Blame - docs/AutomaticReferenceCounting.html - fp2-dev/platform/external/clang

blob: 244f9e6a1fc0a446fd664495690809ec53d11298 [file] [log] [blame]

John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1	<html>
				2	<head>
				3	<title>Objective-C Automatic Reference Counting (ARC)</title>
				4	<link type="text/css" rel="stylesheet" href="../menu.css" />
				5	<link type="text/css" rel="stylesheet" href="../content.css" />
				6	<style type="text/css">
				7	/* Collapse the items in the ToC to the left. */
				8	div#toc ul {
				9	padding-left: 0
				10	}
				11
				12	/* Rationales appear in italic. */
				13	div.rationale {
				14	font-style: italic
				15	}
				16
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	17	div.rationale em {
				18	font-style: normal
				19	}
				20
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	21	div h1 { font-size: 2em; margin: .67em 0 }
				22	div div h1 { font-size: 1.5em; margin: .75em 0 }
				23	div div div h1 { font-size: 1.17em; margin: .83em 0 }
				24	div div div div h1 { margin: 1.12em 0 }
				25
				26	span.term { font-style: italic; font-weight: bold }
				27	</style>
				28
				29	<script lang="javascript">
				30	/// A little script to recursively build a table of contents.
				31	function buildTOC(div, toc, ancestry) {
				32	var children = div.childNodes;
				33	var len = children.length;
				34
				35	var childNumber = 0;
				36
				37	var list = null;
				38	for (var i = 0; i < len; ++i) {
				39	var child = children[i];
				40	if (child.nodeName != "DIV") continue;
				41	if (child.getAttribute("class") == "rationale") continue;
				42	if (child.id == "toc") continue;
				43
				44	// Okay, we're actually going to build a list node.
				45	if (list === null) list = document.createElement("ul");
				46
				47	var childAncestry = ancestry + ++childNumber + ".";
				48
				49	var headerNode = child.childNodes[1];
				50	var title = headerNode.innerHTML;
				51	headerNode.insertBefore(document.createTextNode(childAncestry + " "),
				52	headerNode.firstChild);
				53
				54	var item = document.createElement("li");
				55	item.appendChild(document.createTextNode(childAncestry + " "));
				56
				57	var anchor = document.createElement("a");
				58	anchor.href = "#" + child.id;
				59	anchor.innerHTML = title;
				60	item.appendChild(anchor);
				61
				62	buildTOC(child, item, childAncestry);
				63
				64	list.appendChild(item);
				65	}
				66	if (list) toc.appendChild(list);
				67	}
				68
				69	function onLoad() {
				70	var toc = document.getElementById("toc");
				71	var content = document.getElementById("content");
				72	buildTOC(content, toc, "");
				73	}
				74	window.onload = onLoad;
				75
				76	</script>
				77	</head>
				78	<body>
				79
				80	<!--#include virtual="../menu.html.incl"-->
				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
John McCall	2fad783	2011-07-07 00:03:42 +0000	[diff] [blame^]	731	value.</li>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	732	<li>For <tt>__unsafe_unretained</tt> objects, the new pointee is
				733	stored into the lvalue using primitive semantics.</li>
				734	<li>For <tt>__autoreleasing</tt> objects, the new pointee is retained,
				735	autoreleased, and stored into the lvalue using primitive semantics.</li>
				736	</ul>
				737	</p>
				738
				739	<p><span class="term">Initialization</span> occurs when an object's
				740	lifetime begins, which depends on its storage duration.
				741	Initialization proceeds in two stages:
				742	<ol>
				743	<li>First, a null pointer is stored into the lvalue using primitive
				744	semantics. This step is skipped if the object
				745	is <tt>__unsafe_unretained</tt>.</li>
				746	<li>Second, if the object has an initializer, that expression is
				747	evaluated and then assigned into the object using the usual assignment
				748	semantics.</li>
				749	</ol>
				750	</p>
				751
				752	<p><span class="term">Destruction</span> occurs when an object's
				753	lifetime ends. In all cases it is semantically equivalent to
				754	assigning a null pointer to the object, with the proviso that of
				755	course the object cannot be legally read after the object's lifetime
				756	ends.</p>
				757
				758	<p><span class="term">Moving</span> occurs in specific situations
				759	where an lvalue is <q>moved from</q>, meaning that its current pointee
				760	will be used but the object may be left in a different (but still
				761	valid) state. This arises with <tt>__block</tt> variables and rvalue
				762	references in C++. For <tt>__strong</tt> lvalues, moving is equivalent
				763	to loading the lvalue with primitive semantics, writing a null pointer
				764	to it with primitive semantics, and then releasing the result of the
				765	load at the end of the current full-expression. For all other
				766	lvalues, moving is equivalent to reading the object.</p>
				767
				768	</div> <!-- ownership.semantics -->
				769
				770	<div id="ownership.restrictions">
				771	<h1>Restrictions</h1>
				772
John McCall	2fad783	2011-07-07 00:03:42 +0000	[diff] [blame^]	773	<div id="ownership.restrictions.weak">
				774	<h1>Weak-unavailable types</h1>
				775
				776	<p>It is explicitly permitted for Objective-C classes to not
				777	support <tt>__weak</tt> references. It is undefined behavior to
				778	perform an operation with weak assignment semantics with a pointer to
				779	an Objective-C object whose class does not support <tt>__weak</tt>
				780	references.</p>
				781
				782	<div class="rationale"><p>Rationale: historically, it has been
				783	possible for a class to provide its own reference-count implementation
				784	by overriding <tt>retain</tt>, <tt>release</tt>, etc. However, weak
				785	references to an object require coordination with its class's
				786	reference-count implementation because, among other things, weak loads
				787	and stores must be atomic with respect to the final release.
