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				81
				82	<div id="content">
				83	<h1>Automatic Reference Counting</h1>
				84
				85	<div id="toc">
				86	</div>
				87
				88	<div id="meta">
				89	<h1>About this document</h1>
				90
				91	<div id="meta.purpose">
				92	<h1>Purpose</h1>
				93
				94	<p>The first and primary purpose of this document is to serve as a
				95	complete technical specification of Automatic Reference Counting.
				96	Given a core Objective-C compiler and runtime, it should be possible
				97	to write a compiler and runtime which implements these new
				98	semantics.</p>
				99
				100	<p>The secondary purpose is to act as a rationale for why ARC was
				101	designed in this way. This should remain tightly focused on the
				102	technical design and should not stray into marketing speculation.</p>
				103
				104	</div> <!-- meta.purpose -->
				105
				106	<div id="meta.background">
				107	<h1>Background</h1>
				108
				109	<p>This document assumes a basic familiarity with C.</p>
				110
				111	<p><span class="term">Blocks</span> are a C language extension for
				112	creating anonymous functions. Users interact with and transfer block
				113	objects using <span class="term">block pointers</span>, which are
				114	represented like a normal pointer. A block may capture values from
				115	local variables; when this occurs, memory must be dynamically
				116	allocated. The initial allocation is done on the stack, but the
				117	runtime provides a <tt>Block_copy</tt> function which, given a block
				118	pointer, either copies the underlying block object to the heap,
				119	setting its reference count to 1 and returning the new block pointer,
				120	or (if the block object is already on the heap) increases its
				121	reference count by 1. The paired function is <tt>Block_release</tt>,
				122	which decreases the reference count by 1 and destroys the object if
				123	the count reaches zero and is on the heap.</p>
				124
				125	<p>Objective-C is a set of language extensions, significant enough to
				126	be considered a different language. It is a strict superset of C.
				127	The extensions can also be imposed on C++, producing a language called
				128	Objective-C++. The primary feature is a single-inheritance object
				129	system; we briefly describe the modern dialect.</p>
				130
				131	<p>Objective-C defines a new type kind, collectively called
				132	the <span class="term">object pointer types</span>. This kind has two
				133	notable builtin members, <tt>id</tt> and <tt>Class</tt>; <tt>id</tt>
				134	is the final supertype of all object pointers. The validity of
				135	conversions between object pointer types is not checked at runtime.
				136	Users may define <span class="term">classes</span>; each class is a
				137	type, and the pointer to that type is an object pointer type. A class
				138	may have a superclass; its pointer type is a subtype of its
				139	superclass's pointer type. A class has a set
				140	of <span class="term">ivars</span>, fields which appear on all
				141	instances of that class. For every class <i>T</i> there's an
				142	associated metaclass; it has no fields, its superclass is the
				143	metaclass of <i>T</i>'s superclass, and its metaclass is a global
				144	class. Every class has a global object whose class is the
				145	class's metaclass; metaclasses have no associated type, so pointers to
				146	this object have type <tt>Class</tt>.</p>
				147
				148	<p>A class declaration (<tt>@interface</tt>) declares a set
				149	of <span class="term">methods</span>. A method has a return type, a
				150	list of argument types, and a <span class="term">selector</span>: a
				151	name like <tt>foo:bar:baz:</tt>, where the number of colons
				152	corresponds to the number of formal arguments. A method may be an
				153	instance method, in which case it can be invoked on objects of the
				154	class, or a class method, in which case it can be invoked on objects
				155	of the metaclass. A method may be invoked by providing an object
				156	(called the <span class="term">receiver</span>) and a list of formal
				157	arguments interspersed with the selector, like so:</p>
				158
				159	<pre>[receiver foo: fooArg bar: barArg baz: bazArg]</pre>
				160
				161	<p>This looks in the dynamic class of the receiver for a method with
				162	this name, then in that class's superclass, etc., until it finds
				163	something it can execute. The receiver <q>expression</q> may also be
				164	the name of a class, in which case the actual receiver is the class
				165	object for that class, or (within method definitions) it may
				166	be <tt>super</tt>, in which case the lookup algorithm starts with the
				167	static superclass instead of the dynamic class. The actual methods
				168	dynamically found in a class are not those declared in the
				169	<tt>@interface</tt>, but those defined in a separate
				170	<tt>@implementation</tt> declaration; however, when compiling a
				171	call, typechecking is done based on the methods declared in the
				172	<tt>@interface</tt>.</p>
				173
				174	<p>Method declarations may also be grouped into
				175	<span class="term">protocols</span>, which are not inherently
				176	associated with any class, but which classes may claim to follow.
				177	Object pointer types may be qualified with additional protocols that
				178	the object is known to support.</p>
				179
				180	<p><span class="term">Class extensions</span> are collections of ivars
				181	and methods, designed to allow a class's <tt>@interface</tt> to be
				182	split across multiple files; however, there is still a primary
				183	implementation file which must see the <tt>@interface</tt>s of all
				184	class extensions.
				185	<span class="term">Categories</span> allow methods (but not ivars) to
				186	be declared <i>post hoc</i> on an arbitrary class; the methods in the
				187	category's <tt>@implementation</tt> will be dynamically added to that
				188	class's method tables which the category is loaded at runtime,
				189	replacing those methods in case of a collision.</p>
				190
				191	<p>In the standard environment, objects are allocated on the heap, and
				192	their lifetime is manually managed using a reference count. This is
				193	done using two instance methods which all classes are expected to
				194	implement: <tt>retain</tt> increases the object's reference count by
				195	1, whereas <tt>release</tt> decreases it by 1 and calls the instance
				196	method <tt>dealloc</tt> if the count reaches 0. To simplify certain
				197	operations, there is also an <span class="term">autorelease
				198	pool</span>, a thread-local list of objects to call <tt>release</tt>
				199	on later; an object can be added to this pool by
				200	calling <tt>autorelease</tt> on it.</p>
				201
				202	<p>Block pointers may be converted to type <tt>id</tt>; block objects
				203	are laid out in a way that makes them compatible with Objective-C
				204	objects. There is a builtin class that all block objects are
				205	considered to be objects of; this class implements <tt>retain</tt> by
				206	adjusting the reference count, not by calling <tt>Block_copy</tt>.</p>
				207
				208	</div> <!-- meta.background -->
				209
				210	</div> <!-- meta -->
				211
				212	<div id="general">
				213	<h1>General</h1>
				214
				215	<p>Automatic Reference Counting implements automatic memory management
				216	for Objective-C objects and blocks, freeing the programmer from the
				217	need explicitly insert retains and releases. It does not provide a
				218	cycle collector; users must explicitly manage lifetime instead.</p>
				219
				220	<p>ARC may be explicitly enabled with the compiler
				221	flag <tt>-fobjc-arc</tt>. It may also be explicitly disabled with the
				222	compiler flag <tt>-fno-objc-arc</tt>. The last of these two flags
				223	appearing on the compile line <q>wins</q>.</p>
				224
				225	<p>If ARC is enabled, <tt>__has_feature(objc_arc)</tt> will expand to
				226	1 in the preprocessor. For more information about <tt>__has_feature</tt>,
				227	see the <a href="LanguageExtensions.html#__has_feature_extension">language
				228	extensions</a> document.</p>
				229
				230	</div>
				231
				232	<div id="objects">
				233	<h1>Retainable object pointers</h1>
				234
				235	<p>This section describes retainable object pointers, their basic
				236	operations, and the restrictions imposed on their use under ARC. Note
				237	in particular that it covers the rules for pointer <em>values</em>
				238	(patterns of bits indicating the location of a pointed-to object), not
				239	pointer
				240	<em>objects</em> (locations in memory which store pointer values).
				241	The rules for objects are covered in the next section.</p>
				242
				243	<p>A <span class="term">retainable object pointer</span>
				244	(or <q>retainable pointer</q>) is a value of
				245	a <span class="term">retainable object pointer type</span>
				246	(<q>retainable type</q>). There are three kinds of retainable object
				247	pointer types:</p>
				248	<ul>
				249	<li>block pointers (formed by applying the caret (<tt>^</tt>)
				250	declarator sigil to a function type)</li>
				251	<li>Objective-C object pointers (<tt>id</tt>, <tt>Class</tt>, <tt>NSFoo*</tt>, etc.)</li>
				252	<li>typedefs marked with <tt>__attribute__((NSObject))</tt></li>
				253	</ul>
				254
				255	<p>Other pointer types, such as <tt>int*</tt> and <tt>CFStringRef</tt>,
				256	are not subject to ARC's semantics and restrictions.</p>
				257
				258	<div class="rationale">
				259
				260	<p>Rationale: We are not at liberty to require
				261	all code to be recompiled with ARC; therefore, ARC must interoperate
				262	with Objective-C code which manages retains and releases manually. In
				263	general, there are three requirements in order for a
				264	compiler-supported reference-count system to provide reliable
				265	interoperation:</p>
				266
				267	<ul>
				268	<li>The type system must reliably identify which objects are to be
				269	managed. An <tt>int*</tt> might be a pointer to a <tt>malloc</tt>'ed
				270	array, or it might be a interior pointer to such an array, or it might
				271	point to some field or local variable. In contrast, values of the
				272	retainable object pointer types are never interior.</li>
				273	<li>The type system must reliably indicate how to
				274	manage objects of a type. This usually means that the type must imply
				275	a procedure for incrementing and decrementing retain counts.
