diff options
Diffstat (limited to 'clang/docs/AutomaticReferenceCounting.html')
| -rw-r--r-- | clang/docs/AutomaticReferenceCounting.html | 2187 |
1 files changed, 2187 insertions, 0 deletions
diff --git a/clang/docs/AutomaticReferenceCounting.html b/clang/docs/AutomaticReferenceCounting.html new file mode 100644 index 0000000..1416df5 --- /dev/null +++ b/clang/docs/AutomaticReferenceCounting.html @@ -0,0 +1,2187 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> +<title>Objective-C Automatic Reference Counting (ARC)</title> +<link type="text/css" rel="stylesheet" href="../menu.css"> +<link type="text/css" rel="stylesheet" href="../content.css"> +<style type="text/css"> +/* Collapse the items in the ToC to the left. */ +div#toc ul { + padding-left: 0 +} + +/* Rationales appear in italic. */ +div.rationale { + font-style: italic +} + +div.rationale em { + font-style: normal +} + +/* Revisions are also italicized. */ +span.revision { + font-style: italic +} + +span.whenRevised { + font-weight: bold; + font-style: normal +} + +div h1 { font-size: 2em; margin: .67em 0 } +div div h1 { font-size: 1.5em; margin: .75em 0 } +div div div h1 { font-size: 1.17em; margin: .83em 0 } +div div div div h1 { margin: 1.12em 0 } + +span.term { font-style: italic; font-weight: bold } +</style> + +<script type="text/javascript"> +/// A little script to recursively build a table of contents. +function buildTOC(div, toc, ancestry) { + var children = div.childNodes; + var len = children.length; + + var childNumber = 0; + + var list = null; + for (var i = 0; i < len; ++i) { + var child = children[i]; + if (child.nodeName != "DIV") continue; + if (child.getAttribute("class") == "rationale") continue; + if (child.id == "toc") continue; + + // Okay, we're actually going to build a list node. + if (list === null) list = document.createElement("ul"); + + var childAncestry = ancestry + ++childNumber + "."; + + var headerNode = child.childNodes[1]; + var title = headerNode.innerHTML; + headerNode.insertBefore(document.createTextNode(childAncestry + " "), + headerNode.firstChild); + + var item = document.createElement("li"); + item.appendChild(document.createTextNode(childAncestry + " ")); + + var anchor = document.createElement("a"); + anchor.href = "#" + child.id; + anchor.innerHTML = title; + item.appendChild(anchor); + + buildTOC(child, item, childAncestry); + + list.appendChild(item); + } + if (list) toc.appendChild(list); +} + +function onLoad() { + var toc = document.getElementById("toc"); + var content = document.getElementById("content"); + buildTOC(content, toc, ""); +} +window.onload = onLoad; + +</script> +</head> +<body> + +<!--#include virtual="../menu.html.incl"--> + +<div id="content"> +<h1>Automatic Reference Counting</h1> + +<div id="toc"> +</div> + +<div id="meta"> +<h1>About this document</h1> + +<div id="meta.purpose"> +<h1>Purpose</h1> + +<p>The first and primary purpose of this document is to serve as a +complete technical specification of Automatic Reference Counting. +Given a core Objective-C compiler and runtime, it should be possible +to write a compiler and runtime which implements these new +semantics.</p> + +<p>The secondary purpose is to act as a rationale for why ARC was +designed in this way. This should remain tightly focused on the +technical design and should not stray into marketing speculation.</p> + +</div> <!-- meta.purpose --> + +<div id="meta.background"> +<h1>Background</h1> + +<p>This document assumes a basic familiarity with C.</p> + +<p><span class="term">Blocks</span> are a C language extension for +creating anonymous functions. Users interact with and transfer block +objects using <span class="term">block pointers</span>, which are +represented like a normal pointer. A block may capture values from +local variables; when this occurs, memory must be dynamically +allocated. The initial allocation is done on the stack, but the +runtime provides a <tt>Block_copy</tt> function which, given a block +pointer, either copies the underlying block object to the heap, +setting its reference count to 1 and returning the new block pointer, +or (if the block object is already on the heap) increases its +reference count by 1. The paired function is <tt>Block_release</tt>, +which decreases the reference count by 1 and destroys the object if +the count reaches zero and is on the heap.</p> + +<p>Objective-C is a set of language extensions, significant enough to +be considered a different language. It is a strict superset of C. +The extensions can also be imposed on C++, producing a language called +Objective-C++. The primary feature is a single-inheritance object +system; we briefly describe the modern dialect.</p> + +<p>Objective-C defines a new type kind, collectively called +the <span class="term">object pointer types</span>. This kind has two +notable builtin members, <tt>id</tt> and <tt>Class</tt>; <tt>id</tt> +is the final supertype of all object pointers. The validity of +conversions between object pointer types is not checked at runtime. +Users may define <span class="term">classes</span>; each class is a +type, and the pointer to that type is an object pointer type. A class +may have a superclass; its pointer type is a subtype of its +superclass's pointer type. A class has a set +of <span class="term">ivars</span>, fields which appear on all +instances of that class. For every class <i>T</i> there's an +associated metaclass; it has no fields, its superclass is the +metaclass of <i>T</i>'s superclass, and its metaclass is a global +class. Every class has a global object whose class is the +class's metaclass; metaclasses have no associated type, so pointers to +this object have type <tt>Class</tt>.