				788	Therefore, existing custom reference-count implementations will
				789	generally not support weak references without additional effort. This
				790	is unavoidable without breaking binary compatibility.</p></div>
				791
				792	<p>A class may indicate that it does not support weak references by
				793	providing the <tt>objc_arc_weak_unavailable</tt> attribute on the
				794	class's interface declaration. A retainable object pointer type
				795	is <span class="term">weak-unavailable</span> if is a pointer to an
				796	(optionally protocol-qualified) Objective-C class <tt>T</tt>
				797	where <tt>T</tt> or one of its superclasses has
				798	the <tt>objc_arc_weak_unavailable</tt> attribute. A program is
				799	ill-formed if it applies the <tt>__weak</tt> ownership qualifier to a
				800	weak-unavailable type or if the value operand of a weak assignment
				801	operation has a weak-unavailable type.</p>
				802	</div> <!-- ownership.restrictions.weak -->
				803
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	804	<div id="ownership.restrictions.autoreleasing">
John McCall	2fad783	2011-07-07 00:03:42 +0000	[diff] [blame^]	805	<h1>Storage duration of <tt>__autoreleasing</tt> objects</h1>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	806
				807	<p>A program is ill-formed if it declares an <tt>__autoreleasing</tt>
				808	object of non-automatic storage duration.</p>
				809
				810	<div class="rationale"><p>Rationale: autorelease pools are tied to the
				811	current thread and scope by their nature. While it is possible to
				812	have temporary objects whose instance variables are filled with
				813	autoreleased objects, there is no way that ARC can provide any sort of
				814	safety guarantee there.</p></div>
				815
				816	<p>It is undefined behavior if a non-null pointer is assigned to
				817	an <tt>__autoreleasing</tt> object while an autorelease pool is in
				818	scope and then that object is read after the autorelease pool's scope
				819	is left.</p>
				820
				821	</div>
				822
				823	<div id="ownership.restrictions.conversion.indirect">
				824	<h1>Conversion of pointers to ownership-qualified types</h1>
				825
				826	<p>A program is ill-formed if an expression of type <tt>T*</tt> is
				827	converted, explicitly or implicitly, to the type <tt>U*</tt>,
				828	where <tt>T</tt> and <tt>U</tt> have different ownership
				829	qualification, unless:
				830	<ul>
				831	<li><tt>T</tt> is qualified with <tt>__strong</tt>,
				832	<tt>__autoreleasing</tt>, or <tt>__unsafe_unretained</tt>, and
				833	<tt>U</tt> is qualified with both <tt>const</tt> and
				834	<tt>__unsafe_unretained</tt>; or</li>
				835	<li>either <tt>T</tt> or <tt>U</tt> is <tt>cv void</tt>, where
				836	<tt>cv</tt> is an optional sequence of non-ownership qualifiers; or</li>
				837	<li>the conversion is requested with a <tt>reinterpret_cast</tt> in
				838	Objective-C++; or</li>
				839	<li>the conversion is a
				840	well-formed <a href="#ownership.restrictions.pass_by_writeback">pass-by-writeback</a>.</li>
				841	</ul>
				842	</p>
				843
				844	<p>The analogous rule applies to <tt>T&</tt> and <tt>U&</tt> in
				845	Objective-C++.</p>
				846
				847	<div class="rationale"><p>Rationale: these rules provide a reasonable
				848	level of type-safety for indirect pointers, as long as the underlying
				849	memory is not deallocated. The conversion to <tt>const
				850	__unsafe_unretained</tt> is permitted because the semantics of reads
				851	are equivalent across all these ownership semantics, and that's a very
				852	useful and common pattern. The interconversion with <tt>void*</tt> is
				853	useful for allocating memory or otherwise escaping the type system,
				854	but use it carefully. <tt>reinterpret_cast</tt> is considered to be
				855	an obvious enough sign of taking responsibility for any
				856	problems.</p></div>
				857
				858	<p>It is undefined behavior to access an ownership-qualified object
				859	through an lvalue of a differently-qualified type, except that any
				860	non-<tt>__weak</tt> object may be read through
				861	an <tt>__unsafe_unretained</tt> lvalue.</p>
				862
				863	<p>It is undefined behavior if a managed operation is performed on
				864	a <tt>__strong</tt> or <tt>__weak</tt> object without a guarantee that
				865	it contains a primitive zero bit-pattern, or if the storage for such
				866	an object is freed or reused without the object being first assigned a
				867	null pointer.</p>
				868
				869	<div class="rationale"><p>Rationale: ARC cannot differentiate between
				870	an assignment operator which is intended to <q>initialize</q> dynamic
				871	memory and one which is intended to potentially replace a value.
				872	Therefore the object's pointer must be valid before letting ARC at it.
				873	Similarly, C and Objective-C do not provide any language hooks for
				874	destroying objects held in dynamic memory, so it is the programmer's
				875	responsibility to avoid leaks (<tt>__strong</tt> objects) and
				876	consistency errors (<tt>__weak</tt> objects).</p>
				877
				878	<p>These requirements are followed automatically in Objective-C++ when
				879	creating objects of retainable object owner type with <tt>new</tt>
				880	or <tt>new[]</tt> and destroying them with <tt>delete</tt>,
				881	<tt>delete[]</tt>, or a pseudo-destructor expression. Note that
				882	arrays of nontrivially-ownership-qualified type are not ABI compatible
				883	with non-ARC code because the element type is non-POD: such arrays
				884	that are <tt>new[]</tt>'d in ARC translation units cannot
				885	be <tt>delete[]</tt>'d in non-ARC translation units and
				886	vice-versa.</p></div>
				887
				888	</div>
				889
				890	<div id="ownership.restrictions.pass_by_writeback">
				891	<h1>Passing to an out parameter by writeback</h1>
				892
				893	<p>If the argument passed to a parameter of type
				894	<tt>T __autoreleasing </tt> has type <tt>U oq </tt>,
				895	where <tt>oq</tt> is an ownership qualifier, then the argument is a
				896	candidate for <span class="term">pass-by-writeback</span> if:</p>
				897
				898	<ul>
				899	<li><tt>oq</tt> is <tt>__strong</tt> or <tt>__weak</tt>, and
				900	<li>it would be legal to initialize a <tt>T __strong *</tt> with
				901	a <tt>U __strong *</tt>.</li>
				902	</ul>
				903
				904	<p>For purposes of overload resolution, an implicit conversion
				905	sequence requiring a pass-by-writeback is always worse than an
				906	implicit conversion sequence not requiring a pass-by-writeback.</p>
				907
				908	<p>The pass-by-writeback is ill-formed if the argument expression does
				909	not have a legal form:</p>
				910
				911	<ul>
				912	<li><tt>&var</tt>, where <tt>var</tt> is a scalar variable of
				913	automatic storage duration with retainable object pointer type</li>
				914	<li>a conditional expression where the second and third operands are
				915	both legal forms</li>
				916	<li>a cast whose operand is a legal form</li>
				917	<li>a null pointer constant</li>
				918	</ul>
				919
				920	<div class="rationale"><p>Rationale: the restriction in the form of
				921	the argument serves two purposes. First, it makes it impossible to
				922	pass the address of an array to the argument, which serves to protect
				923	against an otherwise serious risk of mis-inferring an <q>array</q>
				924	argument as an out-parameter. Second, it makes it much less likely
				925	that the user will see confusing aliasing problems due to the
				926	implementation, below, where their store to the writeback temporary is
				927	not immediately seen in the original argument variable.</p></div>
				928
				929	<p>A pass-by-writeback is evaluated as follows:
				930	<ol>
				931	<li>The argument is evaluated to yield a pointer <tt>p</tt> of
				932	type <tt>U oq *</tt>.</li>
				933	<li>If <tt>p</tt> is a null pointer, then a null pointer is passed as
				934	the argument, and no further work is required for the pass-by-writeback.</li>
				935	<li>Otherwise, a temporary of type <tt>T __autoreleasing</tt> is
				936	created and initialized to a null pointer.</li>
				937	<li>If the argument is not an Objective-C method parameter marked
				938	<tt>out</tt>, then <tt>*p</tt> is read, and the result is written
				939	into the temporary with primitive semantics.</li>
				940	<li>The address of the temporary is passed as the argument to the
				941	actual call.</li>
				942	<li>After the call completes, the temporary is loaded with primitive
				943	semantics, and that value is assigned into <tt>*p</tt>.</li>
				944	</ol></p>
				945
				946	<div class="rationale"><p>Rationale: this is all admittedly
				947	convoluted. In an ideal world, we would see that a local variable is
				948	being passed to an out-parameter and retroactively modify its type to
				949	be <tt>__autoreleasing</tt> rather than <tt>__strong</tt>. This would
				950	be remarkably difficult and not always well-founded under the C type
				951	system. However, it was judged unacceptably invasive to require
				952	programmers to write <tt>__autoreleasing</tt> on all the variables
				953	they intend to use for out-parameters. This was the least bad
				954	solution.</p></div>
				955
				956	</div>
				957
				958	<div id="ownership.restrictions.records">
				959	<h1>Ownership-qualified fields of structs and unions</h1>
				960
				961	<p>A program is ill-formed if it declares a member of a C struct or
				962	union to have a nontrivially ownership-qualified type.</p>
				963
				964	<div class="rationale"><p>Rationale: the resulting type would be
				965	non-POD in the C++ sense, but C does not give us very good language
				966	tools for managing the lifetime of aggregates, so it is more
				967	convenient to simply forbid them. It is still possible to manage this
				968	with a <tt>void*</tt> or an <tt>__unsafe_unretained</tt>
				969	object.</p></div>
				970
				971	<p>This restriction does not apply in Objective-C++. However,
				972	nontrivally ownership-qualified types are considered non-POD: in C++0x
				973	terms, they are not trivially default constructible, copy
				974	constructible, move constructible, copy assignable, move assignable,
				975	or destructible. It is a violation of C++ One Definition Rule to use
				976	a class outside of ARC that, under ARC, would have an
				977	ownership-qualified member.</p>
				978
				979	<div class="rationale"><p>Rationale: unlike in C, we can express all
				980	the necessary ARC semantics for ownership-qualified subobjects as
				981	suboperations of the (default) special member functions for the class.
