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<div id="content">
<h1>Automatic Reference Counting</h1>
<div id="toc">
</div>
<div id="meta">
<h1>About this document</h1>
<div id="meta.purpose">
<h1>Purpose</h1>
<p>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.</p>
<p>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.</p>
</div> <!-- meta.purpose -->
<div id="meta.background">
<h1>Background</h1>
<p>This document assumes a basic familiarity with C.</p>
<p><span class="term">Blocks</span> are a C language extension for
creating anonymous functions. Users interact with and transfer block
objects using <span class="term">block pointers</span>, 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 <tt>Block_copy</tt> 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 <tt>Block_release</tt>,
which decreases the reference count by 1 and destroys the object if
the count reaches zero and is on the heap.</p>
<p>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.</p>
<p>Objective-C defines a new type kind, collectively called
the <span class="term">object pointer types</span>. This kind has two
notable builtin members, <tt>id</tt> and <tt>Class</tt>; <tt>id</tt>
is the final supertype of all object pointers. The validity of
conversions between object pointer types is not checked at runtime.
Users may define <span class="term">classes</span>; 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 <span class="term">ivars</span>, fields which appear on all
instances of that class. For every class <i>T</i> there's an
associated metaclass; it has no fields, its superclass is the
metaclass of <i>T</i>'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 <tt>Class</tt>.</p>
<p>A class declaration (<tt>@interface</tt>) declares a set
of <span class="term">methods</span>. A method has a return type, a
list of argument types, and a <span class="term">selector</span>: a
name like <tt>foo:bar:baz:</tt>, 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 <span class="term">receiver</span>) and a list of formal
arguments interspersed with the selector, like so:</p>
<pre>[receiver foo: fooArg bar: barArg baz: bazArg]</pre>
<p>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 <q>expression</q> 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 <tt>super</tt>, 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
<tt>@interface</tt>, but those defined in a separate
<tt>@implementation</tt> declaration; however, when compiling a
call, typechecking is done based on the methods declared in the
<tt>@interface</tt>.</p>
<p>Method declarations may also be grouped into
<span class="term">protocols</span>, 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.</p>
<p><span class="term">Class extensions</span> are collections of ivars
and methods, designed to allow a class's <tt>@interface</tt> to be
split across multiple files; however, there is still a primary
implementation file which must see the <tt>@interface</tt>s of all
class extensions.
<span class="term">Categories</span> allow methods (but not ivars) to
be declared <i>post hoc</i> on an arbitrary class; the methods in the
category's <tt>@implementation</tt> 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.</p>
<p>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: <tt>retain</tt> increases the object's reference count by
1, whereas <tt>release</tt> decreases it by 1 and calls the instance
method <tt>dealloc</tt> if the count reaches 0. To simplify certain
operations, there is also an <span class="term">autorelease
pool</span>, a thread-local list of objects to call <tt>release</tt>
on later; an object can be added to this pool by
calling <tt>autorelease</tt> on it.</p>
<p>Block pointers may be converted to type <tt>id</tt>; 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 <tt>retain</tt> by
adjusting the reference count, not by calling <tt>Block_copy</tt>.</p>
</div> <!-- meta.background -->
</div> <!-- meta -->
<div id="general">
<h1>General</h1>
<p>Automatic Reference Counting implements automatic memory management
for Objective-C objects and blocks, freeing the programmer from the
need explicitly insert retains and releases. It does not provide a
cycle collector; users must explicitly manage lifetime instead.</p>
<p>ARC may be explicitly enabled with the compiler
flag <tt>-fobjc-arc</tt>. It may also be explicitly disabled with the
compiler flag <tt>-fno-objc-arc</tt>. The last of these two flags
appearing on the compile line <q>wins</q>.</p>
<p>If ARC is enabled, <tt>__has_feature(objc_arc)</tt> will expand to
1 in the preprocessor. For more information about <tt>__has_feature</tt>,
see the <a href="LanguageExtensions.html#__has_feature_extension">language
extensions</a> document.</p>
</div>
<div id="objects">
<h1>Retainable object pointers</h1>
<p>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 <em>values</em>
(patterns of bits indicating the location of a pointed-to object), not
pointer
<em>objects</em> (locations in memory which store pointer values).
The rules for objects are covered in the next section.</p>
<p>A <span class="term">retainable object pointer</span>
(or <q>retainable pointer</q>) is a value of
a <span class="term">retainable object pointer type</span>
(<q>retainable type</q>). There are three kinds of retainable object
pointer types:</p>
<ul>
<li>block pointers (formed by applying the caret (<tt>^</tt>)
declarator sigil to a function type)</li>
<li>Objective-C object pointers (<tt>id</tt>, <tt>Class</tt>, <tt>NSFoo*</tt>, etc.)</li>
<li>typedefs marked with <tt>__attribute__((NSObject))</tt></li>
</ul>
<p>Other pointer types, such as <tt>int*</tt> and <tt>CFStringRef</tt>,
are not subject to ARC's semantics and restrictions.</p>
<div class="rationale">
<p>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:</p>
<ul>
<li>The type system must reliably identify which objects are to be
managed. An <tt>int*</tt> might be a pointer to a <tt>malloc</tt>'ed
array, or it might be a 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.</li>
<li>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.</li>
<li>There must be reliable conventions for whether and
when <q>ownership</q> 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.</li>
</ul>
</div> <!-- rationale -->
<p>The use of <tt>__attribute__((NSObject))</tt> 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 <tt>__typeof</tt> and C++
template argument substitution.</p>
<div class="rationale"><p>Rationale: any compiler operation which
incidentally strips type <q>sugar</q> from a type will yield a type
without the attribute, which may result in unexpected
behavior.</p></div>
<div id="objects.retains">
<h1>Retain count semantics</h1>
<p>A retainable object pointer is either a <span class="term">null
pointer</span> or a pointer to a valid object. Furthermore, if it has
block pointer type and is not <tt>null</tt> then it must actually be a
pointer to a block object, and if it has <tt>Class</tt> 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.</p>
<p>For ARC's purposes, a valid object is one with <q>well-behaved</q>
retaining operations. Specifically, the object must be laid out such
that the Objective-C message send machinery can successfully send it
the following messages:</p>
<ul>
<li><tt>retain</tt>, taking no arguments and returning a pointer to
the object.</li>
<li><tt>release</tt>, taking no arguments and returning <tt>void</tt>.</li>
<li><tt>autorelease</tt>, taking no arguments and returning a pointer
to the object.</li>
</ul>
<p>The behavior of these methods is constrained in the following ways.