				276	Supporting single-ownership objects requires a lot more explicit
				277	mediation in the language.</li>
				278	<li>There must be reliable conventions for whether and
				279	when <q>ownership</q> is passed between caller and callee, for both
				280	arguments and return values. Objective-C methods follow such a
				281	convention very reliably, at least for system libraries on Mac OS X,
				282	and functions always pass objects at +0. The C-based APIs for Core
				283	Foundation objects, on the other hand, have much more varied transfer
				284	semantics.</li>
				285	</ul>
				286	</div> <!-- rationale -->
				287
				288	<p>The use of <tt>__attribute__((NSObject))</tt> typedefs is not
				289	recommended. If it's absolutely necessary to use this attribute, be
				290	very explicit about using the typedef, and do not assume that it will
				291	be preserved by language features like <tt>__typeof</tt> and C++
				292	template argument substitution.</p>
				293
				294	<div class="rationale"><p>Rationale: any compiler operation which
				295	incidentally strips type <q>sugar</q> from a type will yield a type
				296	without the attribute, which may result in unexpected
				297	behavior.</p></div>
				298
				299	<div id="objects.retains">
				300	<h1>Retain count semantics</h1>
				301
				302	<p>A retainable object pointer is either a <span class="term">null
				303	pointer</span> or a pointer to a valid object. Furthermore, if it has
				304	block pointer type and is not <tt>null</tt> then it must actually be a
				305	pointer to a block object, and if it has <tt>Class</tt> type (possibly
				306	protocol-qualified) then it must actually be a pointer to a class
				307	object. Otherwise ARC does not enforce the Objective-C type system as
				308	long as the implementing methods follow the signature of the static
				309	type. It is undefined behavior if ARC is exposed to an invalid
				310	pointer.</p>
				311
				312	<p>For ARC's purposes, a valid object is one with <q>well-behaved</q>
				313	retaining operations. Specifically, the object must be laid out such
				314	that the Objective-C message send machinery can successfully send it
				315	the following messages:</p>
				316
				317	<ul>
				318	<li><tt>retain</tt>, taking no arguments and returning a pointer to
				319	the object.</li>
				320	<li><tt>release</tt>, taking no arguments and returning <tt>void</tt>.</li>
				321	<li><tt>autorelease</tt>, taking no arguments and returning a pointer
				322	to the object.</li>
				323	</ul>
				324
				325	<p>The behavior of these methods is constrained in the following ways.
				326	The term <span class="term">high-level semantics</span> is an
				327	intentionally vague term; the intent is that programmers must
				328	implement these methods in a way such that the compiler, modifying
				329	code in ways it deems safe according to these constraints, will not
				330	violate their requirements. For example, if the user puts logging
				331	statements in <tt>retain</tt>, they should not be surprised if those
				332	statements are executed more or less often depending on optimization
				333	settings. These constraints are not exhaustive of the optimization
				334	opportunities: values held in local variables are subject to
				335	additional restrictions, described later in this document.</p>
				336
				337	<p>It is undefined behavior if a computation history featuring a send
				338	of <tt>retain</tt> followed by a send of <tt>release</tt> to the same
				339	object, with no intervening <tt>release</tt> on that object, is not
				340	equivalent under the high-level semantics to a computation
				341	history in which these sends are removed. Note that this implies that
				342	these methods may not raise exceptions.</p>
				343
				344	<p>It is undefined behavior if a computation history features any use
				345	whatsoever of an object following the completion of a send
				346	of <tt>release</tt> that is not preceded by a send of <tt>retain</tt>
				347	to the same object.</p>
				348
				349	<p>The behavior of <tt>autorelease</tt> must be equivalent to sending
				350	<tt>release</tt> when one of the autorelease pools currently in scope
				351	is popped. It may not throw an exception.</p>
				352
				353	<p>When the semantics call for performing one of these operations on a
				354	retainable object pointer, if that pointer is <tt>null</tt> then the
				355	effect is a no-op.</p>
				356
				357	<p>All of the semantics described in this document are subject to
				358	additional <a href="#optimization">optimization rules</a> which permit
				359	the removal or optimization of operations based on local knowledge of
				360	data flow. The semantics describe the high-level behaviors that the
				361	compiler implements, not an exact sequence of operations that a
				362	program will be compiled into.</p>
				363
				364	</div> <!-- objects.retains -->
				365
				366	<div id="objects.operands">
				367	<h1>Retainable object pointers as operands and arguments</h1>
				368
				369	<p>In general, ARC does not perform retain or release operations when
				370	simply using a retainable object pointer as an operand within an
				371	expression. This includes:</p>
				372	<ul>
				373	<li>loading a retainable pointer from an object with non-weak
				374	<a href="#ownership">ownership</a>,</li>
				375	<li>passing a retainable pointer as an argument to a function or
				376	method, and</li>
				377	<li>receiving a retainable pointer as the result of a function or
				378	method call.</li>
				379	</ul>
				380
				381	<div class="rationale"><p>Rationale: while this might seem
				382	uncontroversial, it is actually unsafe when multiple expressions are
				383	evaluated in <q>parallel</q>, as with binary operators and calls,
				384	because (for example) one expression might load from an object while
				385	another writes to it. However, C and C++ already call this undefined
				386	behavior because the evaluations are unsequenced, and ARC simply
				387	exploits that here to avoid needing to retain arguments across a large
				388	number of calls.</p></div>
				389
				390	<p>The remainder of this section describes exceptions to these rules,
				391	how those exceptions are detected, and what those exceptions imply
				392	semantically.</p>
				393
				394	<div id="objects.operands.consumed">
				395	<h1>Consumed parameters</h1>
				396
				397	<p>A function or method parameter of retainable object pointer type
				398	may be marked as <span class="term">consumed</span>, signifying that
				399	the callee expects to take ownership of a +1 retain count. This is
				400	done by adding the <tt>ns_consumed</tt> attribute to the parameter
				401	declaration, like so:</p>
				402
				403	<pre>void foo(__attribute((ns_consumed)) id x);
				404	- (void) foo: (id) __attribute((ns_consumed)) x;</pre>
				405
				406	<p>This attribute is part of the type of the function or method, not
				407	the type of the parameter. It controls only how the argument is
				408	passed and received.</p>
				409
				410	<p>When passing such an argument, ARC retains the argument prior to
				411	making the call.</p>
				412
				413	<p>When receiving such an argument, ARC releases the argument at the
				414	end of the function, subject to the usual optimizations for local
				415	values.</p>
				416
				417	<div class="rationale"><p>Rationale: this formalizes direct transfers
				418	of ownership from a caller to a callee. The most common scenario here
				419	is passing the <tt>self</tt> parameter to <tt>init</tt>, but it is
				420	useful to generalize. Typically, local optimization will remove any
				421	extra retains and releases: on the caller side the retain will be
				422	merged with a +1 source, and on the callee side the release will be
				423	rolled into the initialization of the parameter.</p></div>
				424
				425	<p>The implicit <tt>self</tt> parameter of a method may be marked as
				426	consumed by adding <tt>__attribute__((ns_consumes_self))</tt> to the
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	427	method declaration. Methods in the <tt>init</tt>
				428	<a href="#family">family</a> are treated as if they were implicitly
				429	marked with this attribute.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	430
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	431	<p>It is undefined behavior if an Objective-C message send to a method
				432	with <tt>ns_consumed</tt> parameters (other than self) is made with a
				433	null receiver. It is undefined behavior if the method to which an
				434	Objective-C message send statically resolves to has a different set
				435	of <tt>ns_consumed</tt> parameters than the method it dynamically
				436	resolves to. It is undefined behavior if a block or function call is
				437	made through a static type with a different set of <tt>ns_consumed</tt>
				438	parameters than the implementation of the called block or function.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	439
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	440	<div class="rationale"><p>Rationale: consumed parameters with null
				441	receiver are a guaranteed leak. Mismatches with consumed parameters
				442	will cause over-retains or over-releases, depending on the direction.
				443	The rule about function calls is really just an application of the
				444	existing C/C++ rule about calling functions through an incompatible
				445	function type, but it's useful to state it explicitly.</p></div>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	446
				447	</div>
				448
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	449	<div id="objects.operands.retained_returns">
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	450	<h1>Retained return values</h1>
				451
				452	<p>A function or method which returns a retainable object pointer type
				453	may be marked as returning a retained value, signifying that the
				454	caller expects to take ownership of a +1 retain count. This is done
				455	by adding the <tt>ns_returns_retained</tt> attribute to the function or
				456	method declaration, like so:</p>
				457
				458	<pre>id foo(void) __attribute((ns_returns_retained));
				459	- (id) foo __attribute((ns_returns_retained));</pre>
				460
				461	<p>This attribute is part of the type of the function or method.</p>
				462
				463	<p>When returning from such a function or method, ARC retains the
				464	value at the point of evaluation of the return statement, before
				465	leaving all local scopes.</p>
				466
				467	<p>When receiving a return result from such a function or method, ARC
				468	releases the value at the end of the full-expression it is contained
				469	within, subject to the usual optimizations for local values.</p>
				470
				471	<div class="rationale"><p>Rationale: this formalizes direct transfers of
				472	ownership from a callee to a caller. The most common scenario this
				473	models is the retained return from <tt>init</tt>, <tt>alloc</tt>,
				474	<tt>new</tt>, and <tt>copy</tt> methods, but there are other cases in
				475	the frameworks. After optimization there are typically no extra
				476	retains and releases required.</p></div>
				477
				478	<p>Methods in
				479	the <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>, <tt>mutableCopy</tt>,
				480	and <tt>new</tt> <a href="#family">families</a> are implicitly marked
				481	<tt>__attribute__((ns_returns_retained))</tt>. This may be suppressed
				482	by explicitly marking the
				483	method <tt>__attribute__((ns_returns_not_retained))</tt>.</p>
				484	</div>
				485
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	486	<p>It is undefined behavior if the method to which an Objective-C
				487	message send statically resolves has different retain semantics on its
				488	result from the method it dynamically resolves to. It is undefined
				489	behavior if a block or function call is made through a static type
				490	with different retain semantics on its result from the implementation
				491	of the called block or function.</p>
				492
				493	<div class="rationale"><p>Rationale: Mismatches with returned results
				494	will cause over-retains or over-releases, depending on the direction.
				495	Again, the rule about function calls is really just an application of
				496	the existing C/C++ rule about calling functions through an
				497	incompatible function type.</p></div>
				498
				499
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	500	<div id="objects.operands.other-returns">
				501	<h1>Unretained return values</h1>
				502
				503	<p>A method or function which returns a retainable object type but
				504	does not return a retained value must ensure that the object is
				505	still valid across the return boundary.</p>
				506
				507	<p>When returning from such a function or method, ARC retains the
				508	value at the point of evaluation of the return statement, then leaves
				509	all local scopes, and then balances out the retain while ensuring that
				510	the value lives across the call boundary. In the worst case, this may
				511	involve an <tt>autorelease</tt>, but callers must not assume that the
				512	value is actually in the autorelease pool.</p>
				513
				514	<p>ARC performs no extra mandatory work on the caller side, although
				515	it may elect to do something to shorten the lifetime of the returned
				516	value.</p>
				517
				518	<div class="rationale"><p>Rationale: it is common in non-ARC code to not
				519	return an autoreleased value; therefore the convention does not force
				520	either path. It is convenient to not be required to do unnecessary
				521	retains and autoreleases; this permits optimizations such as eliding
				522	retain/autoreleases when it can be shown that the original pointer
				523	will still be valid at the point of return.</p></div>
				524
				525	<p>A method or function may be marked
				526	with <tt>__attribute__((ns_returns_autoreleased))</tt> to indicate
				527	that it returns a pointer which is guaranteed to be valid at least as
				528	long as the innermost autorelease pool. There are no additional
				529	semantics enforced in the definition of such a method; it merely
				530	enables optimizations in callers.</p>
				531	</div>
				532
				533	<div id="objects.operands.casts">
				534	<h1>Bridged casts</h1>
				535
				536	<p>A <span class="term">bridged cast</span> is a C-style cast
				537	annotated with one of three keywords:</p>
				538
				539	<ul>
				540	<li><tt>(__bridge T) op</tt> casts the operand to the destination
				541	type <tt>T</tt>. If <tt>T</tt> is a retainable object pointer type,
				542	then <tt>op</tt> must have a non-retainable pointer type.