				982	These functions then become non-trivial. This has the non-obvious
				983	repercussion that the class will have a non-trivial copy constructor
				984	and non-trivial destructor; if it wouldn't outside of ARC, this means
				985	that objects of the type will be passed and returned in an
				986	ABI-incompatible manner.</p></div>
				987
				988	</div>
				989
				990	</div>
				991
				992	<div id="ownership.inference">
				993	<h1>Ownership inference</h1>
				994
				995	<div id="ownership.inference.variables">
				996	<h1>Objects</h1>
				997
				998	<p>If an object is declared with retainable object owner type, but
				999	without an explicit ownership qualifier, its type is implicitly
				1000	adjusted to have <tt>__strong</tt> qualification.</p>
				1001
				1002	<p>As a special case, if the object's base type is <tt>Class</tt>
				1003	(possibly protocol-qualified), the type is adjusted to
				1004	have <tt>__unsafe_unretained</tt> qualification instead.</p>
				1005
				1006	</div>
				1007
				1008	<div id="ownership.inference.indirect_parameters">
				1009	<h1>Indirect parameters</h1>
				1010
				1011	<p>If a function or method parameter has type <tt>T*</tt>, where
				1012	<tt>T</tt> is an ownership-unqualified retainable object pointer type,
				1013	then:</p>
				1014
				1015	<ul>
				1016	<li>if <tt>T</tt> is <tt>const</tt>-qualified or <tt>Class</tt>, then
				1017	it is implicitly qualified with <tt>__unsafe_unretained</tt>;</li>
				1018	<li>otherwise, it is implicitly qualified
				1019	with <tt>__autoreleasing</tt>.</li>
				1020	</ul>
				1021	</p>
				1022
				1023	<div class="rationale"><p>Rationale: <tt>__autoreleasing</tt> exists
				1024	mostly for this case, the Cocoa convention for out-parameters. Since
				1025	a pointer to <tt>const</tt> is obviously not an out-parameter, we
				1026	instead use a type more useful for passing arrays. If the user
				1027	instead intends to pass in a <em>mutable</em> array, inferring
				1028	<tt>__autoreleasing</tt> is the wrong thing to do; this directs some
				1029	of the caution in the following rules about writeback.</p></div>
				1030
				1031	<p>Such a type written anywhere else would be ill-formed by the
				1032	general rule requiring ownership qualifiers.</p>
				1033
				1034	<p>This rule does not apply in Objective-C++ if a parameter's type is
				1035	dependent in a template pattern and is only <em>instantiated</em> to
				1036	a type which would be a pointer to an unqualified retainable object
				1037	pointer type. Such code is still ill-formed.</p>
				1038
				1039	<div class="rationale"><p>Rationale: the convention is very unlikely
				1040	to be intentional in template code.</p></div>
				1041
				1042	</div> <!-- ownership.inference.indirect_parameters -->
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1043
				1044	<div id="ownership.inference.template_arguments">
				1045	<h1>Template arguments</h1>
				1046
				1047	<p>If a template argument for a template type parameter is an
				1048	retainable object owner type that does not have an explicit ownership
				1049	qualifier, it is adjusted to have <tt>__strong</tt>
Douglas Gregor	54fb28a	2011-06-17 22:19:27 +0000	[diff] [blame]	1050	qualification. This adjustment occurs regardless of whether the
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1051	template argument was deduced or explicitly specified. </p>
				1052
				1053	<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>
				1054
				1055	</div> <!-- ownership.inference.template_arguments -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1056	</div> <!-- ownership.inference -->
				1057	</div> <!-- ownership -->
				1058
Douglas Gregor	e559ca1	2011-06-17 22:11:49 +0000	[diff] [blame]	1059
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1060	<div id="family">
				1061	<h1>Method families</h1>
				1062
				1063	<p>An Objective-C method may fall into a <span class="term">method
				1064	family</span>, which is a conventional set of behaviors ascribed to it
				1065	by the Cocoa conventions.</p>
				1066
				1067	<p>A method is in a certain method family if:
				1068	<ul>
				1069	<li>it has a <tt>objc_method_family</tt> attribute placing it in that
				1070	family; or if not that,</li>
				1071	<li>it does not have an <tt>objc_method_family</tt> attribute placing
				1072	it in a different or no family, and</li>
				1073	<li>its selector falls into the corresponding selector family, and</li>
				1074	<li>its signature obeys the added restrictions of the method family.</li>
				1075	</ul></p>
				1076
				1077	<p>A selector is in a certain selector family if, ignoring any leading
				1078	underscores, the first component of the selector either consists
				1079	entirely of the name of the method family or it begins with that name
				1080	followed by a character other than a lowercase letter. For
				1081	example, <tt>_perform:with:</tt> and <tt>performWith:</tt> would fall
				1082	into the <tt>perform</tt> family (if we recognized one),
				1083	but <tt>performing:with</tt> would not.</p>
				1084
				1085	<p>The families and their added restrictions are:</p>
				1086
				1087	<ul>
				1088	<li><tt>alloc</tt> methods must return a retainable object pointer type.</li>
				1089	<li><tt>copy</tt> methods must return a retainable object pointer type.</li>
				1090	<li><tt>mutableCopy</tt> methods must return a retainable object pointer type.</li>
				1091	<li><tt>new</tt> methods must return a retainable object pointer type.</li>
				1092	<li><tt>init</tt> methods must be instance methods and must return an
				1093	Objective-C pointer type. Additionally, a program is ill-formed if it
				1094	declares or contains a call to an <tt>init</tt> method whose return
				1095	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]	1096	sub-class of the declaring class (if the method was declared on
				1097	a class) or the static receiver type of the call (if it was declared
				1098	on a protocol).</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1099
				1100	<div class="rationale"><p>Rationale: there are a fair number of existing
				1101	methods with <tt>init</tt>-like selectors which nonetheless don't
				1102	follow the <tt>init</tt> conventions. Typically these are either
				1103	accidental naming collisions or helper methods called during
				1104	initialization. Because of the peculiar retain/release behavior
				1105	of <tt>init</tt> methods, it's very important not to treat these
				1106	methods as <tt>init</tt> methods if they aren't meant to be. It was
				1107	felt that implicitly defining these methods out of the family based on
				1108	the exact relationship between the return type and the declaring class
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1109	would be much too subtle and fragile. Therefore we identify a small
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1110	number of legitimate-seeming return types and call everything else an
				1111	error. This serves the secondary purpose of encouraging programmers
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1112	not to accidentally give methods names in the <tt>init</tt> family.</p>
				1113
				1114	<p>Note that a method with an <tt>init</tt>-family selector which
				1115	returns a non-Objective-C type (e.g. <tt>void</tt>) is perfectly
				1116	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]	1117	</li>
				1118	</ul>
				1119
				1120	<p>A program is ill-formed if a method's declarations,
				1121	implementations, and overrides do not all have the same method
				1122	family.</p>
				1123
				1124	<div id="family.attribute">
				1125	<h1>Explicit method family control</h1>
				1126
				1127	<p>A method may be annotated with the <tt>objc_method_family</tt>
				1128	attribute to precisely control which method family it belongs to. If
				1129	a method in an <tt>@implementation</tt> does not have this attribute,
				1130	but there is a method declared in the corresponding <tt>@interface</tt>
				1131	that does, then the attribute is copied to the declaration in the
				1132	<tt>@implementation</tt>. The attribute is available outside of ARC,
				1133	and may be tested for with the preprocessor query
				1134	<tt>__has_attribute(objc_method_family)</tt>.</p>
				1135
				1136	<p>The attribute is spelled
				1137	<tt>__attribute__((objc_method_family(<i>family</i>)))</tt>.