The term <span class="term">high-level semantics</span> 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 <tt>retain</tt>, 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.</p>
<p>It is undefined behavior if a computation history featuring a send
of <tt>retain</tt> followed by a send of <tt>release</tt> to the same
object, with no intervening <tt>release</tt> 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.</p>
<p>It is undefined behavior if a computation history features any use
whatsoever of an object following the completion of a send
of <tt>release</tt> that is not preceded by a send of <tt>retain</tt>
to the same object.</p>
<p>The behavior of <tt>autorelease</tt> must be equivalent to sending
<tt>release</tt> when one of the autorelease pools currently in scope
is popped. It may not throw an exception.</p>
<p>When the semantics call for performing one of these operations on a
retainable object pointer, if that pointer is <tt>null</tt> then the
effect is a no-op.</p>
<p>All of the semantics described in this document are subject to
additional <a href="#optimization">optimization rules</a> 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.</p>
</div> <!-- objects.retains -->
<div id="objects.operands">
<h1>Retainable object pointers as operands and arguments</h1>
<p>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:</p>
<ul>
<li>loading a retainable pointer from an object with non-weak
<a href="#ownership">ownership</a>,</li>
<li>passing a retainable pointer as an argument to a function or
method, and</li>
<li>receiving a retainable pointer as the result of a function or
method call.</li>
</ul>
<div class="rationale"><p>Rationale: while this might seem
uncontroversial, it is actually unsafe when multiple expressions are
evaluated in <q>parallel</q>, 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.</p></div>
<p>The remainder of this section describes exceptions to these rules,
how those exceptions are detected, and what those exceptions imply
semantically.</p>
<div id="objects.operands.consumed">
<h1>Consumed parameters</h1>
<p>A function or method parameter of retainable object pointer type
may be marked as <span class="term">consumed</span>, signifying that
the callee expects to take ownership of a +1 retain count. This is
done by adding the <tt>ns_consumed</tt> attribute to the parameter
declaration, like so:</p>
<pre>void foo(__attribute((ns_consumed)) id x);
- (void) foo: (id) __attribute((ns_consumed)) x;</pre>
<p>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.</p>
<p>When passing such an argument, ARC retains the argument prior to
making the call.</p>
<p>When receiving such an argument, ARC releases the argument at the
end of the function, subject to the usual optimizations for local
values.</p>
<div class="rationale"><p>Rationale: this formalizes direct transfers
of ownership from a caller to a callee. The most common scenario here
is passing the <tt>self</tt> parameter to <tt>init</tt>, 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.</p></div>
<p>The implicit <tt>self</tt> parameter of a method may be marked as
consumed by adding <tt>__attribute__((ns_consumes_self))</tt> to the
method declaration. Methods in the <tt>init</tt>
<a href="#family">family</a> are treated as if they were implicitly
marked with this attribute.</p>
<p>It is undefined behavior if an Objective-C message send to a method
with <tt>ns_consumed</tt> 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 <tt>ns_consumed</tt> 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 <tt>ns_consumed</tt>
parameters than the implementation of the called block or function.</p>
<div class="rationale"><p>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.</p></div>
</div>
<div id="objects.operands.retained_returns">
<h1>Retained return values</h1>
<p>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 <tt>ns_returns_retained</tt> attribute to the function or
method declaration, like so:</p>
<pre>id foo(void) __attribute((ns_returns_retained));
- (id) foo __attribute((ns_returns_retained));</pre>
<p>This attribute is part of the type of the function or method.</p>
<p>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.</p>
<p>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.</p>
<div class="rationale"><p>Rationale: this formalizes direct transfers of
ownership from a callee to a caller. The most common scenario this
models is the retained return from <tt>init</tt>, <tt>alloc</tt>,
<tt>new</tt>, and <tt>copy</tt> methods, but there are other cases in
the frameworks. After optimization there are typically no extra
retains and releases required.</p></div>
<p>Methods in
the <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>, <tt>mutableCopy</tt>,
and <tt>new</tt> <a href="#family">families</a> are implicitly marked
<tt>__attribute__((ns_returns_retained))</tt>. This may be suppressed
by explicitly marking the
method <tt>__attribute__((ns_returns_not_retained))</tt>.</p>
</div>
<p>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.</p>
<div class="rationale"><p>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.</p></div>
<div id="objects.operands.other-returns">
<h1>Unretained return values</h1>
<p>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.</p>
<p>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 <tt>autorelease</tt>, but callers must not assume that the
value is actually in the autorelease pool.</p>
<p>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.</p>
<div class="rationale"><p>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.</p></div>
<p>A method or function may be marked
with <tt>__attribute__((ns_returns_autoreleased))</tt> 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.</p>
</div>
<div id="objects.operands.casts">
<h1>Bridged casts</h1>
<p>A <span class="term">bridged cast</span> is a C-style cast
annotated with one of three keywords:</p>
<ul>
<li><tt>(__bridge T) op</tt> casts the operand to the destination
type <tt>T</tt>. If <tt>T</tt> is a retainable object pointer type,
then <tt>op</tt> must have a non-retainable pointer type.