				543	If <tt>T</tt> is a non-retainable pointer type, then <tt>op</tt> must
				544	have a retainable object pointer type. Otherwise the cast is
				545	ill-formed. There is no transfer of ownership, and ARC inserts
				546	no retain operations.</li>
				547
				548	<li><tt>(__bridge_retained T) op</tt> casts the operand, which must
				549	have retainable object pointer type, to the destination type, which
				550	must be a non-retainable pointer type. ARC retains the value, subject
				551	to the usual optimizations on local values, and the recipient is
				552	responsible for balancing that +1.</li>
				553
				554	<li><tt>(__bridge_transfer T) op</tt> casts the operand, which must
				555	have non-retainable pointer type, to the destination type, which must
				556	be a retainable object pointer type. ARC will release the value at
				557	the end of the enclosing full-expression, subject to the usual
				558	optimizations on local values.</li>
				559	</ul>
				560
				561	<p>These casts are required in order to transfer objects in and out of
				562	ARC control; see the rationale in the section
				563	on <a href="#objects.restrictions.conversion">conversion of retainable
				564	object pointers</a>.</p>
				565
				566	<p>Using a <tt>__bridge_retained</tt> or <tt>__bridge_transfer</tt>
				567	cast purely to convince ARC to emit an unbalanced retain or release,
				568	respectively, is poor form.</p>
				569
				570	</div>
				571
				572	</div>
				573
				574	<div id="objects.restrictions">
				575	<h1>Restrictions</h1>
				576
				577	<div id="objects.restrictions.conversion">
				578	<h1>Conversion of retainable object pointers</h1>
				579
				580	<p>In general, a program which attempts to implicitly or explicitly
				581	convert a value of retainable object pointer type to any
				582	non-retainable type, or vice-versa, is ill-formed. For example, an
Fariborz Jahanian	a26b2e5	2011-07-06 21:58:44 +0000	[diff] [blame]	583	Objective-C object pointer shall not be converted to <tt>void*</tt>.
				584	As an exception, cast to <tt>intptr_t</tt> is allowed becuase such
				585	casts are not transferring ownership. The <a href="#objects.operands.casts">bridged
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	586	casts</a> may be used to perform these conversions where
				587	necessary.</p>
				588
				589	<div class="rationale"><p>Rationale: we cannot ensure the correct
				590	management of the lifetime of objects if they may be freely passed
				591	around as unmanaged types. The bridged casts are provided so that the
				592	programmer may explicitly describe whether the cast transfers control
				593	into or out of ARC.</p></div>
				594	</div>
				595
				596	<p>An unbridged cast to a retainable object pointer type of the return
				597	value of a Objective-C message send which yields a non-retainable
				598	pointer is treated as a <tt>__bridge_transfer</tt> cast
				599	if:</p>
				600
				601	<ul>
				602	<li>the method has the <tt>cf_returns_retained</tt> attribute, or if
				603	not that,</li>
				604	<li>the method does not have the <tt>cf_returns_not_retained</tt>
				605	attribute and</li>
				606	<li>the method's <a href="#family">selector family</a> would imply
				607	the <tt>ns_returns_retained</tt> attribute on a method which returned
				608	a retainable object pointer type.</li>
				609	</ul>
				610
				611	<p>Otherwise the cast is treated as a <tt>__bridge</tt> cast.</p>
				612
				613	</div>
				614
				615	</div>
				616
				617	<div id="ownership">
				618	<h1>Ownership qualification</h1>
				619
				620	<p>This section describes the behavior of <em>objects</em> of
				621	retainable object pointer type; that is, locations in memory which
				622	store retainable object pointers.</p>
				623
				624	<p>A type is a <span class="term">retainable object owner type</span>
				625	if it is a retainable object pointer type or an array type whose
				626	element type is a retainable object owner type.</p>
				627
				628	<p>An <span class="term">ownership qualifier</span> is a type
Douglas Gregor	4020cae	2011-06-17 23:16:24 +0000	[diff] [blame]	629	qualifier which applies only to retainable object owner types. An array type is
				630	ownership-qualified according to its element type, and adding an ownership
				631	qualifier to an array type so qualifies its element type.</p>
				632
				633	<p>A program is ill-formed if it attempts to apply an ownership qualifier
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	634	to a type which is already ownership-qualified, even if it is the same
Douglas Gregor	4020cae	2011-06-17 23:16:24 +0000	[diff] [blame]	635	qualifier. There is a single exception to this rule: an ownership qualifier
				636	may be applied to a substituted template type parameter, which overrides the
				637	ownership qualifier provided by the template argument.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	638
				639	<p>Except as described under
				640	the <a href="#ownership.inference">inference rules</a>, a program is
				641	ill-formed if it attempts to form a pointer or reference type to a
				642	retainable object owner type which lacks an ownership qualifier.</p>
				643
				644	<div class="rationale"><p>Rationale: these rules, together with the
				645	inference rules, ensure that all objects and lvalues of retainable
Douglas Gregor	4020cae	2011-06-17 23:16:24 +0000	[diff] [blame]	646	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>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	647
				648	<p>There are four ownership qualifiers:</p>
				649
				650	<ul>
				651	<li><tt>__autoreleasing</tt></li>
				652	<li><tt>__strong</tt></li>
				653	<li><tt>__unsafe_unretained</tt></li>
				654	<li><tt>__weak</tt></li>
				655	</ul>
				656
				657	<p>A type is <span class="term">nontrivially ownership-qualified</span>
				658	if it is qualified with <tt>__autoreleasing</tt>, <tt>__strong</tt>, or
				659	<tt>__weak</tt>.</p>
				660
				661	<div id="ownership.spelling">
				662	<h1>Spelling</h1>
				663
				664	<p>The names of the ownership qualifiers are reserved for the
				665	implementation. A program may not assume that they are or are not
				666	implemented with macros, or what those macros expand to.</p>
				667
				668	<p>An ownership qualifier may be written anywhere that any other type
				669	qualifier may be written.</p>
				670
				671	<p>If an ownership qualifier appears in
				672	the <i>declaration-specifiers</i>, the following rules apply:</p>
				673
				674	<ul>
				675	<li>if the type specifier is a retainable object owner type, the
				676	qualifier applies to that type;</li>
				677	<li>if the outermost non-array part of the declarator is a pointer or
				678	block pointer, the qualifier applies to that type;</li>
				679	<li>otherwise the program is ill-formed.</li>
				680	</ul>
				681
				682	<p>If an ownership qualifier appears on the declarator name, or on the
				683	declared object, it is applied to outermost pointer or block-pointer
				684	type.</p>
				685
				686	<p>If an ownership qualifier appears anywhere else in a declarator, it
				687	applies to the type there.</p>
				688
				689	</div> <!-- ownership.spelling -->
				690
				691	<div id="ownership.semantics">
				692	<h1>Semantics</h1>
				693
				694	<p>There are five <span class="term">managed operations</span> which
				695	may be performed on an object of retainable object pointer type. Each
				696	qualifier specifies different semantics for each of these operations.
				697	It is still undefined behavior to access an object outside of its
				698	lifetime.</p>
				699
				700	<p>A load or store with <q>primitive semantics</q> has the same
				701	semantics as the respective operation would have on an <tt>void*</tt>
				702	lvalue with the same alignment and non-ownership qualification.</p>
				703
				704	<p><span class="term">Reading</span> occurs when performing a
				705	lvalue-to-rvalue conversion on an object lvalue.
				706
				707	<ul>
				708	<li>For <tt>__weak</tt> objects, the current pointee is retained and
				709	then released at the end of the current full-expression. This must
				710	execute atomically with respect to assignments and to the final
				711	release of the pointee.</li>
				712	<li>For all other objects, the lvalue is loaded with primitive
				713	semantics.</li>
				714	</ul>
				715	</p>
				716
				717	<p><span class="term">Assignment</span> occurs when evaluating
				718	an assignment operator. The semantics vary based on the qualification:
				719	<ul>
				720	<li>For <tt>__strong</tt> objects, the new pointee is first retained;
				721	second, the lvalue is loaded with primitive semantics; third, the new
				722	pointee is stored into the lvalue with primitive semantics; and
				723	finally, the old pointee is released. This is not performed
				724	atomically; external synchronization must be used to make this safe in
				725	the face of concurrent loads and stores.</li>
				726	<li>For <tt>__weak</tt> objects, the lvalue is updated to point to the
				727	new pointee, unless that object is currently undergoing deallocation,
				728	in which case it the lvalue is updated to a null pointer. This must
				729	execute atomically with respect to other assignments to the object, to
				730	reads from the object, and to the final release of the new pointed-to
Fariborz Jahanian	a236c0e	2011-07-06 22:56:44 +0000	[diff] [blame]	731	value. If class is not compatible with weak references,
				732	<tt>objc_arc_weak_unavailable</tt>
				733	attribute must be specified on class declaration to disallow weak
				734	references to their objects.</li>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	735	<li>For <tt>__unsafe_unretained</tt> objects, the new pointee is
				736	stored into the lvalue using primitive semantics.</li>
				737	<li>For <tt>__autoreleasing</tt> objects, the new pointee is retained,
				738	autoreleased, and stored into the lvalue using primitive semantics.</li>
				739	</ul>
				740	</p>
				741
				742	<p><span class="term">Initialization</span> occurs when an object's
				743	lifetime begins, which depends on its storage duration.