				1138	If <i>family</i> is <tt>none</tt>, the method has no family, even if
				1139	it would otherwise be considered to have one based on its selector and
				1140	type. Otherwise, <i>family</i> must be one
				1141	of <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>,
				1142	<tt>mutableCopy</tt>, or <tt>new</tt>, in which case the method is
				1143	considered to belong to the corresponding family regardless of its
				1144	selector. It is an error if a method that is explicitly added to a
				1145	family in this way does not meet the requirements of the family other
				1146	than the selector naming convention.</p>
				1147
				1148	<div class="rationale"><p>Rationale: the rules codified in this document
				1149	describe the standard conventions of Objective-C. However, as these
				1150	conventions have not heretofore been enforced by an unforgiving
				1151	mechanical system, they are only imperfectly kept, especially as they
				1152	haven't always even been precisely defined. While it is possible to
				1153	define low-level ownership semantics with attributes like
				1154	<tt>ns_returns_retained</tt>, this attribute allows the user to
				1155	communicate semantic intent, which of use both to ARC (which, e.g.,
				1156	treats calls to <tt>init</tt> specially) and the static analyzer.</p></div>
				1157	</div>
				1158
				1159	<div id="family.semantics">
				1160	<h1>Semantics of method families</h1>
				1161
				1162	<p>A method's membership in a method family may imply non-standard
				1163	semantics for its parameters and return type.</p>
				1164
				1165	<p>Methods in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>,
				1166	and <tt>new</tt> families — that is, methods in all the
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1167	currently-defined families except <tt>init</tt> — implicitly
				1168	<a href="#objects.operands.retained_returns">return a retained
				1169	object</a> as if they were annotated with
				1170	the <tt>ns_returns_retained</tt> attribute. This can be overridden by
				1171	annotating the method with either of
				1172	the <tt>ns_returns_autoreleased</tt> or
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1173	<tt>ns_returns_not_retained</tt> attributes.</p>
				1174
Fariborz Jahanian	acd4aaf	2011-07-06 22:47:46 +0000	[diff] [blame]	1175	<p>Properties also follow same naming rules as methods. This means that
				1176	those in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>,
				1177	and <tt>new</tt> families provide access to
				1178	<a href="#objects.operands.retained_returns">retained objects</a>.
				1179	This can be overridden by annotating the property with
				1180	<tt>ns_returns_not_retained</tt> attribute.</p>
				1181
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1182	<div id="family.semantics.init">
				1183	<h1>Semantics of <tt>init</tt></h1>
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1184	<p>Methods in the <tt>init</tt> family implicitly
				1185	<a href="#objects.operands.consumed">consume</a> their <tt>self</tt>
				1186	parameter and <a href="#objects.operands.retained_returns">return a
				1187	retained object</a>. Neither of these properties can be altered
				1188	through attributes.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1189
				1190	<p>A call to an <tt>init</tt> method with a receiver that is either
				1191	<tt>self</tt> (possibly parenthesized or casted) or <tt>super</tt> is
				1192	called a <span class="term">delegate init call</span>. It is an error
				1193	for a delegate init call to be made except from an <tt>init</tt>
				1194	method, and excluding blocks within such methods.</p>
				1195
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1196	<p>As an exception to the <a href="misc.self">usual rule</a>, the
				1197	variable <tt>self</tt> is mutable in an <tt>init</tt> method and has
				1198	the usual semantics for a <tt>__strong</tt> variable. However, it is
				1199	undefined behavior and the program is ill-formed, no diagnostic
				1200	required, if an <tt>init</tt> method attempts to use the previous
				1201	value of <tt>self</tt> after the completion of a delegate init call.