If <tt>T</tt> is a non-retainable pointer type, then <tt>op</tt> 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.</li>
<li><tt>(__bridge_retained T) op</tt> 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.</li>
<li><tt>(__bridge_transfer T) op</tt> 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.</li>
</ul>
<p>These casts are required in order to transfer objects in and out of
ARC control; see the rationale in the section
on <a href="#objects.restrictions.conversion">conversion of retainable
object pointers</a>.</p>
<p>Using a <tt>__bridge_retained</tt> or <tt>__bridge_transfer</tt>
cast purely to convince ARC to emit an unbalanced retain or release,
respectively, is poor form.</p>
</div>
</div>
<div id="objects.restrictions">
<h1>Restrictions</h1>
<div id="objects.restrictions.conversion">
<h1>Conversion of retainable object pointers</h1>
<p>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 <tt>intptr_t</tt>
or <tt>void*</tt>. The <a href="#objects.operands.casts">bridged
casts</a> may be used to perform these conversions where
necessary.</p>
<div class="rationale"><p>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.</p></div>
</div>
<p>An unbridged cast to a retainable object pointer type of the return
value of a Objective-C message send which yields a non-retainable
pointer is treated as a <tt>__bridge_transfer</tt> cast
if:</p>
<ul>
<li>the method has the <tt>cf_returns_retained</tt> attribute, or if
not that,</li>
<li>the method does not have the <tt>cf_returns_not_retained</tt>
attribute and</li>
<li>the method's <a href="#family">selector family</a> would imply
the <tt>ns_returns_retained</tt> attribute on a method which returned
a retainable object pointer type.</li>
</ul>
<p>Otherwise the cast is treated as a <tt>__bridge</tt> cast.</p>
</div>
</div>
<div id="ownership">
<h1>Ownership qualification</h1>
<p>This section describes the behavior of <em>objects</em> of
retainable object pointer type; that is, locations in memory which
store retainable object pointers.</p>
<p>A type is a <span class="term">retainable object owner type</span>
if it is a retainable object pointer type or an array type whose
element type is a retainable object owner type.</p>
<p>An <span class="term">ownership qualifier</span> 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.</p>
<p>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.</p>
<p>Except as described under
the <a href="#ownership.inference">inference rules</a>, 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.</p>
<div class="rationale"><p>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 <a href="#ownership.inference.template_arguments">inference of <tt>__strong</tt> for template type arguments</a>. </p></div>
<p>There are four ownership qualifiers:</p>
<ul>
<li><tt>__autoreleasing</tt></li>
<li><tt>__strong</tt></li>
<li><tt>__unsafe_unretained</tt></li>
<li><tt>__weak</tt></li>
</ul>
<p>A type is <span class="term">nontrivially ownership-qualified</span>
if it is qualified with <tt>__autoreleasing</tt>, <tt>__strong</tt>, or
<tt>__weak</tt>.</p>
<div id="ownership.spelling">
<h1>Spelling</h1>
<p>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.</p>
<p>An ownership qualifier may be written anywhere that any other type
qualifier may be written.</p>
<p>If an ownership qualifier appears in
the <i>declaration-specifiers</i>, the following rules apply:</p>
<ul>
<li>if the type specifier is a retainable object owner type, the
qualifier applies to that type;</li>
<li>if the outermost non-array part of the declarator is a pointer or
block pointer, the qualifier applies to that type;</li>
<li>otherwise the program is ill-formed.</li>
</ul>
<p>If an ownership qualifier appears on the declarator name, or on the
declared object, it is applied to outermost pointer or block-pointer
type.</p>
<p>If an ownership qualifier appears anywhere else in a declarator, it
applies to the type there.</p>
</div> <!-- ownership.spelling -->
<div id="ownership.semantics">
<h1>Semantics</h1>
<p>There are five <span class="term">managed operations</span> 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.</p>
<p>A load or store with <q>primitive semantics</q> has the same
semantics as the respective operation would have on an <tt>void*</tt>
lvalue with the same alignment and non-ownership qualification.</p>
<p><span class="term">Reading</span> occurs when performing a
lvalue-to-rvalue conversion on an object lvalue.
<ul>
<li>For <tt>__weak</tt> 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.</li>
<li>For all other objects, the lvalue is loaded with primitive
semantics.</li>
</ul>
</p>
<p><span class="term">Assignment</span> occurs when evaluating
an assignment operator. The semantics vary based on the qualification:
<ul>
<li>For <tt>__strong</tt> 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.</li>
<li>For <tt>__weak</tt> objects, the lvalue is updated to point to the
new pointee, unless that object is currently undergoing deallocation,
in which case it 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 pointed-to
value.</li>
<li>For <tt>__unsafe_unretained</tt> objects, the new pointee is
stored into the lvalue using primitive semantics.</li>
<li>For <tt>__autoreleasing</tt> objects, the new pointee is retained,
autoreleased, and stored into the lvalue using primitive semantics.</li>
</ul>
</p>
<p><span class="term">Initialization</span> occurs when an object's
lifetime begins, which depends on its storage duration.