				744	Initialization proceeds in two stages:
				745	<ol>
				746	<li>First, a null pointer is stored into the lvalue using primitive
				747	semantics. This step is skipped if the object
				748	is <tt>__unsafe_unretained</tt>.</li>
				749	<li>Second, if the object has an initializer, that expression is
				750	evaluated and then assigned into the object using the usual assignment
				751	semantics.</li>
				752	</ol>
				753	</p>
				754
				755	<p><span class="term">Destruction</span> occurs when an object's
				756	lifetime ends. In all cases it is semantically equivalent to
				757	assigning a null pointer to the object, with the proviso that of
				758	course the object cannot be legally read after the object's lifetime
				759	ends.</p>
				760
				761	<p><span class="term">Moving</span> occurs in specific situations
				762	where an lvalue is <q>moved from</q>, meaning that its current pointee
				763	will be used but the object may be left in a different (but still
				764	valid) state. This arises with <tt>__block</tt> variables and rvalue
				765	references in C++. For <tt>__strong</tt> lvalues, moving is equivalent
				766	to loading the lvalue with primitive semantics, writing a null pointer
				767	to it with primitive semantics, and then releasing the result of the
				768	load at the end of the current full-expression. For all other
				769	lvalues, moving is equivalent to reading the object.</p>
				770
				771	</div> <!-- ownership.semantics -->
				772
				773	<div id="ownership.restrictions">
				774	<h1>Restrictions</h1>
				775
				776	<div id="ownership.restrictions.autoreleasing">
				777	<h1>Storage duration of<tt> __autoreleasing</tt> objects</h1>
				778
				779	<p>A program is ill-formed if it declares an <tt>__autoreleasing</tt>
				780	object of non-automatic storage duration.</p>
				781
				782	<div class="rationale"><p>Rationale: autorelease pools are tied to the
				783	current thread and scope by their nature. While it is possible to
				784	have temporary objects whose instance variables are filled with
				785	autoreleased objects, there is no way that ARC can provide any sort of
				786	safety guarantee there.</p></div>
				787
				788	<p>It is undefined behavior if a non-null pointer is assigned to
				789	an <tt>__autoreleasing</tt> object while an autorelease pool is in
				790	scope and then that object is read after the autorelease pool's scope
				791	is left.</p>
				792
				793	</div>
				794
				795	<div id="ownership.restrictions.conversion.indirect">
				796	<h1>Conversion of pointers to ownership-qualified types</h1>
				797
				798	<p>A program is ill-formed if an expression of type <tt>T*</tt> is
				799	converted, explicitly or implicitly, to the type <tt>U*</tt>,
				800	where <tt>T</tt> and <tt>U</tt> have different ownership
				801	qualification, unless:
				802	<ul>
				803	<li><tt>T</tt> is qualified with <tt>__strong</tt>,
				804	<tt>__autoreleasing</tt>, or <tt>__unsafe_unretained</tt>, and
				805	<tt>U</tt> is qualified with both <tt>const</tt> and
				806	<tt>__unsafe_unretained</tt>; or</li>
				807	<li>either <tt>T</tt> or <tt>U</tt> is <tt>cv void</tt>, where
				808	<tt>cv</tt> is an optional sequence of non-ownership qualifiers; or</li>
				809	<li>the conversion is requested with a <tt>reinterpret_cast</tt> in
				810	Objective-C++; or</li>
				811	<li>the conversion is a
				812	well-formed <a href="#ownership.restrictions.pass_by_writeback">pass-by-writeback</a>.</li>
				813	</ul>
				814	</p>
				815
				816	<p>The analogous rule applies to <tt>T&</tt> and <tt>U&</tt> in
				817	Objective-C++.</p>
				818
				819	<div class="rationale"><p>Rationale: these rules provide a reasonable
				820	level of type-safety for indirect pointers, as long as the underlying
				821	memory is not deallocated. The conversion to <tt>const
				822	__unsafe_unretained</tt> is permitted because the semantics of reads
				823	are equivalent across all these ownership semantics, and that's a very
				824	useful and common pattern. The interconversion with <tt>void*</tt> is
				825	useful for allocating memory or otherwise escaping the type system,
				826	but use it carefully. <tt>reinterpret_cast</tt> is considered to be
				827	an obvious enough sign of taking responsibility for any
				828	problems.</p></div>
				829
				830	<p>It is undefined behavior to access an ownership-qualified object
				831	through an lvalue of a differently-qualified type, except that any
				832	non-<tt>__weak</tt> object may be read through
				833	an <tt>__unsafe_unretained</tt> lvalue.</p>
				834
				835	<p>It is undefined behavior if a managed operation is performed on
				836	a <tt>__strong</tt> or <tt>__weak</tt> object without a guarantee that
				837	it contains a primitive zero bit-pattern, or if the storage for such
				838	an object is freed or reused without the object being first assigned a
				839	null pointer.</p>
				840
				841	<div class="rationale"><p>Rationale: ARC cannot differentiate between
				842	an assignment operator which is intended to <q>initialize</q> dynamic
				843	memory and one which is intended to potentially replace a value.
				844	Therefore the object's pointer must be valid before letting ARC at it.
				845	Similarly, C and Objective-C do not provide any language hooks for
				846	destroying objects held in dynamic memory, so it is the programmer's
				847	responsibility to avoid leaks (<tt>__strong</tt> objects) and
				848	consistency errors (<tt>__weak</tt> objects).</p>
				849
				850	<p>These requirements are followed automatically in Objective-C++ when
				851	creating objects of retainable object owner type with <tt>new</tt>
				852	or <tt>new[]</tt> and destroying them with <tt>delete</tt>,
				853	<tt>delete[]</tt>, or a pseudo-destructor expression. Note that
				854	arrays of nontrivially-ownership-qualified type are not ABI compatible
				855	with non-ARC code because the element type is non-POD: such arrays
				856	that are <tt>new[]</tt>'d in ARC translation units cannot
				857	be <tt>delete[]</tt>'d in non-ARC translation units and
				858	vice-versa.</p></div>
				859
				860	</div>
				861
				862	<div id="ownership.restrictions.pass_by_writeback">
				863	<h1>Passing to an out parameter by writeback</h1>
				864
				865	<p>If the argument passed to a parameter of type
				866	<tt>T __autoreleasing </tt> has type <tt>U oq </tt>,
				867	where <tt>oq</tt> is an ownership qualifier, then the argument is a
				868	candidate for <span class="term">pass-by-writeback</span> if:</p>
				869
				870	<ul>
				871	<li><tt>oq</tt> is <tt>__strong</tt> or <tt>__weak</tt>, and
				872	<li>it would be legal to initialize a <tt>T __strong *</tt> with
				873	a <tt>U __strong *</tt>.</li>
				874	</ul>
				875
				876	<p>For purposes of overload resolution, an implicit conversion
				877	sequence requiring a pass-by-writeback is always worse than an
				878	implicit conversion sequence not requiring a pass-by-writeback.</p>
				879
				880	<p>The pass-by-writeback is ill-formed if the argument expression does
				881	not have a legal form:</p>
				882
				883	<ul>
				884	<li><tt>&var</tt>, where <tt>var</tt> is a scalar variable of
				885	automatic storage duration with retainable object pointer type</li>
				886	<li>a conditional expression where the second and third operands are
				887	both legal forms</li>
				888	<li>a cast whose operand is a legal form</li>
				889	<li>a null pointer constant</li>
				890	</ul>
				891
				892	<div class="rationale"><p>Rationale: the restriction in the form of
				893	the argument serves two purposes. First, it makes it impossible to
				894	pass the address of an array to the argument, which serves to protect
				895	against an otherwise serious risk of mis-inferring an <q>array</q>
				896	argument as an out-parameter. Second, it makes it much less likely
				897	that the user will see confusing aliasing problems due to the
				898	implementation, below, where their store to the writeback temporary is
				899	not immediately seen in the original argument variable.</p></div>
				900
				901	<p>A pass-by-writeback is evaluated as follows:
				902	<ol>
				903	<li>The argument is evaluated to yield a pointer <tt>p</tt> of
				904	type <tt>U oq *</tt>.</li>
				905	<li>If <tt>p</tt> is a null pointer, then a null pointer is passed as
				906	the argument, and no further work is required for the pass-by-writeback.</li>
				907	<li>Otherwise, a temporary of type <tt>T __autoreleasing</tt> is
				908	created and initialized to a null pointer.</li>
				909	<li>If the argument is not an Objective-C method parameter marked
				910	<tt>out</tt>, then <tt>*p</tt> is read, and the result is written
				911	into the temporary with primitive semantics.</li>
				912	<li>The address of the temporary is passed as the argument to the
				913	actual call.</li>
				914	<li>After the call completes, the temporary is loaded with primitive
				915	semantics, and that value is assigned into <tt>*p</tt>.</li>
				916	</ol></p>
				917
				918	<div class="rationale"><p>Rationale: this is all admittedly
				919	convoluted. In an ideal world, we would see that a local variable is
				920	being passed to an out-parameter and retroactively modify its type to
				921	be <tt>__autoreleasing</tt> rather than <tt>__strong</tt>. This would
				922	be remarkably difficult and not always well-founded under the C type
				923	system. However, it was judged unacceptably invasive to require
				924	programmers to write <tt>__autoreleasing</tt> on all the variables
				925	they intend to use for out-parameters. This was the least bad
				926	solution.</p></div>
				927
				928	</div>
				929
				930	<div id="ownership.restrictions.records">
				931	<h1>Ownership-qualified fields of structs and unions</h1>
				932
				933	<p>A program is ill-formed if it declares a member of a C struct or
				934	union to have a nontrivially ownership-qualified type.</p>
				935
				936	<div class="rationale"><p>Rationale: the resulting type would be
				937	non-POD in the C++ sense, but C does not give us very good language
				938	tools for managing the lifetime of aggregates, so it is more
				939	convenient to simply forbid them. It is still possible to manage this
				940	with a <tt>void*</tt> or an <tt>__unsafe_unretained</tt>
				941	object.</p></div>
				942
				943	<p>This restriction does not apply in Objective-C++. However,
				944	nontrivally ownership-qualified types are considered non-POD: in C++0x
				945	terms, they are not trivially default constructible, copy
				946	constructible, move constructible, copy assignable, move assignable,
				947	or destructible. It is a violation of C++ One Definition Rule to use
				948	a class outside of ARC that, under ARC, would have an
				949	ownership-qualified member.</p>
				950
				951	<div class="rationale"><p>Rationale: unlike in C, we can express all
				952	the necessary ARC semantics for ownership-qualified subobjects as
				953	suboperations of the (default) special member functions for the class.