				1202	It is conventional, but not required, for an <tt>init</tt> method to
				1203	return <tt>self</tt>.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1204
John McCall	be16b89	2011-06-18 08:15:19 +0000	[diff] [blame]	1205	<p>It is undefined behavior for a program to cause two or more calls
				1206	to <tt>init</tt> methods on the same object, except that
				1207	each <tt>init</tt> method invocation may perform at most one delegate
				1208	init call.</p>
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1209
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1210	</div> <!-- family.semantics.init -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1211
				1212	<div id="family.semantics.result_type">
				1213	<h1>Related result types</h1>
				1214
				1215	<p>Certain methods are candidates to have <span class="term">related
				1216	result types</span>:</p>
				1217	<ul>
				1218	<li>class methods in the <tt>alloc</tt> and <tt>new</tt> method families</li>
				1219	<li>instance methods in the <tt>init</tt> family</li>
				1220	<li>the instance method <tt>self</tt></li>
				1221	<li>outside of ARC, the instance methods <tt>retain</tt> and <tt>autorelease</tt></li>
				1222	</ul>
				1223
				1224	<p>If the formal result type of such a method is <tt>id</tt> or
				1225	protocol-qualified <tt>id</tt>, or a type equal to the declaring class
				1226	or a superclass, then it is said to have a related result type. In
				1227	this case, when invoked in an explicit message send, it is assumed to
				1228	return a type related to the type of the receiver:</p>
				1229
				1230	<ul>
				1231	<li>if it is a class method, and the receiver is a class
				1232	name <tt>T</tt>, the message send expression has type <tt>T*</tt>;
				1233	otherwise</li>
				1234	<li>if it is an instance method, and the receiver has type <tt>T</tt>,
				1235	the message send expression has type <tt>T</tt>; otherwise</li>
				1236	<li>the message send expression has the normal result type of the
				1237	method.</li>
				1238	</ul>
				1239
				1240	<p>This is a new rule of the Objective-C language and applies outside
				1241	of ARC.</p>
				1242
				1243	<div class="rationale"><p>Rationale: ARC's automatic code emission is
				1244	more prone than most code to signature errors, i.e. errors where a
				1245	call was emitted against one method signature, but the implementing
				1246	method has an incompatible signature. Having more precise type
				1247	information helps drastically lower this risks, as well as catching
				1248	a number of latent bugs.</p></div>
				1249
				1250	</div> <!-- family.semantics.result_type -->
				1251	</div> <!-- family.semantics -->
				1252	</div> <!-- family -->
				1253
				1254	<div id="optimization">
				1255	<h1>Optimization</h1>
				1256
				1257	<p>ARC applies aggressive rules for the optimization of local
				1258	behavior. These rules are based around a core assumption of
				1259	<span class="term">local balancing</span>: that other code will
				1260	perform retains and releases as necessary (and only as necessary) for
				1261	its own safety, and so the optimizer does not need to consider global
				1262	properties of the retain and release sequence. For example, if a
				1263	retain and release immediately bracket a call, the optimizer can
				1264	delete the retain and release on the assumption that the called
				1265	function will not do a constant number of unmotivated releases
				1266	followed by a constant number of <q>balancing</q> retains, such that
				1267	the local retain/release pair is the only thing preventing the called
				1268	function from ending up with a dangling reference.</p>
				1269
				1270	<p>The optimizer assumes that when a new value enters local control,
				1271	e.g. from a load of a non-local object or as the result of a function
				1272	call, it is instaneously valid. Subsequently, a retain and release of
				1273	a value are necessary on a computation path only if there is a use of
				1274	that value before the release and after any operation which might
				1275	cause a release of the value (including indirectly or non-locally),
				1276	and only if the value is not demonstrably already retained.</p>
				1277
				1278	<p>The complete optimization rules are quite complicated, but it would
				1279	still be useful to document them here.</p>
				1280
				1281	</div>
				1282
				1283	<div id="misc">
				1284	<h1>Miscellaneous</h1>
				1285
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1286	<div id="misc.special_methods">
				1287	<h1>Special methods</h1>
				1288
				1289	<div id="misc.special_methods.retain">
				1290	<h1>Memory management methods</h1>
				1291
				1292	<p>A program is ill-formed if it contains a method definition, message
				1293	send, or <tt>@selector</tt> expression for any of the following
				1294	selectors:
				1295	<ul>
				1296	<li><tt>autorelease</tt></li>
				1297	<li><tt>release</tt></li>
				1298	<li><tt>retain</tt></li>
				1299	<li><tt>retainCount</tt></li>
				1300	</ul>
				1301	</p>
				1302
				1303	<div class="rationale"><p>Rationale: <tt>retainCount</tt> is banned
				1304	because ARC robs it of consistent semantics. The others were banned
				1305	after weighing three options for how to deal with message sends:</p>
				1306
				1307	<p><b>Honoring</b> them would work out very poorly if a programmer
				1308	naively or accidentally tried to incorporate code written for manual
				1309	retain/release code into an ARC program. At best, such code would do
				1310	twice as much work as necessary; quite frequently, however, ARC and
				1311	the explicit code would both try to balance the same retain, leading
				1312	to crashes. The cost is losing the ability to perform <q>unrooted</q>
				1313	retains, i.e. retains not logically corresponding to a strong
				1314	reference in the object graph.</p>
				1315
				1316	<p><b>Ignoring</b> them would badly violate user expectations about their
				1317	code. While it <em>would</em> make it easier to develop code simultaneously
				1318	for ARC and non-ARC, there is very little reason to do so except for
				1319	certain library developers. ARC and non-ARC translation units share
				1320	an execution model and can seamlessly interoperate. Within a
				1321	translation unit, a developer who faithfully maintains their code in
				1322	non-ARC mode is suffering all the restrictions of ARC for zero
				1323	benefit, while a developer who isn't testing the non-ARC mode is
				1324	likely to be unpleasantly surprised if they try to go back to it.</p>
				1325
				1326	<p><b>Banning</b> them has the disadvantage of making it very awkward
				1327	to migrate existing code to ARC. The best answer to that, given a
				1328	number of other changes and restrictions in ARC, is to provide a
				1329	specialized tool to assist users in that migration.</p>
				1330
				1331	<p>Implementing these methods was banned because they are too integral
				1332	to the semantics of ARC; many tricks which worked tolerably under
				1333	manual reference counting will misbehave if ARC performs an ephemeral
				1334	extra retain or two. If absolutely required, it is still possible to
				1335	implement them in non-ARC code, for example in a category; the
				1336	implementations must obey the <a href="#objects.retains">semantics</a>
				1337	laid out elsewhere in this document.</p>
				1338
				1339	</div>
				1340	</div> <!-- misc.special_methods.retain -->
				1341
				1342	<div id="misc.special_methods.dealloc">
				1343	<h1><tt>dealloc</tt></h1>
				1344
				1345	<p>A program is ill-formed if it contains a message send
				1346	or <tt>@selector</tt> expression for the selector <tt>dealloc</tt>.</p>
				1347
				1348	<div class="rationale"><p>Rationale: there are no legitimate reasons
				1349	to call <tt>dealloc</tt> directly.</p></div>
				1350
				1351	<p>A class may provide a method definition for an instance method
				1352	named <tt>dealloc</tt>. This method will be called after the final
				1353	<tt>release</tt> of the object but before it is deallocated or any of
				1354	its instance variables are destroyed. The superclass's implementation
				1355	of <tt>dealloc</tt> will be called automatically when the method
				1356	returns.</p>
				1357
				1358	<div class="rationale"><p>Rationale: even though ARC destroys instance
				1359	variables automatically, there are still legitimate reasons to write
				1360	a <tt>dealloc</tt> method, such as freeing non-retainable resources.