Initialization proceeds in two stages:
<ol>
<li>First, a null pointer is stored into the lvalue using primitive
semantics. This step is skipped if the object
is <tt>__unsafe_unretained</tt>.</li>
<li>Second, if the object has an initializer, that expression is
evaluated and then assigned into the object using the usual assignment
semantics.</li>
</ol>
</p>
<p><span class="term">Destruction</span> 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.</p>
<p><span class="term">Moving</span> occurs in specific situations
where an lvalue is <q>moved from</q>, meaning that its current pointee
will be used but the object may be left in a different (but still
valid) state. This arises with <tt>__block</tt> variables and rvalue
references in C++. For <tt>__strong</tt> 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.</p>
</div> <!-- ownership.semantics -->
<div id="ownership.restrictions">
<h1>Restrictions</h1>
<div id="ownership.restrictions.autoreleasing">
<h1>Storage duration of<tt> __autoreleasing</tt> objects</h1>
<p>A program is ill-formed if it declares an <tt>__autoreleasing</tt>
object of non-automatic storage duration.</p>
<div class="rationale"><p>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.</p></div>
<p>It is undefined behavior if a non-null pointer is assigned to
an <tt>__autoreleasing</tt> object while an autorelease pool is in
scope and then that object is read after the autorelease pool's scope
is left.</p>
</div>
<div id="ownership.restrictions.conversion.indirect">
<h1>Conversion of pointers to ownership-qualified types</h1>
<p>A program is ill-formed if an expression of type <tt>T*</tt> is
converted, explicitly or implicitly, to the type <tt>U*</tt>,
where <tt>T</tt> and <tt>U</tt> have different ownership
qualification, unless:
<ul>
<li><tt>T</tt> is qualified with <tt>__strong</tt>,
<tt>__autoreleasing</tt>, or <tt>__unsafe_unretained</tt>, and
<tt>U</tt> is qualified with both <tt>const</tt> and
<tt>__unsafe_unretained</tt>; or</li>
<li>either <tt>T</tt> or <tt>U</tt> is <tt>cv void</tt>, where
<tt>cv</tt> is an optional sequence of non-ownership qualifiers; or</li>
<li>the conversion is requested with a <tt>reinterpret_cast</tt> in
Objective-C++; or</li>
<li>the conversion is a
well-formed <a href="#ownership.restrictions.pass_by_writeback">pass-by-writeback</a>.</li>
</ul>
</p>
<p>The analogous rule applies to <tt>T&</tt> and <tt>U&</tt> in
Objective-C++.</p>
<div class="rationale"><p>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 <tt>const
__unsafe_unretained</tt> 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 <tt>void*</tt> is
useful for allocating memory or otherwise escaping the type system,
but use it carefully. <tt>reinterpret_cast</tt> is considered to be
an obvious enough sign of taking responsibility for any
problems.</p></div>
<p>It is undefined behavior to access an ownership-qualified object
through an lvalue of a differently-qualified type, except that any
non-<tt>__weak</tt> object may be read through
an <tt>__unsafe_unretained</tt> lvalue.</p>
<p>It is undefined behavior if a managed operation is performed on
a <tt>__strong</tt> or <tt>__weak</tt> 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.</p>
<div class="rationale"><p>Rationale: ARC cannot differentiate between
an assignment operator which is intended to <q>initialize</q> 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 (<tt>__strong</tt> objects) and
consistency errors (<tt>__weak</tt> objects).</p>
<p>These requirements are followed automatically in Objective-C++ when
creating objects of retainable object owner type with <tt>new</tt>
or <tt>new[]</tt> and destroying them with <tt>delete</tt>,
<tt>delete[]</tt>, 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 <tt>new[]</tt>'d in ARC translation units cannot
be <tt>delete[]</tt>'d in non-ARC translation units and
vice-versa.</p></div>
</div>
<div id="ownership.restrictions.pass_by_writeback">
<h1>Passing to an out parameter by writeback</h1>
<p>If the argument passed to a parameter of type
<tt>T __autoreleasing *</tt> has type <tt>U oq *</tt>,
where <tt>oq</tt> is an ownership qualifier, then the argument is a
candidate for <span class="term">pass-by-writeback</span> if:</p>
<ul>
<li><tt>oq</tt> is <tt>__strong</tt> or <tt>__weak</tt>, and
<li>it would be legal to initialize a <tt>T __strong *</tt> with
a <tt>U __strong *</tt>.</li>
</ul>
<p>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.</p>
<p>The pass-by-writeback is ill-formed if the argument expression does
not have a legal form:</p>
<ul>
<li><tt>&var</tt>, where <tt>var</tt> is a scalar variable of
automatic storage duration with retainable object pointer type</li>
<li>a conditional expression where the second and third operands are
both legal forms</li>
<li>a cast whose operand is a legal form</li>
<li>a null pointer constant</li>
</ul>
<div class="rationale"><p>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 <q>array</q>
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.</p></div>
<p>A pass-by-writeback is evaluated as follows:
<ol>
<li>The argument is evaluated to yield a pointer <tt>p</tt> of
type <tt>U oq *</tt>.</li>
<li>If <tt>p</tt> is a null pointer, then a null pointer is passed as
the argument, and no further work is required for the pass-by-writeback.</li>
<li>Otherwise, a temporary of type <tt>T __autoreleasing</tt> is
created and initialized to a null pointer.</li>
<li>If the argument is not an Objective-C method parameter marked
<tt>out</tt>, then <tt>*p</tt> is read, and the result is written
into the temporary with primitive semantics.</li>
<li>The address of the temporary is passed as the argument to the
actual call.</li>
<li>After the call completes, the temporary is loaded with primitive
semantics, and that value is assigned into <tt>*p</tt>.</li>
</ol></p>
<div class="rationale"><p>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 <tt>__autoreleasing</tt> rather than <tt>__strong</tt>. 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 <tt>__autoreleasing</tt> on all the variables
they intend to use for out-parameters. This was the least bad
solution.</p></div>
</div>
<div id="ownership.restrictions.records">
<h1>Ownership-qualified fields of structs and unions</h1>
<p>A program is ill-formed if it declares a member of a C struct or
union to have a nontrivially ownership-qualified type.</p>
<div class="rationale"><p>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 <tt>void*</tt> or an <tt>__unsafe_unretained</tt>
object.</p></div>
<p>This restriction does not apply in Objective-C++. However,
nontrivally ownership-qualified types are considered non-POD: in C++0x
terms, they are not trivially default constructible, copy
constructible, move constructible, copy assignable, move assignable,
or destructible. It is a violation of C++ One Definition Rule to use
a class outside of ARC that, under ARC, would have an
ownership-qualified member.</p>
<div class="rationale"><p>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
repercussion that the class will have a non-trivial copy constructor
and non-trivial destructor; if it wouldn't outside of ARC, this means
that objects of the type will be passed and returned in an
ABI-incompatible manner.</p></div>
</div>
</div>
<div id="ownership.inference">
<h1>Ownership inference</h1>
<div id="ownership.inference.variables">
<h1>Objects</h1>
<p>If an object is declared with retainable object owner type, but
without an explicit ownership qualifier, its type is implicitly