				954	These functions then become non-trivial. This has the non-obvious
				955	repercussion that the class will have a non-trivial copy constructor
				956	and non-trivial destructor; if it wouldn't outside of ARC, this means
				957	that objects of the type will be passed and returned in an
				958	ABI-incompatible manner.</p></div>
				959
				960	</div>
				961
				962	</div>
				963
				964	<div id="ownership.inference">
				965	<h1>Ownership inference</h1>
				966
				967	<div id="ownership.inference.variables">
				968	<h1>Objects</h1>
				969
				970	<p>If an object is declared with retainable object owner type, but
				971	without an explicit ownership qualifier, its type is implicitly
				972	adjusted to have <tt>__strong</tt> qualification.</p>
				973
				974	<p>As a special case, if the object's base type is <tt>Class</tt>
				975	(possibly protocol-qualified), the type is adjusted to
				976	have <tt>__unsafe_unretained</tt> qualification instead.</p>
				977
				978	</div>
				979
				980	<div id="ownership.inference.indirect_parameters">
				981	<h1>Indirect parameters</h1>
				982
				983	<p>If a function or method parameter has type <tt>T*</tt>, where
				984	<tt>T</tt> is an ownership-unqualified retainable object pointer type,
				985	then:</p>
				986
				987	<ul>
				988	<li>if <tt>T</tt> is <tt>const</tt>-qualified or <tt>Class</tt>, then
				989	it is implicitly qualified with <tt>__unsafe_unretained</tt>;</li>
				990	<li>otherwise, it is implicitly qualified
				991	with <tt>__autoreleasing</tt>.</li>
				992	</ul>
				993	</p>
				994
				995	<div class="rationale"><p>Rationale: <tt>__autoreleasing</tt> exists
				996	mostly for this case, the Cocoa convention for out-parameters. Since
				997	a pointer to <tt>const</tt> is obviously not an out-parameter, we
				998	instead use a type more useful for passing arrays. If the user
				999	instead intends to pass in a <em>mutable</em> array, inferring
				1000	<tt>__autoreleasing</tt> is the wrong thing to do; this directs some
				1001	of the caution in the following rules about writeback.</p></div>
				1002
				1003	<p>Such a type written anywhere else would be ill-formed by the
				1004	general rule requiring ownership qualifiers.</p>
				1005
				1006	<p>This rule does not apply in Objective-C++ if a parameter's type is
				1007	dependent in a template pattern and is only <em>instantiated</em> to
				1008	a type which would be a pointer to an unqualified retainable object
				1009	pointer type. Such code is still ill-formed.</p>
				1010
				1011	<div class="rationale"><p>Rationale: the convention is very unlikely
				1012	to be intentional in template code.</p></div>
				1013
				1014	</div> <!-- ownership.inference.indirect_parameters -->
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1015
				1016	<div id="ownership.inference.template_arguments">
				1017	<h1>Template arguments</h1>
				1018
				1019	<p>If a template argument for a template type parameter is an
				1020	retainable object owner type that does not have an explicit ownership
				1021	qualifier, it is adjusted to have <tt>__strong</tt>
Douglas Gregor	54fb28a	2011-06-17 22:19:27 +0000	[diff] [blame]	1022	qualification. This adjustment occurs regardless of whether the
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1023	template argument was deduced or explicitly specified. </p>
				1024
				1025	<div class="rationale"><p>Rationale: <tt>__strong</tt> is a useful default for containers (e.g., <tt>std::vector<id></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>
				1026
				1027	</div> <!-- ownership.inference.template_arguments -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1028	</div> <!-- ownership.inference -->
				1029	</div> <!-- ownership -->
				1030
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1031
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1032	<div id="family">
				1033	<h1>Method families</h1>
				1034
				1035	<p>An Objective-C method may fall into a <span class="term">method
				1036	family</span>, which is a conventional set of behaviors ascribed to it
				1037	by the Cocoa conventions.</p>
				1038
				1039	<p>A method is in a certain method family if:
				1040	<ul>
				1041	<li>it has a <tt>objc_method_family</tt> attribute placing it in that
				1042	family; or if not that,</li>
				1043	<li>it does not have an <tt>objc_method_family</tt> attribute placing
				1044	it in a different or no family, and</li>
				1045	<li>its selector falls into the corresponding selector family, and</li>
				1046	<li>its signature obeys the added restrictions of the method family.</li>
				1047	</ul></p>
				1048
				1049	<p>A selector is in a certain selector family if, ignoring any leading
				1050	underscores, the first component of the selector either consists
				1051	entirely of the name of the method family or it begins with that name
				1052	followed by a character other than a lowercase letter. For
				1053	example, <tt>_perform:with:</tt> and <tt>performWith:</tt> would fall
				1054	into the <tt>perform</tt> family (if we recognized one),
				1055	but <tt>performing:with</tt> would not.</p>
				1056
				1057	<p>The families and their added restrictions are:</p>
				1058
				1059	<ul>
				1060	<li><tt>alloc</tt> methods must return a retainable object pointer type.</li>
				1061	<li><tt>copy</tt> methods must return a retainable object pointer type.</li>
				1062	<li><tt>mutableCopy</tt> methods must return a retainable object pointer type.</li>
				1063	<li><tt>new</tt> methods must return a retainable object pointer type.</li>
				1064	<li><tt>init</tt> methods must be instance methods and must return an
				1065	Objective-C pointer type. Additionally, a program is ill-formed if it
				1066	declares or contains a call to an <tt>init</tt> method whose return
				1067	type is neither <tt>id</tt> nor a pointer to a super-class or
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1068	sub-class of the declaring class (if the method was declared on
				1069	a class) or the static receiver type of the call (if it was declared
				1070	on a protocol).</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1071
				1072	<div class="rationale"><p>Rationale: there are a fair number of existing
				1073	methods with <tt>init</tt>-like selectors which nonetheless don't
				1074	follow the <tt>init</tt> conventions. Typically these are either
				1075	accidental naming collisions or helper methods called during
				1076	initialization. Because of the peculiar retain/release behavior
				1077	of <tt>init</tt> methods, it's very important not to treat these
				1078	methods as <tt>init</tt> methods if they aren't meant to be. It was
				1079	felt that implicitly defining these methods out of the family based on
				1080	the exact relationship between the return type and the declaring class
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1081	would be much too subtle and fragile. Therefore we identify a small
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1082	number of legitimate-seeming return types and call everything else an
				1083	error. This serves the secondary purpose of encouraging programmers
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1084	not to accidentally give methods names in the <tt>init</tt> family.</p>
				1085
				1086	<p>Note that a method with an <tt>init</tt>-family selector which
				1087	returns a non-Objective-C type (e.g. <tt>void</tt>) is perfectly
				1088	well-formed; it simply isn't in the <tt>init</tt> family.</p></div>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1089	</li>
				1090	</ul>
				1091
				1092	<p>A program is ill-formed if a method's declarations,
				1093	implementations, and overrides do not all have the same method
				1094	family.</p>
				1095
				1096	<div id="family.attribute">
				1097	<h1>Explicit method family control</h1>
				1098
				1099	<p>A method may be annotated with the <tt>objc_method_family</tt>
				1100	attribute to precisely control which method family it belongs to. If
				1101	a method in an <tt>@implementation</tt> does not have this attribute,
				1102	but there is a method declared in the corresponding <tt>@interface</tt>
				1103	that does, then the attribute is copied to the declaration in the
				1104	<tt>@implementation</tt>. The attribute is available outside of ARC,
				1105	and may be tested for with the preprocessor query
				1106	<tt>__has_attribute(objc_method_family)</tt>.</p>
				1107
				1108	<p>The attribute is spelled
				1109	<tt>__attribute__((objc_method_family(<i>family</i>)))</tt>.
				1110	If <i>family</i> is <tt>none</tt>, the method has no family, even if
				1111	it would otherwise be considered to have one based on its selector and
				1112	type. Otherwise, <i>family</i> must be one
				1113	of <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>,
				1114	<tt>mutableCopy</tt>, or <tt>new</tt>, in which case the method is
				1115	considered to belong to the corresponding family regardless of its
				1116	selector. It is an error if a method that is explicitly added to a
				1117	family in this way does not meet the requirements of the family other
				1118	than the selector naming convention.</p>
				1119
				1120	<div class="rationale"><p>Rationale: the rules codified in this document
				1121	describe the standard conventions of Objective-C. However, as these
				1122	conventions have not heretofore been enforced by an unforgiving
				1123	mechanical system, they are only imperfectly kept, especially as they
				1124	haven't always even been precisely defined. While it is possible to
				1125	define low-level ownership semantics with attributes like
				1126	<tt>ns_returns_retained</tt>, this attribute allows the user to
				1127	communicate semantic intent, which of use both to ARC (which, e.g.,
				1128	treats calls to <tt>init</tt> specially) and the static analyzer.</p></div>
				1129	</div>
				1130
				1131	<div id="family.semantics">
				1132	<h1>Semantics of method families</h1>
				1133
				1134	<p>A method's membership in a method family may imply non-standard
				1135	semantics for its parameters and return type.</p>
				1136
				1137	<p>Methods in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>,
				1138	and <tt>new</tt> families — that is, methods in all the
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1139	currently-defined families except <tt>init</tt> — implicitly
				1140	<a href="#objects.operands.retained_returns">return a retained
				1141	object</a> as if they were annotated with
				1142	the <tt>ns_returns_retained</tt> attribute. This can be overridden by
				1143	annotating the method with either of
				1144	the <tt>ns_returns_autoreleased</tt> or
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1145	<tt>ns_returns_not_retained</tt> attributes.</p>
				1146
Fariborz Jahanian	acd4aaf	2011-07-06 22:47:46 +0000	[diff] [blame]	1147	<p>Properties also follow same naming rules as methods. This means that
				1148	those in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>,
				1149	and <tt>new</tt> families provide access to
				1150	<a href="#objects.operands.retained_returns">retained objects</a>.