				1361	Failing to call <tt>[super dealloc]</tt> in such a method is nearly
				1362	always a bug. Sometimes, the object is simply trying to prevent
				1363	itself from being destroyed, but <tt>dealloc</tt> is really far too
				1364	late for the object to be raising such objections. Somewhat more
				1365	legitimately, an object may have been pool-allocated and should not be
				1366	deallocated with <tt>free</tt>; for now, this can only be supported
				1367	with a <tt>dealloc</tt> implementation outside of ARC. Such an
				1368	implementation must be very careful to do all the other work
				1369	that <tt>NSObject</tt>'s <tt>dealloc</tt> would, which is outside the
				1370	scope of this document to describe.</p></div>
				1371
				1372	</div>
				1373
				1374	</div> <!-- misc.special_methods -->
				1375
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1376	<div id="autoreleasepool">
				1377	<h1><tt>@autoreleasepool</tt></h1>
				1378
				1379	<p>To simplify the use of autorelease pools, and to bring them under
				1380	the control of the compiler, a new kind of statement is available in
				1381	Objective-C. It is written <tt>@autoreleasepool</tt> followed by
				1382	a <i>compound-statement</i>, i.e. by a new scope delimited by curly
				1383	braces. Upon entry to this block, the current state of the
				1384	autorelease pool is captured. When the block is exited normally,
				1385	whether by fallthrough or directed control flow (such
				1386	as <tt>return</tt> or <tt>break</tt>), the autorelease pool is
				1387	restored to the saved state, releasing all the objects in it. When
				1388	the block is exited with an exception, the pool is not drained.</p>
				1389
John McCall	f3d08a6	2011-06-18 07:31:30 +0000	[diff] [blame]	1390	<p><tt>@autoreleasepool</tt> may be used in non-ARC translation units,
				1391	with equivalent semantics.</p>
				1392
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1393	<p>A program is ill-formed if it refers to the
				1394	<tt>NSAutoreleasePool</tt> class.</p>
				1395
				1396	<div class="rationale"><p>Rationale: autorelease pools are clearly
				1397	important for the compiler to reason about, but it is far too much to
				1398	expect the compiler to accurately reason about control dependencies
				1399	between two calls. It is also very easy to accidentally forget to
				1400	drain an autorelease pool when using the manual API, and this can
				1401	significantly inflate the process's high-water-mark. The introduction
				1402	of a new scope is unfortunate but basically required for sane
				1403	interaction with the rest of the language. Not draining the pool
				1404	during an unwind is apparently required by the Objective-C exceptions
				1405	implementation.</p></div>
				1406
				1407	</div> <!-- autoreleasepool -->
				1408
				1409	<div id="misc.self">
				1410	<h1><tt>self</tt></h1>
				1411
				1412	<p>The <tt>self</tt> parameter variable of an Objective-C method is
				1413	never actually retained by the implementation. It is undefined
				1414	behavior, or at least dangerous, to cause an object to be deallocated
				1415	during a message send to that object. To make this
				1416	safe, <tt>self</tt> is implicitly <tt>const</tt> unless the method is
				1417	in the <a href="#family.semantics.init"><tt>init</tt> family</a>.</p>
				1418
				1419	<div class="rationale"><p>Rationale: the cost of
				1420	retaining <tt>self</tt> in all methods was found to be prohibitive, as
				1421	it tends to be live across calls, preventing the optimizer from
				1422	proving that the retain and release are unnecessary — for good
				1423	reason, as it's quite possible in theory to cause an object to be
				1424	deallocated during its execution without this retain and release.
				1425	Since it's extremely uncommon to actually do so, even unintentionally,
				1426	and since there's no natural way for the programmer to remove this
				1427	retain/release pair otherwise (as there is for other parameters by,
				1428	say, making the variable <tt>__unsafe_unretained</tt>), we chose to
				1429	make this optimizing assumption and shift some amount of risk to the
				1430	user.</p></div>
				1431
				1432	</div> <!-- misc.self -->
				1433
				1434	<div id="misc.enumeration">
				1435	<h1>Fast enumeration iteration variables</h1>
				1436
				1437	<p>If a variable is declared in the condition of an Objective-C fast
				1438	enumeration loop, and the variable has no explicit ownership
				1439	qualifier, then it is qualified with <tt>const __strong</tt> and
				1440	objects encountered during the enumeration are not actually
				1441	retained.</p>
				1442
				1443	<div class="rationale"><p>Rationale: this is an optimization made
				1444	possible because fast enumeration loops promise to keep the objects
				1445	retained during enumeration, and the collection itself cannot be
				1446	synchronously modified. It can be overridden by explicitly qualifying
				1447	the variable with <tt>__strong</tt>, which will make the variable
				1448	mutable again and cause the loop to retain the objects it
				1449	encounters.</div>
				1450
				1451	</div>
				1452
				1453	<div id="misc.blocks">
				1454	<h1>Blocks</h1>
				1455
				1456	<p>The implicit <tt>const</tt> capture variables created when
				1457	evaluating a block literal expression have the same ownership
				1458	semantics as the local variables they capture. The capture is
				1459	performed by reading from the captured variable and initializing the
				1460	capture variable with that value; the capture variable is destroyed
				1461	when the block literal is, i.e. at the end of the enclosing scope.</p>
				1462
				1463	<p>The <a href="#ownership.inference">inference</a> rules apply
				1464	equally to <tt>__block</tt> variables, which is a shift in semantics
				1465	from non-ARC, where <tt>__block</tt> variables did not implicitly
				1466	retain during capture.</p>
				1467
				1468	<p><tt>__block</tt> variables of retainable object owner type are
				1469	moved off the stack by initializing the heap copy with the result of
				1470	moving from the stack copy.</tt></p>
				1471
				1472	<p>With the exception of retains done as part of initializing
				1473	a <tt>__strong</tt> parameter variable or reading a <tt>__weak</tt>
				1474	variable, whenever these semantics call for retaining a value of
				1475	block-pointer type, it has the effect of a <tt>Block_copy</tt>. The
				1476	optimizer may remove such copies when it sees that the result is
				1477	used only as an argument to a call.</p>
				1478
				1479	</div> <!-- misc.blocks -->
				1480
				1481	<div id="misc.exceptions">
				1482	<h1>Exceptions</h1>
				1483
				1484	<p>By default in Objective C, ARC is not exception-safe for normal
				1485	releases:
				1486	<ul>
				1487	<li>It does not end the lifetime of <tt>__strong</tt> variables when
				1488	their scopes are abnormally terminated by an exception.</li>
				1489	<li>It does not perform releases which would occur at the end of
				1490	a full-expression if that full-expression throws an exception.</li>
				1491	</ul>
				1492
				1493	<p>A program may be compiled with the option
				1494	<tt>-fobjc-arc-exceptions</tt> in order to enable these, or with the
				1495	option <tt>-fno-objc-arc-exceptions</tt> to explicitly disable them,
				1496	with the last such argument <q>winning</q>.</p>
				1497
				1498	<div class="rationale"><p>Rationale: the standard Cocoa convention is
				1499	that exceptions signal programmer error and are not intended to be
				1500	recovered from. Making code exceptions-safe by default would impose
				1501	severe runtime and code size penalties on code that typically does not
				1502	actually care about exceptions safety. Therefore, ARC-generated code
				1503	leaks by default on exceptions, which is just fine if the process is
				1504	going to be immediately terminated anyway. Programs which do care
				1505	about recovering from exceptions should enable the option.</p></div>
				1506
				1507	<p>In Objective-C++, <tt>-fobjc-arc-exceptions</tt> is enabled by
				1508	default.</p>
				1509
				1510	<div class="rationale"><p>Rationale: C++ already introduces pervasive
				1511	exceptions-cleanup code of the sort that ARC introduces. C++
				1512	programmers who have not already disabled exceptions are much more
				1513	likely to actual require exception-safety.</p></div>
				1514
				1515	<p>ARC does end the lifetimes of <tt>__weak</tt> objects when an
				1516	exception terminates their scope unless exceptions are disabled in the
				1517	compiler.</p>
				1518
				1519	<div class="rationale"><p>Rationale: the consequence of a
				1520	local <tt>__weak</tt> object not being destroyed is very likely to be
				1521	corruption of the Objective-C runtime, so we want to be safer here.