adjusted to have <tt>__strong</tt> qualification.</p>
<p>As a special case, if the object's base type is <tt>Class</tt>
(possibly protocol-qualified), the type is adjusted to
have <tt>__unsafe_unretained</tt> qualification instead.</p>
</div>
<div id="ownership.inference.indirect_parameters">
<h1>Indirect parameters</h1>
<p>If a function or method parameter has type <tt>T*</tt>, where
<tt>T</tt> is an ownership-unqualified retainable object pointer type,
then:</p>
<ul>
<li>if <tt>T</tt> is <tt>const</tt>-qualified or <tt>Class</tt>, then
it is implicitly qualified with <tt>__unsafe_unretained</tt>;</li>
<li>otherwise, it is implicitly qualified
with <tt>__autoreleasing</tt>.</li>
</ul>
</p>
<div class="rationale"><p>Rationale: <tt>__autoreleasing</tt> exists
mostly for this case, the Cocoa convention for out-parameters. Since
a pointer to <tt>const</tt> 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 <em>mutable</em> array, inferring
<tt>__autoreleasing</tt> is the wrong thing to do; this directs some
of the caution in the following rules about writeback.</p></div>
<p>Such a type written anywhere else would be ill-formed by the
general rule requiring ownership qualifiers.</p>
<p>This rule does not apply in Objective-C++ if a parameter's type is
dependent in a template pattern and is only <em>instantiated</em> to
a type which would be a pointer to an unqualified retainable object
pointer type. Such code is still ill-formed.</p>
<div class="rationale"><p>Rationale: the convention is very unlikely
to be intentional in template code.</p></div>
</div> <!-- ownership.inference.indirect_parameters -->
<div id="ownership.inference.template_arguments">
<h1>Template arguments</h1>
<p>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 <tt>__strong</tt>
qualification. This adjustment occurs regardless of whether the
template argument was deduced or explicitly specified. </p>
<div class="rationale"><p>Rationale: <tt>__strong</tt> is a useful default for containers (e.g., <tt>std::vector&lt;id&gt;</tt>), 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.</p></div>
</div> <!-- ownership.inference.template_arguments -->
</div> <!-- ownership.inference -->
</div> <!-- ownership -->
<div id="family">
<h1>Method families</h1>
<p>An Objective-C method may fall into a <span class="term">method
family</span>, which is a conventional set of behaviors ascribed to it
by the Cocoa conventions.</p>
<p>A method is in a certain method family if:
<ul>
<li>it has a <tt>objc_method_family</tt> attribute placing it in that
family; or if not that,</li>
<li>it does not have an <tt>objc_method_family</tt> attribute placing
it in a different or no family, and</li>
<li>its selector falls into the corresponding selector family, and</li>
<li>its signature obeys the added restrictions of the method family.</li>
</ul></p>
<p>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, <tt>_perform:with:</tt> and <tt>performWith:</tt> would fall
into the <tt>perform</tt> family (if we recognized one),
but <tt>performing:with</tt> would not.</p>
<p>The families and their added restrictions are:</p>
<ul>
<li><tt>alloc</tt> methods must return a retainable object pointer type.</li>
<li><tt>copy</tt> methods must return a retainable object pointer type.</li>
<li><tt>mutableCopy</tt> methods must return a retainable object pointer type.</li>
<li><tt>new</tt> methods must return a retainable object pointer type.</li>
<li><tt>init</tt> 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 <tt>init</tt> method whose return
type is neither <tt>id</tt> 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).</p>
<div class="rationale"><p>Rationale: there are a fair number of existing
methods with <tt>init</tt>-like selectors which nonetheless don't
follow the <tt>init</tt> conventions. Typically these are either
accidental naming collisions or helper methods called during
initialization. Because of the peculiar retain/release behavior
of <tt>init</tt> methods, it's very important not to treat these
methods as <tt>init</tt> 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 <tt>init</tt> family.</p>
<p>Note that a method with an <tt>init</tt>-family selector which
returns a non-Objective-C type (e.g. <tt>void</tt>) is perfectly
well-formed; it simply isn't in the <tt>init</tt> family.</p></div>
</li>
</ul>
<p>A program is ill-formed if a method's declarations,
implementations, and overrides do not all have the same method
family.</p>
<div id="family.attribute">
<h1>Explicit method family control</h1>
<p>A method may be annotated with the <tt>objc_method_family</tt>
attribute to precisely control which method family it belongs to. If
a method in an <tt>@implementation</tt> does not have this attribute,
but there is a method declared in the corresponding <tt>@interface</tt>
that does, then the attribute is copied to the declaration in the
<tt>@implementation</tt>. The attribute is available outside of ARC,
and may be tested for with the preprocessor query
<tt>__has_attribute(objc_method_family)</tt>.</p>
<p>The attribute is spelled
<tt>__attribute__((objc_method_family(<i>family</i>)))</tt>.
If <i>family</i> is <tt>none</tt>, the method has no family, even if
it would otherwise be considered to have one based on its selector and
type. Otherwise, <i>family</i> must be one
of <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>,
<tt>mutableCopy</tt>, or <tt>new</tt>, 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.</p>
<div class="rationale"><p>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
<tt>ns_returns_retained</tt>, this attribute allows the user to
communicate semantic intent, which of use both to ARC (which, e.g.,
treats calls to <tt>init</tt> specially) and the static analyzer.</p></div>
</div>
<div id="family.semantics">
<h1>Semantics of method families</h1>
<p>A method's membership in a method family may imply non-standard
semantics for its parameters and return type.</p>
<p>Methods in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>,
and <tt>new</tt> families &mdash; that is, methods in all the
currently-defined families except <tt>init</tt> &mdash; implicitly
<a href="#objects.operands.retained_returns">return a retained
object</a> as if they were annotated with
the <tt>ns_returns_retained</tt> attribute. This can be overridden by
annotating the method with either of
the <tt>ns_returns_autoreleased</tt> or
<tt>ns_returns_not_retained</tt> attributes.</p>
<div id="family.semantics.init">
<h1>Semantics of <tt>init</tt></h1>
<p>Methods in the <tt>init</tt> family implicitly
<a href="#objects.operands.consumed">consume</a> their <tt>self</tt>
parameter and <a href="#objects.operands.retained_returns">return a
retained object</a>. Neither of these properties can be altered
through attributes.</p>
<p>A call to an <tt>init</tt> method with a receiver that is either
<tt>self</tt> (possibly parenthesized or casted) or <tt>super</tt> is
called a <span class="term">delegate init call</span>. It is an error
for a delegate init call to be made except from an <tt>init</tt>
method, and excluding blocks within such methods.</p>
<p>As an exception to the <a href="misc.self">usual rule</a>, the
variable <tt>self</tt> is mutable in an <tt>init</tt> method and has
the usual semantics for a <tt>__strong</tt> variable. However, it is
undefined behavior and the program is ill-formed, no diagnostic
required, if an <tt>init</tt> method attempts to use the previous
value of <tt>self</tt> after the completion of a delegate init call.