				1151	This can be overridden by annotating the property with
				1152	<tt>ns_returns_not_retained</tt> attribute.</p>
				1153
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1154	<div id="family.semantics.init">
				1155	<h1>Semantics of <tt>init</tt></h1>
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1156	<p>Methods in the <tt>init</tt> family implicitly
				1157	<a href="#objects.operands.consumed">consume</a> their <tt>self</tt>
				1158	parameter and <a href="#objects.operands.retained_returns">return a
				1159	retained object</a>. Neither of these properties can be altered
				1160	through attributes.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1161
				1162	<p>A call to an <tt>init</tt> method with a receiver that is either
				1163	<tt>self</tt> (possibly parenthesized or casted) or <tt>super</tt> is
				1164	called a <span class="term">delegate init call</span>. It is an error
				1165	for a delegate init call to be made except from an <tt>init</tt>
				1166	method, and excluding blocks within such methods.</p>
				1167
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1168	<p>As an exception to the <a href="misc.self">usual rule</a>, the
				1169	variable <tt>self</tt> is mutable in an <tt>init</tt> method and has
				1170	the usual semantics for a <tt>__strong</tt> variable. However, it is
				1171	undefined behavior and the program is ill-formed, no diagnostic
				1172	required, if an <tt>init</tt> method attempts to use the previous
				1173	value of <tt>self</tt> after the completion of a delegate init call.
				1174	It is conventional, but not required, for an <tt>init</tt> method to
				1175	return <tt>self</tt>.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1176
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1177	<p>It is undefined behavior for a program to cause two or more calls
				1178	to <tt>init</tt> methods on the same object, except that
				1179	each <tt>init</tt> method invocation may perform at most one delegate
				1180	init call.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1181
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1182	</div> <!-- family.semantics.init -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1183
				1184	<div id="family.semantics.result_type">
				1185	<h1>Related result types</h1>
				1186
				1187	<p>Certain methods are candidates to have <span class="term">related
				1188	result types</span>:</p>
				1189	<ul>
				1190	<li>class methods in the <tt>alloc</tt> and <tt>new</tt> method families</li>
				1191	<li>instance methods in the <tt>init</tt> family</li>
				1192	<li>the instance method <tt>self</tt></li>
				1193	<li>outside of ARC, the instance methods <tt>retain</tt> and <tt>autorelease</tt></li>
				1194	</ul>
				1195
				1196	<p>If the formal result type of such a method is <tt>id</tt> or
				1197	protocol-qualified <tt>id</tt>, or a type equal to the declaring class
				1198	or a superclass, then it is said to have a related result type. In
				1199	this case, when invoked in an explicit message send, it is assumed to
				1200	return a type related to the type of the receiver:</p>
				1201
				1202	<ul>
				1203	<li>if it is a class method, and the receiver is a class
				1204	name <tt>T</tt>, the message send expression has type <tt>T*</tt>;
				1205	otherwise</li>
				1206	<li>if it is an instance method, and the receiver has type <tt>T</tt>,
				1207	the message send expression has type <tt>T</tt>; otherwise</li>
				1208	<li>the message send expression has the normal result type of the
				1209	method.</li>
				1210	</ul>
				1211
				1212	<p>This is a new rule of the Objective-C language and applies outside
				1213	of ARC.</p>
				1214
				1215	<div class="rationale"><p>Rationale: ARC's automatic code emission is
				1216	more prone than most code to signature errors, i.e. errors where a
				1217	call was emitted against one method signature, but the implementing
				1218	method has an incompatible signature. Having more precise type
				1219	information helps drastically lower this risks, as well as catching
				1220	a number of latent bugs.</p></div>
				1221
				1222	</div> <!-- family.semantics.result_type -->
				1223	</div> <!-- family.semantics -->
				1224	</div> <!-- family -->
				1225
				1226	<div id="optimization">
				1227	<h1>Optimization</h1>
				1228
				1229	<p>ARC applies aggressive rules for the optimization of local
				1230	behavior. These rules are based around a core assumption of
				1231	<span class="term">local balancing</span>: that other code will
				1232	perform retains and releases as necessary (and only as necessary) for
				1233	its own safety, and so the optimizer does not need to consider global
				1234	properties of the retain and release sequence. For example, if a
				1235	retain and release immediately bracket a call, the optimizer can
				1236	delete the retain and release on the assumption that the called
				1237	function will not do a constant number of unmotivated releases
				1238	followed by a constant number of <q>balancing</q> retains, such that
				1239	the local retain/release pair is the only thing preventing the called
				1240	function from ending up with a dangling reference.</p>
				1241
				1242	<p>The optimizer assumes that when a new value enters local control,
				1243	e.g. from a load of a non-local object or as the result of a function
				1244	call, it is instaneously valid. Subsequently, a retain and release of
				1245	a value are necessary on a computation path only if there is a use of
				1246	that value before the release and after any operation which might
				1247	cause a release of the value (including indirectly or non-locally),
				1248	and only if the value is not demonstrably already retained.</p>
				1249
				1250	<p>The complete optimization rules are quite complicated, but it would
				1251	still be useful to document them here.</p>
				1252
				1253	</div>
				1254
				1255	<div id="misc">
				1256	<h1>Miscellaneous</h1>
				1257
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1258	<div id="misc.special_methods">
				1259	<h1>Special methods</h1>
				1260
				1261	<div id="misc.special_methods.retain">
				1262	<h1>Memory management methods</h1>
				1263
				1264	<p>A program is ill-formed if it contains a method definition, message
				1265	send, or <tt>@selector</tt> expression for any of the following
				1266	selectors:
				1267	<ul>
				1268	<li><tt>autorelease</tt></li>
				1269	<li><tt>release</tt></li>
				1270	<li><tt>retain</tt></li>
				1271	<li><tt>retainCount</tt></li>
				1272	</ul>
				1273	</p>
				1274
				1275	<div class="rationale"><p>Rationale: <tt>retainCount</tt> is banned
				1276	because ARC robs it of consistent semantics. The others were banned
				1277	after weighing three options for how to deal with message sends:</p>
				1278
				1279	<p><b>Honoring</b> them would work out very poorly if a programmer
				1280	naively or accidentally tried to incorporate code written for manual
				1281	retain/release code into an ARC program. At best, such code would do
				1282	twice as much work as necessary; quite frequently, however, ARC and
				1283	the explicit code would both try to balance the same retain, leading
				1284	to crashes. The cost is losing the ability to perform <q>unrooted</q>
				1285	retains, i.e. retains not logically corresponding to a strong
				1286	reference in the object graph.</p>
				1287
				1288	<p><b>Ignoring</b> them would badly violate user expectations about their
				1289	code. While it <em>would</em> make it easier to develop code simultaneously
				1290	for ARC and non-ARC, there is very little reason to do so except for
				1291	certain library developers. ARC and non-ARC translation units share
				1292	an execution model and can seamlessly interoperate. Within a
				1293	translation unit, a developer who faithfully maintains their code in
				1294	non-ARC mode is suffering all the restrictions of ARC for zero
				1295	benefit, while a developer who isn't testing the non-ARC mode is
				1296	likely to be unpleasantly surprised if they try to go back to it.</p>
				1297
				1298	<p><b>Banning</b> them has the disadvantage of making it very awkward
				1299	to migrate existing code to ARC. The best answer to that, given a
				1300	number of other changes and restrictions in ARC, is to provide a
				1301	specialized tool to assist users in that migration.</p>
				1302
				1303	<p>Implementing these methods was banned because they are too integral
				1304	to the semantics of ARC; many tricks which worked tolerably under
				1305	manual reference counting will misbehave if ARC performs an ephemeral
				1306	extra retain or two. If absolutely required, it is still possible to
				1307	implement them in non-ARC code, for example in a category; the
				1308	implementations must obey the <a href="#objects.retains">semantics</a>
				1309	laid out elsewhere in this document.</p>
				1310
				1311	</div>
				1312	</div> <!-- misc.special_methods.retain -->
				1313
				1314	<div id="misc.special_methods.dealloc">
				1315	<h1><tt>dealloc</tt></h1>
				1316
				1317	<p>A program is ill-formed if it contains a message send
				1318	or <tt>@selector</tt> expression for the selector <tt>dealloc</tt>.</p>
				1319
				1320	<div class="rationale"><p>Rationale: there are no legitimate reasons
				1321	to call <tt>dealloc</tt> directly.</p></div>
				1322
				1323	<p>A class may provide a method definition for an instance method
				1324	named <tt>dealloc</tt>. This method will be called after the final
				1325	<tt>release</tt> of the object but before it is deallocated or any of
				1326	its instance variables are destroyed. The superclass's implementation
				1327	of <tt>dealloc</tt> will be called automatically when the method
				1328	returns.</p>
				1329
				1330	<div class="rationale"><p>Rationale: even though ARC destroys instance
				1331	variables automatically, there are still legitimate reasons to write
				1332	a <tt>dealloc</tt> method, such as freeing non-retainable resources.
				1333	Failing to call <tt>[super dealloc]</tt> in such a method is nearly
				1334	always a bug. Sometimes, the object is simply trying to prevent
				1335	itself from being destroyed, but <tt>dealloc</tt> is really far too
				1336	late for the object to be raising such objections. Somewhat more
				1337	legitimately, an object may have been pool-allocated and should not be
				1338	deallocated with <tt>free</tt>; for now, this can only be supported
				1339	with a <tt>dealloc</tt> implementation outside of ARC. Such an
				1340	implementation must be very careful to do all the other work
				1341	that <tt>NSObject</tt>'s <tt>dealloc</tt> would, which is outside the
				1342	scope of this document to describe.</p></div>
				1343
				1344	</div>
				1345
				1346	</div> <!-- misc.special_methods -->
				1347
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1348	<div id="autoreleasepool">
				1349	<h1><tt>@autoreleasepool</tt></h1>
				1350
				1351	<p>To simplify the use of autorelease pools, and to bring them under
				1352	the control of the compiler, a new kind of statement is available in
				1353	Objective-C. It is written <tt>@autoreleasepool</tt> followed by
				1354	a <i>compound-statement</i>, i.e. by a new scope delimited by curly
				1355	braces. Upon entry to this block, the current state of the
				1356	autorelease pool is captured. When the block is exited normally,
				1357	whether by fallthrough or directed control flow (such
				1358	as <tt>return</tt> or <tt>break</tt>), the autorelease pool is
				1359	restored to the saved state, releasing all the objects in it. When
				1360	the block is exited with an exception, the pool is not drained.</p>
				1361
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1362	<p><tt>@autoreleasepool</tt> may be used in non-ARC translation units,
				1363	with equivalent semantics.</p>
				1364
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1365	<p>A program is ill-formed if it refers to the
				1366	<tt>NSAutoreleasePool</tt> class.</p>
				1367
				1368	<div class="rationale"><p>Rationale: autorelease pools are clearly
				1369	important for the compiler to reason about, but it is far too much to
				1370	expect the compiler to accurately reason about control dependencies
				1371	between two calls. It is also very easy to accidentally forget to
				1372	drain an autorelease pool when using the manual API, and this can
				1373	significantly inflate the process's high-water-mark. The introduction
				1374	of a new scope is unfortunate but basically required for sane
				1375	interaction with the rest of the language. Not draining the pool
				1376	during an unwind is apparently required by the Objective-C exceptions
				1377	implementation.</p></div>
				1378
				1379	</div> <!-- autoreleasepool -->
				1380
				1381	<div id="misc.self">
				1382	<h1><tt>self</tt></h1>
				1383
				1384	<p>The <tt>self</tt> parameter variable of an Objective-C method is
				1385	never actually retained by the implementation. It is undefined
				1386	behavior, or at least dangerous, to cause an object to be deallocated
				1387	during a message send to that object. To make this
				1388	safe, <tt>self</tt> is implicitly <tt>const</tt> unless the method is
				1389	in the <a href="#family.semantics.init"><tt>init</tt> family</a>.</p>
				1390
				1391	<div class="rationale"><p>Rationale: the cost of
				1392	retaining <tt>self</tt> in all methods was found to be prohibitive, as
				1393	it tends to be live across calls, preventing the optimizer from
				1394	proving that the retain and release are unnecessary — for good
				1395	reason, as it's quite possible in theory to cause an object to be
				1396	deallocated during its execution without this retain and release.