				1522	Of course, potentially massive leaks are about as likely to take down
				1523	the process as this corruption is if the program does try to recover
				1524	from exceptions.</p></div>
				1525
				1526	</div> <!-- misc.exceptions -->
				1527
				1528	</div> <!-- misc -->
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1529
				1530	<div id="runtime">
				1531	<h1>Runtime support</h1>
				1532
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1533	<p>This section describes the interaction between the ARC runtime and
				1534	the code generated by the ARC compiler. This is not part of the ARC
				1535	language specification; instead, it is effectively a language-specific
				1536	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]	1537
				1538	<p>Ownership qualification does not alter the storage requirements for
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1539	objects, except that it is undefined behavior if a <tt>__weak</tt>
				1540	object is inadequately aligned for an object of type <tt>id</tt>. The
				1541	other qualifiers may be used on explicitly under-aligned memory.</p>
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1542
				1543	<p>The runtime tracks <tt>__weak</tt> objects which holds non-null
John McCall	3914a30	2011-06-19 09:59:33 +0000	[diff] [blame]	1544	values. It is undefined behavior to direct modify a <tt>__weak</tt>
				1545	object which is being tracked by the runtime except through an
				1546	<a href="#runtime.objc_storeWeak"><tt>objc_storeWeak</tt></a>,
				1547	<a href="#runtime.objc_destroyWeak"><tt>objc_destroyWeak</tt></a>,
				1548	or <a href="#runtime.objc_moveWeak"><tt>objc_moveWeak</tt></a>
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1549	call.</p>
				1550
John McCall	3914a30	2011-06-19 09:59:33 +0000	[diff] [blame]	1551	<p>The runtime must provide a number of new entrypoints which the
				1552	compiler may emit, which are described in the remainder of this
				1553	section.</p>
				1554
				1555	<div class="rationale"><p>Rationale: Several of these functions are
				1556	semantically equivalent to a message send; we emit calls to C
				1557	functions instead because:</p>
				1558	<ul>
				1559	<li>the machine code to do so is significantly smaller,</li>
				1560	<li>it is much easier to recognize the C functions in the ARC optimizer, and</li>
				1561	<li>a sufficient sophisticated runtime may be able to avoid the
				1562	message send in common cases.</li>
				1563	</ul>
				1564
				1565	<p>Several other of these functions are <q>fused</q> operations which
				1566	can be described entirely in terms of other operations. We use the
				1567	fused operations primarily as a code-size optimization, although in
				1568	some cases there is also a real potential for avoiding redundant
				1569	operations in the runtime.</p>
				1570
				1571	</div>
				1572
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1573	<div id="runtime.objc_autorelease">
				1574	<h1><tt>id objc_autorelease(id value);</tt></h1>
				1575	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1576	valid object.</p>
				1577	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1578	adds the object to the innermost autorelease pool exactly as if the
				1579	object had been sent the <tt>autorelease</tt> message.</p>
				1580	<p>Always returns <tt>value</tt>.</p>
				1581	</div> <!-- runtime.objc_autorelease -->
				1582
				1583	<div id="runtime.objc_autoreleasePoolPop">
				1584	<h1><tt>void objc_autoreleasePoolPop(void *pool);</tt></h1>
				1585	<p><i>Precondition:</i> <tt>pool</tt> is the result of a previous call to
				1586	<a href="runtime.objc_autoreleasePoolPush"><tt>objc_autoreleasePoolPush</tt></a>
				1587	on the current thread, where neither <tt>pool</tt> nor any enclosing
				1588	pool have previously been popped.</p>
				1589	<p>Releases all the objects added to the given autorelease pool and
				1590	any autorelease pools it encloses, then sets the current autorelease
				1591	pool to the pool directly enclosing <tt>pool</tt>.</p>
				1592	</div> <!-- runtime.objc_autoreleasePoolPop -->
				1593
				1594	<div id="runtime.objc_autoreleasePoolPush">
				1595	<h1><tt>void *objc_autoreleasePoolPush(void);</tt></h1>
				1596	<p>Creates a new autorelease pool that is enclosed by the current
				1597	pool, makes that the current pool, and returns an opaque <q>handle</q>
				1598	to it.</p>
				1599
				1600	<div class="rationale"><p>Rationale: while the interface is described
				1601	as an explicit hierarchy of pools, the rules allow the implementation
				1602	to just keep a stack of objects, using the stack depth as the opaque
				1603	pool handle.</p></div>
				1604
				1605	</div> <!-- runtime.objc_autoreleasePoolPush -->
				1606
				1607	<div id="runtime.objc_autoreleaseReturnValue">
				1608	<h1><tt>id objc_autoreleaseReturnValue(id value);</tt></h1>
				1609	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1610	valid object.</p>
				1611	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1612	makes a best effort to hand off ownership of a retain count on the
				1613	object to a call
				1614	to <a href="runtime.objc_retainAutoreleasedReturnValue"><tt>objc_retainAutoreleasedReturnValue</tt></a>
				1615	for the same object in an enclosing call frame. If this is not
				1616	possible, the object is autoreleased as above.</p>
				1617	<p>Always returns <tt>value</tt>.</p>
				1618	</div> <!-- runtime.objc_autoreleaseReturnValue -->
				1619
				1620	<div id="runtime.objc_copyWeak">
				1621	<h1><tt>void objc_copyWeak(id dest, id src);</tt></h1>
				1622	<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
				1623	contains a null pointer or has been registered as a <tt>__weak</tt>
				1624	object. <tt>dest</tt> is a valid pointer which has not been
				1625	registered as a <tt>__weak</tt> object.</p>
				1626	<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
				1627	potentially registering it with the runtime. Equivalent to the
				1628	following code:</p>
				1629	<pre>void objc_copyWeak(id dest, id src) {
				1630	objc_release(objc_initWeak(dest, objc_loadWeakRetained(src)));
				1631	}</pre>
				1632	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1633	on <tt>src</tt>.</p>
				1634	</div> <!-- runtime.objc_copyWeak -->
				1635
				1636	<div id="runtime.objc_destroyWeak">
				1637	<h1><tt>void objc_destroyWeak(id *object);</tt></h1>
				1638	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1639	either contains a null pointer or has been registered as
				1640	a <tt>__weak</tt> object.</p>
				1641	<p><tt>object</tt> is unregistered as a weak object, if it ever was.
				1642	The current value of <tt>object</tt> is left unspecified; otherwise,
				1643	equivalent to the following code:</p>
				1644	<pre>void objc_destroyWeak(id *object) {
				1645	objc_storeWeak(object, nil);
				1646	}</pre>
				1647	<p>Does not need to be atomic with respect to calls
				1648	to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
				1649	</div> <!-- runtime.objc_destroyWeak -->
				1650
				1651	<div id="runtime.objc_initWeak">
				1652	<h1><tt>id objc_initWeak(id *object, id value);</tt></h1>
				1653	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which has
				1654	not been registered as a <tt>__weak</tt> object. <tt>value</tt> is
				1655	null or a pointer to a valid object.</p>
				1656	<p>If <tt>value</tt> is a null pointer or the object to which it
				1657	points has begun deallocation, <tt>object</tt> is zero-initialized.
