==========================================

*Precondition:* ``src`` is a valid pointer which either contains a null pointer
or has been registered as a ``__weak`` object.  ``dest`` is a valid pointer
which has not been registered as a ``__weak`` object.

``dest`` is initialized to be equivalent to ``src``, potentially registering it
with the runtime.  ``src`` may then be left in its original state, in which
case this call is equivalent to :ref:`objc_copyWeak
<arc.runtime.objc_copyWeak>`, or it may be left as null.

Must be atomic with respect to calls to ``objc_storeWeak`` on ``src``.

.. _arc.runtime.objc_release:

``void objc_release(id value);``
================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it performs a
release operation exactly as if the object had been sent the ``release``
message.

.. _arc.runtime.objc_retain:

``id objc_retain(id value);``
=============================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it performs a retain
operation exactly as if the object had been sent the ``retain`` message.

Always returns ``value``.

.. _arc.runtime.objc_retainAutorelease:

``id objc_retainAutorelease(id value);``
========================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it performs a retain
operation followed by an autorelease operation.  Equivalent to the following
code:

.. code-block:: objc

  id objc_retainAutorelease(id value) {
    return objc_autorelease(objc_retain(value));
  }

Always returns ``value``.

.. _arc.runtime.objc_retainAutoreleaseReturnValue:

``id objc_retainAutoreleaseReturnValue(id value);``
===================================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it performs a retain
operation followed by the operation described in
:ref:`objc_autoreleaseReturnValue <arc.runtime.objc_autoreleaseReturnValue>`.
Equivalent to the following code:

.. code-block:: objc

  id objc_retainAutoreleaseReturnValue(id value) {
    return objc_autoreleaseReturnValue(objc_retain(value));
  }

Always returns ``value``.

.. _arc.runtime.objc_retainAutoreleasedReturnValue:

``id objc_retainAutoreleasedReturnValue(id value);``
====================================================

*Precondition:* ``value`` is null or a pointer to a valid object.

If ``value`` is null, this call has no effect.  Otherwise, it attempts to
accept a hand off of a retain count from a call to
:ref:`objc_autoreleaseReturnValue <arc.runtime.objc_autoreleaseReturnValue>` on
``value`` in a recently-called function or something it calls.  If that fails,
it performs a retain operation exactly like :ref:`objc_retain
<arc.runtime.objc_retain>`.

Always returns ``value``.

.. _arc.runtime.objc_retainBlock:

``id objc_retainBlock(id value);``
==================================

*Precondition:* ``value`` is null or a pointer to a valid block object.

If ``value`` is null, this call has no effect.  Otherwise, if the block pointed
to by ``value`` is still on the stack, it is copied to the heap and the address
of the copy is returned.  Otherwise a retain operation is performed on the
block exactly as if it had been sent the ``retain`` message.

.. _arc.runtime.objc_storeStrong:

``id objc_storeStrong(id *object, id value);``
==============================================

*Precondition:* ``object`` is a valid pointer to a ``__strong`` object which is
adequately aligned for a pointer.  ``value`` is null or a pointer to a valid
object.

Performs the complete sequence for assigning to a ``__strong`` object of
non-block type.  Equivalent to the following code:

.. code-block:: objc

  id objc_storeStrong(id *object, id value) {
    value = [value retain];
    id oldValue = *object;
    *object = value;
    [oldValue release];
    return value;
  }

Always returns ``value``.

.. _arc.runtime.objc_storeWeak:

``id objc_storeWeak(id *object, id value);``
============================================

*Precondition:* ``object`` is a valid pointer which either contains a null
pointer or has been registered as a ``__weak`` object.  ``value`` is null or a
pointer to a valid object.

If ``value`` is a null pointer or the object to which it points has begun
deallocation, ``object`` is assigned null and unregistered as a ``__weak``
object.  Otherwise, ``object`` is registered as a ``__weak`` object or has its
registration updated to point to ``value``.

Returns the value of ``object`` after the call.