				1397	Since it's extremely uncommon to actually do so, even unintentionally,
				1398	and since there's no natural way for the programmer to remove this
				1399	retain/release pair otherwise (as there is for other parameters by,
				1400	say, making the variable <tt>__unsafe_unretained</tt>), we chose to
				1401	make this optimizing assumption and shift some amount of risk to the
				1402	user.</p></div>
				1403
				1404	</div> <!-- misc.self -->
				1405
				1406	<div id="misc.enumeration">
				1407	<h1>Fast enumeration iteration variables</h1>
				1408
				1409	<p>If a variable is declared in the condition of an Objective-C fast
				1410	enumeration loop, and the variable has no explicit ownership
				1411	qualifier, then it is qualified with <tt>const __strong</tt> and
				1412	objects encountered during the enumeration are not actually
				1413	retained.</p>
				1414
				1415	<div class="rationale"><p>Rationale: this is an optimization made
				1416	possible because fast enumeration loops promise to keep the objects
				1417	retained during enumeration, and the collection itself cannot be
				1418	synchronously modified. It can be overridden by explicitly qualifying
				1419	the variable with <tt>__strong</tt>, which will make the variable
				1420	mutable again and cause the loop to retain the objects it
				1421	encounters.</div>
				1422
				1423	</div>
				1424
				1425	<div id="misc.blocks">
				1426	<h1>Blocks</h1>
				1427
				1428	<p>The implicit <tt>const</tt> capture variables created when
				1429	evaluating a block literal expression have the same ownership
				1430	semantics as the local variables they capture. The capture is
				1431	performed by reading from the captured variable and initializing the
				1432	capture variable with that value; the capture variable is destroyed
				1433	when the block literal is, i.e. at the end of the enclosing scope.</p>
				1434
				1435	<p>The <a href="#ownership.inference">inference</a> rules apply
				1436	equally to <tt>__block</tt> variables, which is a shift in semantics
				1437	from non-ARC, where <tt>__block</tt> variables did not implicitly
				1438	retain during capture.</p>
				1439
				1440	<p><tt>__block</tt> variables of retainable object owner type are
				1441	moved off the stack by initializing the heap copy with the result of
				1442	moving from the stack copy.</tt></p>
				1443
				1444	<p>With the exception of retains done as part of initializing
				1445	a <tt>__strong</tt> parameter variable or reading a <tt>__weak</tt>
				1446	variable, whenever these semantics call for retaining a value of
				1447	block-pointer type, it has the effect of a <tt>Block_copy</tt>. The
				1448	optimizer may remove such copies when it sees that the result is
				1449	used only as an argument to a call.</p>
				1450
				1451	</div> <!-- misc.blocks -->
				1452
				1453	<div id="misc.exceptions">
				1454	<h1>Exceptions</h1>
				1455
				1456	<p>By default in Objective C, ARC is not exception-safe for normal
				1457	releases:
				1458	<ul>
				1459	<li>It does not end the lifetime of <tt>__strong</tt> variables when
				1460	their scopes are abnormally terminated by an exception.</li>
				1461	<li>It does not perform releases which would occur at the end of
				1462	a full-expression if that full-expression throws an exception.</li>
				1463	</ul>
				1464
				1465	<p>A program may be compiled with the option
				1466	<tt>-fobjc-arc-exceptions</tt> in order to enable these, or with the
				1467	option <tt>-fno-objc-arc-exceptions</tt> to explicitly disable them,
				1468	with the last such argument <q>winning</q>.</p>
				1469
				1470	<div class="rationale"><p>Rationale: the standard Cocoa convention is
				1471	that exceptions signal programmer error and are not intended to be
				1472	recovered from. Making code exceptions-safe by default would impose
				1473	severe runtime and code size penalties on code that typically does not
				1474	actually care about exceptions safety. Therefore, ARC-generated code
				1475	leaks by default on exceptions, which is just fine if the process is
				1476	going to be immediately terminated anyway. Programs which do care
				1477	about recovering from exceptions should enable the option.</p></div>
				1478
				1479	<p>In Objective-C++, <tt>-fobjc-arc-exceptions</tt> is enabled by
				1480	default.</p>
				1481
				1482	<div class="rationale"><p>Rationale: C++ already introduces pervasive
				1483	exceptions-cleanup code of the sort that ARC introduces. C++
				1484	programmers who have not already disabled exceptions are much more
				1485	likely to actual require exception-safety.</p></div>
				1486
				1487	<p>ARC does end the lifetimes of <tt>__weak</tt> objects when an
				1488	exception terminates their scope unless exceptions are disabled in the
				1489	compiler.</p>
				1490
				1491	<div class="rationale"><p>Rationale: the consequence of a
				1492	local <tt>__weak</tt> object not being destroyed is very likely to be
				1493	corruption of the Objective-C runtime, so we want to be safer here.
				1494	Of course, potentially massive leaks are about as likely to take down
				1495	the process as this corruption is if the program does try to recover
				1496	from exceptions.</p></div>
				1497
				1498	</div> <!-- misc.exceptions -->
				1499
				1500	</div> <!-- misc -->
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1501
				1502	<div id="runtime">
				1503	<h1>Runtime support</h1>
				1504
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1505	<p>This section describes the interaction between the ARC runtime and
				1506	the code generated by the ARC compiler. This is not part of the ARC
				1507	language specification; instead, it is effectively a language-specific
				1508	ABI supplement, akin to the <q>Itanium</q> generic ABI for C++.</p>
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1509
				1510	<p>Ownership qualification does not alter the storage requirements for
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1511	objects, except that it is undefined behavior if a <tt>__weak</tt>
				1512	object is inadequately aligned for an object of type <tt>id</tt>. The
				1513	other qualifiers may be used on explicitly under-aligned memory.</p>
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1514
				1515	<p>The runtime tracks <tt>__weak</tt> objects which holds non-null
John McCall	3914a30	2011-06-19 09:59:33 +0000	[diff] [blame]	1516	values. It is undefined behavior to direct modify a <tt>__weak</tt>
				1517	object which is being tracked by the runtime except through an
				1518	<a href="#runtime.objc_storeWeak"><tt>objc_storeWeak</tt></a>,
				1519	<a href="#runtime.objc_destroyWeak"><tt>objc_destroyWeak</tt></a>,
				1520	or <a href="#runtime.objc_moveWeak"><tt>objc_moveWeak</tt></a>
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1521	call.</p>
				1522
John McCall	3914a30	2011-06-19 09:59:33 +0000	[diff] [blame]	1523	<p>The runtime must provide a number of new entrypoints which the
				1524	compiler may emit, which are described in the remainder of this
				1525	section.</p>
				1526
				1527	<div class="rationale"><p>Rationale: Several of these functions are
				1528	semantically equivalent to a message send; we emit calls to C
				1529	functions instead because:</p>
				1530	<ul>
				1531	<li>the machine code to do so is significantly smaller,</li>
				1532	<li>it is much easier to recognize the C functions in the ARC optimizer, and</li>
				1533	<li>a sufficient sophisticated runtime may be able to avoid the
				1534	message send in common cases.</li>
				1535	</ul>
				1536
				1537	<p>Several other of these functions are <q>fused</q> operations which
				1538	can be described entirely in terms of other operations. We use the
				1539	fused operations primarily as a code-size optimization, although in
				1540	some cases there is also a real potential for avoiding redundant
				1541	operations in the runtime.</p>
				1542
				1543	</div>
				1544
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1545	<div id="runtime.objc_autorelease">
				1546	<h1><tt>id objc_autorelease(id value);</tt></h1>
				1547	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1548	valid object.</p>
				1549	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1550	adds the object to the innermost autorelease pool exactly as if the
				1551	object had been sent the <tt>autorelease</tt> message.</p>
				1552	<p>Always returns <tt>value</tt>.</p>
				1553	</div> <!-- runtime.objc_autorelease -->
				1554
				1555	<div id="runtime.objc_autoreleasePoolPop">
				1556	<h1><tt>void objc_autoreleasePoolPop(void *pool);</tt></h1>
				1557	<p><i>Precondition:</i> <tt>pool</tt> is the result of a previous call to
				1558	<a href="runtime.objc_autoreleasePoolPush"><tt>objc_autoreleasePoolPush</tt></a>
				1559	on the current thread, where neither <tt>pool</tt> nor any enclosing
				1560	pool have previously been popped.</p>
				1561	<p>Releases all the objects added to the given autorelease pool and
				1562	any autorelease pools it encloses, then sets the current autorelease
				1563	pool to the pool directly enclosing <tt>pool</tt>.</p>
				1564	</div> <!-- runtime.objc_autoreleasePoolPop -->
				1565
				1566	<div id="runtime.objc_autoreleasePoolPush">
				1567	<h1><tt>void *objc_autoreleasePoolPush(void);</tt></h1>
				1568	<p>Creates a new autorelease pool that is enclosed by the current
				1569	pool, makes that the current pool, and returns an opaque <q>handle</q>
				1570	to it.</p>
				1571
				1572	<div class="rationale"><p>Rationale: while the interface is described
				1573	as an explicit hierarchy of pools, the rules allow the implementation
				1574	to just keep a stack of objects, using the stack depth as the opaque
				1575	pool handle.</p></div>
				1576
				1577	</div> <!-- runtime.objc_autoreleasePoolPush -->
				1578
				1579	<div id="runtime.objc_autoreleaseReturnValue">
				1580	<h1><tt>id objc_autoreleaseReturnValue(id value);</tt></h1>
				1581	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1582	valid object.</p>
				1583	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1584	makes a best effort to hand off ownership of a retain count on the
				1585	object to a call
				1586	to <a href="runtime.objc_retainAutoreleasedReturnValue"><tt>objc_retainAutoreleasedReturnValue</tt></a>
				1587	for the same object in an enclosing call frame. If this is not
				1588	possible, the object is autoreleased as above.</p>
				1589	<p>Always returns <tt>value</tt>.</p>
				1590	</div> <!-- runtime.objc_autoreleaseReturnValue -->
				1591
				1592	<div id="runtime.objc_copyWeak">
				1593	<h1><tt>void objc_copyWeak(id dest, id src);</tt></h1>
				1594	<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
				1595	contains a null pointer or has been registered as a <tt>__weak</tt>
				1596	object. <tt>dest</tt> is a valid pointer which has not been
				1597	registered as a <tt>__weak</tt> object.</p>
				1598	<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
				1599	potentially registering it with the runtime. Equivalent to the
				1600	following code:</p>
				1601	<pre>void objc_copyWeak(id dest, id src) {
				1602	objc_release(objc_initWeak(dest, objc_loadWeakRetained(src)));
				1603	}</pre>
				1604	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1605	on <tt>src</tt>.</p>
				1606	</div> <!-- runtime.objc_copyWeak -->
				1607
				1608	<div id="runtime.objc_destroyWeak">
				1609	<h1><tt>void objc_destroyWeak(id *object);</tt></h1>
				1610	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1611	either contains a null pointer or has been registered as
				1612	a <tt>__weak</tt> object.</p>
				1613	<p><tt>object</tt> is unregistered as a weak object, if it ever was.