				1658	Otherwise, <tt>object</tt> is registered as a <tt>__weak</tt> object
				1659	pointing to <tt>value</tt>. Equivalent to the following code:</p>
				1660	<pre>id objc_initWeak(id *object, id value) {
				1661	*object = nil;
				1662	return objc_storeWeak(object, value);
				1663	}</pre>
				1664	<p>Returns the value of <tt>object</tt> after the call.</p>
				1665	<p>Does not need to be atomic with respect to calls
				1666	to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p>
				1667	</div> <!-- runtime.objc_initWeak -->
				1668
				1669	<div id="runtime.objc_loadWeak">
				1670	<h1><tt>id objc_loadWeak(id *object);</tt></h1>
				1671	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1672	either contains a null pointer or has been registered as
				1673	a <tt>__weak</tt> object.</p>
				1674	<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
				1675	the last value stored into <tt>object</tt> has not yet been
				1676	deallocated or begun deallocation, retains and autoreleases that value
				1677	and returns it. Otherwise returns null. Equivalent to the following
				1678	code:</p>
				1679	<pre>id objc_loadWeak(id *object) {
				1680	return objc_autorelease(objc_loadWeakRetained(object));
				1681	}</pre>
				1682	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1683	on <tt>object</tt>.</p>
				1684	<div class="rationale">Rationale: loading weak references would be
				1685	inherently prone to race conditions without the retain.</div>
				1686	</div> <!-- runtime.objc_loadWeak -->
				1687
				1688	<div id="runtime.objc_loadWeakRetained">
				1689	<h1><tt>id objc_loadWeakRetained(id *object);</tt></h1>
				1690	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1691	either contains a null pointer or has been registered as
				1692	a <tt>__weak</tt> object.</p>
				1693	<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and
				1694	the last value stored into <tt>object</tt> has not yet been
				1695	deallocated or begun deallocation, retains that value and returns it.
				1696	Otherwise returns null.</p>
				1697	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1698	on <tt>object</tt>.</p>
				1699	</div> <!-- runtime.objc_loadWeakRetained -->
				1700
				1701	<div id="runtime.objc_moveWeak">
				1702	<h1><tt>void objc_moveWeak(id dest, id src);</tt></h1>
				1703	<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either
				1704	contains a null pointer or has been registered as a <tt>__weak</tt>
				1705	object. <tt>dest</tt> is a valid pointer which has not been
				1706	registered as a <tt>__weak</tt> object.</p>
				1707	<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>,
				1708	potentially registering it with the runtime. <tt>src</tt> may then be
				1709	left in its original state, in which case this call is equivalent
				1710	to <a href="#runtime.objc_copyWeak"><tt>objc_copyWeak</tt></a>, or it
				1711	may be left as null.</p>
				1712	<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt>
				1713	on <tt>src</tt>.</p>
				1714	</div> <!-- runtime.objc_moveWeak -->
				1715
John McCall	085d09d	2011-06-19 10:12:24 +0000	[diff] [blame]	1716	<div id="runtime.objc_release">
				1717	<h1><tt>void objc_release(id value);</tt></h1>
				1718	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1719	valid object.</p>
				1720	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1721	performs a release operation exactly as if the object had been sent
				1722	the <tt>release</tt> message.</p>
				1723	</div> <!-- runtime.objc_release -->
				1724
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1725	<div id="runtime.objc_retain">
				1726	<h1><tt>id objc_retain(id value);</tt></h1>
				1727	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1728	valid object.</p>
				1729	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1730	performs a retain operation exactly as if the object had been sent
				1731	the <tt>retain</tt> message.</p>
				1732	<p>Always returns <tt>value</tt>.</p>
				1733	</div> <!-- runtime.objc_retain -->
				1734
				1735	<div id="runtime.objc_retainAutorelease">
				1736	<h1><tt>id objc_retainAutorelease(id value);</tt></h1>
				1737	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1738	valid object.</p>
				1739	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1740	performs a retain operation followed by an autorelease operation.
				1741	Equivalent to the following code:</p>
				1742	<pre>id objc_retainAutorelease(id value) {
				1743	return objc_autorelease(objc_retain(value));
				1744	}</pre>
				1745	<p>Always returns <tt>value</tt>.</p>
				1746	</div> <!-- runtime.objc_retainAutorelease -->
				1747
				1748	<div id="runtime.objc_retainAutoreleaseReturnValue">
				1749	<h1><tt>id objc_retainAutoreleaseReturnValue(id value);</tt></h1>
				1750	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1751	valid object.</p>
				1752	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1753	performs a retain operation followed by the operation described in
				1754	<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>.
				1755	Equivalent to the following code:</p>
				1756	<pre>id objc_retainAutoreleaseReturnValue(id value) {
				1757	return objc_autoreleaseReturnValue(objc_retain(value));
				1758	}</pre>
				1759	<p>Always returns <tt>value</tt>.</p>
				1760	</div> <!-- runtime.objc_retainAutoreleaseReturnValue -->
				1761
				1762	<div id="runtime.objc_retainAutoreleasedReturnValue">
				1763	<h1><tt>id objc_retainAutoreleasedReturnValue(id value);</tt></h1>
				1764	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1765	valid object.</p>
				1766	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it
				1767	attempts to accept a hand off of a retain count from a call to
				1768	<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>
				1769	on <tt>value</tt> in a recently-called function or something it
				1770	calls. If that fails, it performs a retain operation exactly
				1771	like <a href="#runtime.objc_retain"><tt>objc_retain</tt></a>.</p>
				1772	<p>Always returns <tt>value</tt>.</p>
				1773	</div> <!-- runtime.objc_retainAutoreleasedReturnValue -->
				1774
				1775	<div id="runtime.objc_retainBlock">
				1776	<h1><tt>id objc_retainBlock(id value);</tt></h1>
				1777	<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a
				1778	valid block object.</p>
				1779	<p>If <tt>value</tt> is null, this call has no effect. Otherwise, if
				1780	the block pointed to by <tt>value</tt> is still on the stack, it is
				1781	copied to the heap and the address of the copy is returned. Otherwise
				1782	a retain operation is performed on the block exactly as if it had been
				1783	sent the <tt>retain</tt> message.</p>
				1784	</div> <!-- runtime.objc_retainBlock -->
				1785
John McCall	98a48cf	2011-06-19 09:36:02 +0000	[diff] [blame]	1786	<div id="runtime.objc_storeStrong">
				1787	<h1><tt>id objc_storeStrong(id *object, id value);</tt></h1>
				1788	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer to
				1789	a <tt>__strong</tt> object which is adequately aligned for a
				1790	pointer. <tt>value</tt> is null or a pointer to a valid object.</p>
				1791	<p>Performs the complete sequence for assigning to a <tt>__strong</tt>
				1792	object of non-block type. Equivalent to the following code:</p>
				1793	<pre>id objc_storeStrong(id *object, id value) {
				1794	value = [value retain];
				1795	id oldValue = *object;
				1796	*object = value;
				1797	[oldValue release];
				1798	return value;
				1799	}</pre>
				1800	<p>Always returns <tt>value</tt>.</p>
				1801	</div> <!-- runtime.objc_storeStrong -->
				1802
				1803	<div id="runtime.objc_storeWeak">
				1804	<h1><tt>id objc_storeWeak(id *object, id value);</tt></h1>
				1805	<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which
				1806	either contains a null pointer or has been registered as
				1807	a <tt>__weak</tt> object. <tt>value</tt> is null or a pointer to a
				1808	valid object.</p>
				1809	<p>If <tt>value</tt> is a null pointer or the object to which it
				1810	points has begun deallocation, <tt>object</tt> is assigned null
				1811	and unregistered as a <tt>__weak</tt> object. Otherwise,
				1812	<tt>object</tt> is registered as a <tt>__weak</tt> object or has its
				1813	registration updated to point to <tt>value</tt>.</p>
				1814	<p>Returns the value of <tt>object</tt> after the call.</p>
				1815	</div> <!-- runtime.objc_storeWeak -->
				1816
				1817	</div> <!-- runtime -->
John McCall	8246702	2011-06-15 21:21:53 +0000	[diff] [blame]	1818	</div> <!-- root -->
				1819	</body>
				1820	</html>