It is conventional, but not required, for an <tt>init</tt> method to
return <tt>self</tt>.</p>
<p>It is undefined behavior for a program to cause two or more calls
to <tt>init</tt> methods on the same object, except that
each <tt>init</tt> method invocation may perform at most one delegate
init call.</p>
</div> <!-- family.semantics.init -->
<div id="family.semantics.result_type">
<h1>Related result types</h1>
<p>Certain methods are candidates to have <span class="term">related
result types</span>:</p>
<ul>
<li>class methods in the <tt>alloc</tt> and <tt>new</tt> method families</li>
<li>instance methods in the <tt>init</tt> family</li>
<li>the instance method <tt>self</tt></li>
<li>outside of ARC, the instance methods <tt>retain</tt> and <tt>autorelease</tt></li>
</ul>
<p>If the formal result type of such a method is <tt>id</tt> or
protocol-qualified <tt>id</tt>, 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:</p>
<ul>
<li>if it is a class method, and the receiver is a class
name <tt>T</tt>, the message send expression has type <tt>T*</tt>;
otherwise</li>
<li>if it is an instance method, and the receiver has type <tt>T</tt>,
the message send expression has type <tt>T</tt>; otherwise</li>
<li>the message send expression has the normal result type of the
method.</li>
</ul>
<p>This is a new rule of the Objective-C language and applies outside
of ARC.</p>
<div class="rationale"><p>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 risks, as well as catching
a number of latent bugs.</p></div>
</div> <!-- family.semantics.result_type -->
</div> <!-- family.semantics -->
</div> <!-- family -->
<div id="optimization">
<h1>Optimization</h1>
<p>ARC applies aggressive rules for the optimization of local
behavior. These rules are based around a core assumption of
<span class="term">local balancing</span>: 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 <q>balancing</q> retains, such that
the local retain/release pair is the only thing preventing the called
function from ending up with a dangling reference.</p>
<p>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.</p>
<p>The complete optimization rules are quite complicated, but it would
still be useful to document them here.</p>
</div>
<div id="misc">
<h1>Miscellaneous</h1>
<div id="misc.special_methods">
<h1>Special methods</h1>
<div id="misc.special_methods.retain">
<h1>Memory management methods</h1>
<p>A program is ill-formed if it contains a method definition, message
send, or <tt>@selector</tt> expression for any of the following
selectors:
<ul>
<li><tt>autorelease</tt></li>
<li><tt>release</tt></li>
<li><tt>retain</tt></li>
<li><tt>retainCount</tt></li>
</ul>
</p>
<div class="rationale"><p>Rationale: <tt>retainCount</tt> is banned
because ARC robs it of consistent semantics. The others were banned
after weighing three options for how to deal with message sends:</p>
<p><b>Honoring</b> 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 <q>unrooted</q>
retains, i.e. retains not logically corresponding to a strong
reference in the object graph.</p>
<p><b>Ignoring</b> them would badly violate user expectations about their
code. While it <em>would</em> 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.</p>
<p><b>Banning</b> 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.</p>
<p>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 <a href="#objects.retains">semantics</a>
laid out elsewhere in this document.</p>
</div>
</div> <!-- misc.special_methods.retain -->
<div id="misc.special_methods.dealloc">
<h1><tt>dealloc</tt></h1>
<p>A program is ill-formed if it contains a message send
or <tt>@selector</tt> expression for the selector <tt>dealloc</tt>.</p>
<div class="rationale"><p>Rationale: there are no legitimate reasons
to call <tt>dealloc</tt> directly.</p></div>
<p>A class may provide a method definition for an instance method
named <tt>dealloc</tt>. This method will be called after the final
<tt>release</tt> of the object but before it is deallocated or any of
its instance variables are destroyed. The superclass's implementation
of <tt>dealloc</tt> will be called automatically when the method
returns.</p>
<div class="rationale"><p>Rationale: even though ARC destroys instance
variables automatically, there are still legitimate reasons to write
a <tt>dealloc</tt> method, such as freeing non-retainable resources.