				1614	The current value of <tt>object</tt> is left unspecified; otherwise,
				1615	equivalent to the following code:</p>
				1616	<pre>void objc_destroyWeak(id *object) {
				1617	objc_storeWeak(object, nil);
				1618	}</pre>
				1619	<p>Does not need to be atomic with respect to calls
				1620	to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
				1621	</div> <!-- runtime.objc_destroyWeak -->
				1622
				1623	<div id="runtime.objc_initWeak">
				1624	<h1><tt>id objc_initWeak(id *object, id value);</tt></h1>
				1625	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which has
				1626	not been registered as a <tt>__weak</tt> object. <tt>value</tt> is
				1627	null or a pointer to a valid object.</p>
				1628	<p>If <tt>value</tt> is a null pointer or the object to which it
				1629	points has begun deallocation, <tt>object</tt> is zero-initialized.
				1630	Otherwise, <tt>object</tt> is registered as a <tt>__weak</tt> object
				1631	pointing to <tt>value</tt>. Equivalent to the following code:</p>
				1632	<pre>id objc_initWeak(id *object, id value) {
				1633	*object = nil;
				1634	return objc_storeWeak(object, value);
				1635	}</pre>
				1636	<p>Returns the value of <tt>object</tt> after the call.</p>
				1637	<p>Does not need to be atomic with respect to calls
				1638	to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
				1639	</div> <!-- runtime.objc_initWeak -->
				1640
				1641	<div id="runtime.objc_loadWeak">
				1642	<h1><tt>id objc_loadWeak(id *object);</tt></h1>
				1643	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1644	either contains a null pointer or has been registered as
				1645	a <tt>__weak</tt> object.</p>
				1646	<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
				1647	the last value stored into <tt>object</tt> has not yet been
				1648	deallocated or begun deallocation, retains and autoreleases that value
				1649	and returns it. Otherwise returns null. Equivalent to the following
				1650	code:</p>
				1651	<pre>id objc_loadWeak(id *object) {
				1652	return objc_autorelease(objc_loadWeakRetained(object));
				1653	}</pre>
				1654	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1655	on <tt>object</tt>.</p>
				1656	<div class="rationale">Rationale: loading weak references would be
				1657	inherently prone to race conditions without the retain.</div>
				1658	</div> <!-- runtime.objc_loadWeak -->
				1659
				1660	<div id="runtime.objc_loadWeakRetained">
				1661	<h1><tt>id objc_loadWeakRetained(id *object);</tt></h1>
				1662	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1663	either contains a null pointer or has been registered as
				1664	a <tt>__weak</tt> object.</p>
				1665	<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
				1666	the last value stored into <tt>object</tt> has not yet been
				1667	deallocated or begun deallocation, retains that value and returns it.
				1668	Otherwise returns null.</p>
				1669	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1670	on <tt>object</tt>.</p>
				1671	</div> <!-- runtime.objc_loadWeakRetained -->
				1672
				1673	<div id="runtime.objc_moveWeak">
				1674	<h1><tt>void objc_moveWeak(id dest, id src);</tt></h1>
				1675	<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
				1676	contains a null pointer or has been registered as a <tt>__weak</tt>
				1677	object. <tt>dest</tt> is a valid pointer which has not been
				1678	registered as a <tt>__weak</tt> object.</p>
				1679	<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
				1680	potentially registering it with the runtime. <tt>src</tt> may then be
				1681	left in its original state, in which case this call is equivalent
				1682	to <a href="#runtime.objc_copyWeak"><tt>objc_copyWeak</tt></a>, or it
				1683	may be left as null.</p>
				1684	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1685	on <tt>src</tt>.</p>
				1686	</div> <!-- runtime.objc_moveWeak -->
				1687
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1688	<div id="runtime.objc_release">
				1689	<h1><tt>void objc_release(id value);</tt></h1>
				1690	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1691	valid object.</p>
				1692	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1693	performs a release operation exactly as if the object had been sent
				1694	the <tt>release</tt> message.</p>
				1695	</div> <!-- runtime.objc_release -->
				1696
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1697	<div id="runtime.objc_retain">
				1698	<h1><tt>id objc_retain(id value);</tt></h1>
				1699	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1700	valid object.</p>
				1701	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1702	performs a retain operation exactly as if the object had been sent
				1703	the <tt>retain</tt> message.</p>
				1704	<p>Always returns <tt>value</tt>.</p>
				1705	</div> <!-- runtime.objc_retain -->
				1706
				1707	<div id="runtime.objc_retainAutorelease">
				1708	<h1><tt>id objc_retainAutorelease(id value);</tt></h1>
				1709	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1710	valid object.</p>
				1711	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1712	performs a retain operation followed by an autorelease operation.
				1713	Equivalent to the following code:</p>
				1714	<pre>id objc_retainAutorelease(id value) {
				1715	return objc_autorelease(objc_retain(value));
				1716	}</pre>
				1717	<p>Always returns <tt>value</tt>.</p>
				1718	</div> <!-- runtime.objc_retainAutorelease -->
				1719
				1720	<div id="runtime.objc_retainAutoreleaseReturnValue">
				1721	<h1><tt>id objc_retainAutoreleaseReturnValue(id value);</tt></h1>
				1722	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1723	valid object.</p>
				1724	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1725	performs a retain operation followed by the operation described in
				1726	<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>.
				1727	Equivalent to the following code:</p>
				1728	<pre>id objc_retainAutoreleaseReturnValue(id value) {
				1729	return objc_autoreleaseReturnValue(objc_retain(value));
				1730	}</pre>
				1731	<p>Always returns <tt>value</tt>.</p>
				1732	</div> <!-- runtime.objc_retainAutoreleaseReturnValue -->
				1733
				1734	<div id="runtime.objc_retainAutoreleasedReturnValue">
				1735	<h1><tt>id objc_retainAutoreleasedReturnValue(id value);</tt></h1>
				1736	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1737	valid object.</p>
				1738	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1739	attempts to accept a hand off of a retain count from a call to
				1740	<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>
				1741	on <tt>value</tt> in a recently-called function or something it
				1742	calls. If that fails, it performs a retain operation exactly
				1743	like <a href="#runtime.objc_retain"><tt>objc_retain</tt></a>.</p>
				1744	<p>Always returns <tt>value</tt>.</p>
				1745	</div> <!-- runtime.objc_retainAutoreleasedReturnValue -->
				1746
				1747	<div id="runtime.objc_retainBlock">
				1748	<h1><tt>id objc_retainBlock(id value);</tt></h1>
				1749	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1750	valid block object.</p>
				1751	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, if
				1752	the block pointed to by <tt>value</tt> is still on the stack, it is
				1753	copied to the heap and the address of the copy is returned. Otherwise
				1754	a retain operation is performed on the block exactly as if it had been
				1755	sent the <tt>retain</tt> message.</p>
				1756	</div> <!-- runtime.objc_retainBlock -->
				1757
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1758	<div id="runtime.objc_storeStrong">
				1759	<h1><tt>id objc_storeStrong(id *object, id value);</tt></h1>
				1760	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer to
				1761	a <tt>__strong</tt> object which is adequately aligned for a
				1762	pointer. <tt>value</tt> is null or a pointer to a valid object.</p>
				1763	<p>Performs the complete sequence for assigning to a <tt>__strong</tt>
				1764	object of non-block type. Equivalent to the following code:</p>
				1765	<pre>id objc_storeStrong(id *object, id value) {
				1766	value = [value retain];
				1767	id oldValue = *object;
				1768	*object = value;
				1769	[oldValue release];
				1770	return value;
				1771	}</pre>
				1772	<p>Always returns <tt>value</tt>.</p>
				1773	</div> <!-- runtime.objc_storeStrong -->
				1774
				1775	<div id="runtime.objc_storeWeak">
				1776	<h1><tt>id objc_storeWeak(id *object, id value);</tt></h1>
				1777	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1778	either contains a null pointer or has been registered as
				1779	a <tt>__weak</tt> object. <tt>value</tt> is null or a pointer to a
				1780	valid object.</p>
				1781	<p>If <tt>value</tt> is a null pointer or the object to which it
				1782	points has begun deallocation, <tt>object</tt> is assigned null
				1783	and unregistered as a <tt>__weak</tt> object. Otherwise,
				1784	<tt>object</tt> is registered as a <tt>__weak</tt> object or has its
				1785	registration updated to point to <tt>value</tt>.</p>
				1786	<p>Returns the value of <tt>object</tt> after the call.</p>
				1787	</div> <!-- runtime.objc_storeWeak -->
				1788
				1789	</div> <!-- runtime -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1790	</div> <!-- root -->
				1791	</body>
				1792	</html>