Failing to call <tt>[super&nbsp;dealloc]</tt> in such a method is nearly
always a bug. Sometimes, the object is simply trying to prevent
itself from being destroyed, but <tt>dealloc</tt> 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 <tt>free</tt>; for now, this can only be supported
with a <tt>dealloc</tt> implementation outside of ARC. Such an
implementation must be very careful to do all the other work
that <tt>NSObject</tt>'s <tt>dealloc</tt> would, which is outside the
scope of this document to describe.</p></div>
</div>
</div> <!-- misc.special_methods -->
<div id="autoreleasepool">
<h1><tt>@autoreleasepool</tt></h1>
<p>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 <tt>@autoreleasepool</tt> followed by
a <i>compound-statement</i>, 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 <tt>return</tt> or <tt>break</tt>), 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.</p>
<p><tt>@autoreleasepool</tt> may be used in non-ARC translation units,
with equivalent semantics.</p>
<p>A program is ill-formed if it refers to the
<tt>NSAutoreleasePool</tt> class.</p>
<div class="rationale"><p>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.</p></div>
</div> <!-- autoreleasepool -->
<div id="misc.self">
<h1><tt>self</tt></h1>
<p>The <tt>self</tt> 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, <tt>self</tt> is implicitly <tt>const</tt> unless the method is
in the <a href="#family.semantics.init"><tt>init</tt> family</a>.</p>
<div class="rationale"><p>Rationale: the cost of
retaining <tt>self</tt> 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 &mdash; 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 <tt>__unsafe_unretained</tt>), we chose to
make this optimizing assumption and shift some amount of risk to the
user.</p></div>
</div> <!-- misc.self -->
<div id="misc.enumeration">
<h1>Fast enumeration iteration variables</h1>
<p>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 <tt>const __strong</tt> and
objects encountered during the enumeration are not actually
retained.</p>
<div class="rationale"><p>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 <tt>__strong</tt>, which will make the variable
mutable again and cause the loop to retain the objects it
encounters.</div>
</div>
<div id="misc.blocks">
<h1>Blocks</h1>
<p>The implicit <tt>const</tt> 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.</p>
<p>The <a href="#ownership.inference">inference</a> rules apply
equally to <tt>__block</tt> variables, which is a shift in semantics
from non-ARC, where <tt>__block</tt> variables did not implicitly
retain during capture.</p>
<p><tt>__block</tt> 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.</tt></p>
<p>With the exception of retains done as part of initializing
a <tt>__strong</tt> parameter variable or reading a <tt>__weak</tt>
variable, whenever these semantics call for retaining a value of
block-pointer type, it has the effect of a <tt>Block_copy</tt>. The
optimizer may remove such copies when it sees that the result is
used only as an argument to a call.</p>
</div> <!-- misc.blocks -->
<div id="misc.exceptions">
<h1>Exceptions</h1>
<p>By default in Objective C, ARC is not exception-safe for normal
releases:
<ul>
<li>It does not end the lifetime of <tt>__strong</tt> variables when
their scopes are abnormally terminated by an exception.</li>
<li>It does not perform releases which would occur at the end of
a full-expression if that full-expression throws an exception.</li>
</ul>
<p>A program may be compiled with the option
<tt>-fobjc-arc-exceptions</tt> in order to enable these, or with the
option <tt>-fno-objc-arc-exceptions</tt> to explicitly disable them,
with the last such argument <q>winning</q>.</p>
<div class="rationale"><p>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.</p></div>
<p>In Objective-C++, <tt>-fobjc-arc-exceptions</tt> is enabled by
default.</p>
<div class="rationale"><p>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.</p></div>
<p>ARC does end the lifetimes of <tt>__weak</tt> objects when an
exception terminates their scope unless exceptions are disabled in the
compiler.</p>
<div class="rationale"><p>Rationale: the consequence of a
local <tt>__weak</tt> 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.</p></div>
</div> <!-- misc.exceptions -->
</div> <!-- misc -->
<div id="runtime">
<h1>Runtime support</h1>
<p>This section describes the requirements of the current ARC
implementation in Clang on the Objective-C runtime. It is not part of
the ARC language specification; instead, it is effectively a
language-specific ABI supplement, akin to the Itanium ABI for C++.</p>
<p>Ownership qualification does not alter the storage requirements for
an object, except that <tt>__weak</tt> objects must always be
appropriately aligned for an object of type <tt>id</tt>.</p>
<p>The runtime tracks <tt>__weak</tt> objects which holds non-null
values. It is undefined behavior to direct modify a <tt>__weak</tt>
object which is being tracked by the runtime except through an
<a href="#runtime.objc_storeWeak"><tt>objc_storeWeak</tt></a>,
<a href="#runtime.objc_destroyWeak"><tt>objc_destroyWeak</tt></a>,
or <a href="#runtime.objc_moveWeak"><tt>objc_moveWeak</tt></a>
call.</p>
<p>The runtime must provide a number of new entrypoints which the
compiler may emit, which are described in the remainder of this
section.</p>
<div class="rationale"><p>Rationale: Several of these functions are
semantically equivalent to a message send; we emit calls to C
functions instead because:</p>
<ul>
<li>the machine code to do so is significantly smaller,</li>
<li>it is much easier to recognize the C functions in the ARC optimizer, and</li>
<li>a sufficient sophisticated runtime may be able to avoid the
message send in common cases.</li>
</ul>
<p>Several other of these functions are <q>fused</q> 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.</p>
</div>
<div id="runtime.objc_autorelease">
<h1><tt>id objc_autorelease(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> 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 <tt>autorelease</tt> message.</p>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_autorelease -->
<div id="runtime.objc_autoreleasePoolPop">
<h1><tt>void objc_autoreleasePoolPop(void *pool);</tt></h1>
<p><i>Precondition:</i> <tt>pool</tt> is the result of a previous call to
<a href="runtime.objc_autoreleasePoolPush"><tt>objc_autoreleasePoolPush</tt></a>
on the current thread, where neither <tt>pool</tt> nor any enclosing
pool have previously been popped.</p>
<p>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 <tt>pool</tt>.</p>
</div> <!-- runtime.objc_autoreleasePoolPop -->
<div id="runtime.objc_autoreleasePoolPush">
<h1><tt>void *objc_autoreleasePoolPush(void);</tt></h1>
<p>Creates a new autorelease pool that is enclosed by the current
pool, makes that the current pool, and returns an opaque <q>handle</q>
to it.</p>
<div class="rationale"><p>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.</p></div>
</div> <!-- runtime.objc_autoreleasePoolPush -->
<div id="runtime.objc_autoreleaseReturnValue">
<h1><tt>id objc_autoreleaseReturnValue(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> 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 <a href="runtime.objc_retainAutoreleasedReturnValue"><tt>objc_retainAutoreleasedReturnValue</tt></a>
for the same object in an enclosing call frame. If this is not
possible, the object is autoreleased as above.</p>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_autoreleaseReturnValue -->
<div id="runtime.objc_copyWeak">
<h1><tt>void objc_copyWeak(id *dest, id *src);</tt></h1>
<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
contains a null pointer or has been registered as a <tt>__weak</tt>
object. <tt>dest</tt> is a valid pointer which has not been
registered as a <tt>__weak</tt> object.</p>
<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
potentially registering it with the runtime. Equivalent to the
following code:</p>
<pre>void objc_copyWeak(id *dest, id *src) {
objc_release(objc_initWeak(dest, objc_loadWeakRetained(src)));
}</pre>
<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
on <tt>src</tt>.</p>
</div> <!-- runtime.objc_copyWeak -->
<div id="runtime.objc_destroyWeak">
<h1><tt>void objc_destroyWeak(id *object);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
either contains a null pointer or has been registered as
a <tt>__weak</tt> object.</p>
<p><tt>object</tt> is unregistered as a weak object, if it ever was.
The current value of <tt>object</tt> is left unspecified; otherwise,
equivalent to the following code:</p>
<pre>void objc_destroyWeak(id *object) {
objc_storeWeak(object, nil);
}</pre>
<p>Does not need to be atomic with respect to calls
to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
</div> <!-- runtime.objc_destroyWeak -->
<div id="runtime.objc_initWeak">
<h1><tt>id objc_initWeak(id *object, id value);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which has
not been registered as a <tt>__weak</tt> object. <tt>value</tt> is
null or a pointer to a valid object.</p>
<p>If <tt>value</tt> is a null pointer or the object to which it
points has begun deallocation, <tt>object</tt> is zero-initialized.
Otherwise, <tt>object</tt> is registered as a <tt>__weak</tt> object
pointing to <tt>value</tt>. Equivalent to the following code:</p>
<pre>id objc_initWeak(id *object, id value) {
*object = nil;
return objc_storeWeak(object, value);
}</pre>
<p>Returns the value of <tt>object</tt> after the call.</p>
<p>Does not need to be atomic with respect to calls
to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
</div> <!-- runtime.objc_initWeak -->
<div id="runtime.objc_loadWeak">
<h1><tt>id objc_loadWeak(id *object);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
either contains a null pointer or has been registered as
a <tt>__weak</tt> object.</p>
<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
the last value stored into <tt>object</tt> has not yet been
deallocated or begun deallocation, retains and autoreleases that value
and returns it. Otherwise returns null. Equivalent to the following
code:</p>
<pre>id objc_loadWeak(id *object) {
return objc_autorelease(objc_loadWeakRetained(object));
}</pre>
<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
on <tt>object</tt>.</p>
<div class="rationale">Rationale: loading weak references would be
inherently prone to race conditions without the retain.</div>
</div> <!-- runtime.objc_loadWeak -->
<div id="runtime.objc_loadWeakRetained">
<h1><tt>id objc_loadWeakRetained(id *object);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
either contains a null pointer or has been registered as
a <tt>__weak</tt> object.</p>
<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
the last value stored into <tt>object</tt> has not yet been
deallocated or begun deallocation, retains that value and returns it.
Otherwise returns null.</p>
<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
on <tt>object</tt>.</p>
</div> <!-- runtime.objc_loadWeakRetained -->
<div id="runtime.objc_moveWeak">
<h1><tt>void objc_moveWeak(id *dest, id *src);</tt></h1>
<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
contains a null pointer or has been registered as a <tt>__weak</tt>
object. <tt>dest</tt> is a valid pointer which has not been
registered as a <tt>__weak</tt> object.</p>
<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
potentially registering it with the runtime. <tt>src</tt> may then be
left in its original state, in which case this call is equivalent
to <a href="#runtime.objc_copyWeak"><tt>objc_copyWeak</tt></a>, or it
may be left as null.</p>
<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
on <tt>src</tt>.</p>
</div> <!-- runtime.objc_moveWeak -->
<div id="runtime.objc_retain">
<h1><tt>id objc_retain(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
performs a retain operation exactly as if the object had been sent
the <tt>retain</tt> message.</p>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_retain -->
<div id="runtime.objc_retainAutorelease">
<h1><tt>id objc_retainAutorelease(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
performs a retain operation followed by an autorelease operation.
Equivalent to the following code:</p>
<pre>id objc_retainAutorelease(id value) {
return objc_autorelease(objc_retain(value));
}</pre>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_retainAutorelease -->
<div id="runtime.objc_retainAutoreleaseReturnValue">
<h1><tt>id objc_retainAutoreleaseReturnValue(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
performs a retain operation followed by the operation described in
<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>.
Equivalent to the following code:</p>
<pre>id objc_retainAutoreleaseReturnValue(id value) {
return objc_autoreleaseReturnValue(objc_retain(value));
}</pre>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_retainAutoreleaseReturnValue -->
<div id="runtime.objc_retainAutoreleasedReturnValue">
<h1><tt>id objc_retainAutoreleasedReturnValue(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
attempts to accept a hand off of a retain count from a call to
<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>
on <tt>value</tt> in a recently-called function or something it
calls. If that fails, it performs a retain operation exactly
like <a href="#runtime.objc_retain"><tt>objc_retain</tt></a>.</p>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_retainAutoreleasedReturnValue -->
<div id="runtime.objc_retainBlock">
<h1><tt>id objc_retainBlock(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid block object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, if
the block pointed to by <tt>value</tt> 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 <tt>retain</tt> message.</p>
</div> <!-- runtime.objc_retainBlock -->
<div id="runtime.objc_release">
<h1><tt>void objc_release(id value);</tt></h1>
<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
performs a release operation exactly as if the object had been sent
the <tt>release</tt> message.</p>
</div> <!-- runtime.objc_release -->
<div id="runtime.objc_storeStrong">
<h1><tt>id objc_storeStrong(id *object, id value);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer to
a <tt>__strong</tt> object which is adequately aligned for a
pointer. <tt>value</tt> is null or a pointer to a valid object.</p>
<p>Performs the complete sequence for assigning to a <tt>__strong</tt>
object of non-block type. Equivalent to the following code:</p>
<pre>id objc_storeStrong(id *object, id value) {
value = [value retain];
id oldValue = *object;
*object = value;
[oldValue release];
return value;
}</pre>
<p>Always returns <tt>value</tt>.</p>
</div> <!-- runtime.objc_storeStrong -->
<div id="runtime.objc_storeWeak">
<h1><tt>id objc_storeWeak(id *object, id value);</tt></h1>
<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
either contains a null pointer or has been registered as
a <tt>__weak</tt> object. <tt>value</tt> is null or a pointer to a
valid object.</p>
<p>If <tt>value</tt> is a null pointer or the object to which it
points has begun deallocation, <tt>object</tt> is assigned null
and unregistered as a <tt>__weak</tt> object. Otherwise,
<tt>object</tt> is registered as a <tt>__weak</tt> object or has its
registration updated to point to <tt>value</tt>.</p>
<p>Returns the value of <tt>object</tt> after the call.</p>
</div> <!-- runtime.objc_storeWeak -->
</div> <!-- runtime -->
</div> <!-- root -->
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