</p> + +<p>A class declaration (<tt>@interface</tt>) declares a set +of <span class="term">methods</span>. A method has a return type, a +list of argument types, and a <span class="term">selector</span>: a +name like <tt>foo:bar:baz:</tt>, where the number of colons +corresponds to the number of formal arguments. A method may be an +instance method, in which case it can be invoked on objects of the +class, or a class method, in which case it can be invoked on objects +of the metaclass. A method may be invoked by providing an object +(called the <span class="term">receiver</span>) and a list of formal +arguments interspersed with the selector, like so:</p> + +<pre>[receiver foo: fooArg bar: barArg baz: bazArg]</pre> + +<p>This looks in the dynamic class of the receiver for a method with +this name, then in that class's superclass, etc., until it finds +something it can execute. The receiver <q>expression</q> may also be +the name of a class, in which case the actual receiver is the class +object for that class, or (within method definitions) it may +be <tt>super</tt>, in which case the lookup algorithm starts with the +static superclass instead of the dynamic class. The actual methods +dynamically found in a class are not those declared in the +<tt>@interface</tt>, but those defined in a separate +<tt>@implementation</tt> declaration; however, when compiling a +call, typechecking is done based on the methods declared in the +<tt>@interface</tt>.</p> + +<p>Method declarations may also be grouped into +<span class="term">protocols</span>, which are not inherently +associated with any class, but which classes may claim to follow. +Object pointer types may be qualified with additional protocols that +the object is known to support.</p> + +<p><span class="term">Class extensions</span> are collections of ivars +and methods, designed to allow a class's <tt>@interface</tt> to be +split across multiple files; however, there is still a primary +implementation file which must see the <tt>@interface</tt>s of all +class extensions. +<span class="term">Categories</span> allow methods (but not ivars) to +be declared <i>post hoc</i> on an arbitrary class; the methods in the +category's <tt>@implementation</tt> will be dynamically added to that +class's method tables which the category is loaded at runtime, +replacing those methods in case of a collision.</p> + +<p>In the standard environment, objects are allocated on the heap, and +their lifetime is manually managed using a reference count. This is +done using two instance methods which all classes are expected to +implement: <tt>retain</tt> increases the object's reference count by +1, whereas <tt>release</tt> decreases it by 1 and calls the instance +method <tt>dealloc</tt> if the count reaches 0. To simplify certain +operations, there is also an <span class="term">autorelease +pool</span>, a thread-local list of objects to call <tt>release</tt> +on later; an object can be added to this pool by +calling <tt>autorelease</tt> on it.</p> + +<p>Block pointers may be converted to type <tt>id</tt>; block objects +are laid out in a way that makes them compatible with Objective-C +objects. There is a builtin class that all block objects are +considered to be objects of; this class implements <tt>retain</tt> by +adjusting the reference count, not by calling <tt>Block_copy</tt>.</p> + +</div> <!-- meta.background --> + +<div id="meta.evolution"> +<h1>Evolution</h1> + +<p>ARC is under continual evolution, and this document must be updated +as the language progresses.</p> + +<p>If a change increases the expressiveness of the language, for +example by lifting a restriction or by adding new syntax, the change +will be annotated with a revision marker, like so:</p> + +<blockquote> + ARC applies to Objective-C pointer types, block pointer types, and + <span class="revision"><span class="whenRevised">[beginning Apple + 8.0, LLVM 3.8]</span> BPTRs declared within <code>extern + "BCPL"</code> blocks</span>. +</blockquote> + +<p>For now, it is sensible to version this document by the releases of +its sole implementation (and its host project), clang. +<q>LLVM X.Y</q> refers to an open-source release of clang from the +LLVM project. <q>Apple X.Y</q> refers to an Apple-provided release of +the Apple LLVM Compiler. Other organizations that prepare their own, +separately-versioned clang releases and wish to maintain similar +information in this document should send requests to cfe-dev.</p> + +<p>If a change decreases the expressiveness of the language, for +example by imposing a new restriction, this should be taken as an +oversight in the original specification and something to be avoided +in all versions. Such changes are generally to be avoided.</p> + +</div> <!-- meta.evolution --> + +</div> <!-- meta --> + +<div id="general"> +<h1>General</h1> + +<p>Automatic Reference Counting implements automatic memory management +for Objective-C objects and blocks, freeing the programmer from the +need to explicitly insert retains and releases. It does not provide a +cycle collector; users must explicitly manage the lifetime of their +objects, breaking cycles manually or with weak or unsafe +references.</p> + +<p>ARC may be explicitly enabled with the compiler +flag <tt>-fobjc-arc</tt>. It may also be explicitly disabled with the +compiler flag <tt>-fno-objc-arc</tt>. The last of these two flags +appearing on the compile line <q>wins</q>.</p> + +<p>If ARC is enabled, <tt>__has_feature(objc_arc)</tt> will expand to +1 in the preprocessor. For more information about <tt>__has_feature</tt>, +see the <a href="LanguageExtensions.html#__has_feature_extension">language +extensions</a> document.</p> + +</div> <!-- general --> + +<div id="objects"> +<h1>Retainable object pointers</h1> + +<p>This section describes retainable object pointers, their basic +operations, and the restrictions imposed on their use under ARC. Note +in particular that it covers the rules for pointer <em>values</em> +(patterns of bits indicating the location of a pointed-to object), not +pointer +<em>objects</em> (locations in memory which store pointer values). +The rules for objects are covered in the next section.</p> + +<p>A <span class="term">retainable object pointer</span> +(or <q>retainable pointer</q>) is a value of +a <span class="term">retainable object pointer type</span> +(<q>retainable type</q>). There are three kinds of retainable object +pointer types:</p> +<ul> +<li>block pointers (formed by applying the caret (<tt>^</tt>) +declarator sigil to a function type)</li> +<li>Objective-C object pointers (<tt>id</tt>, <tt>Class</tt>, <tt>NSFoo*</tt>, etc.)</li> +<li>typedefs marked with <tt>__attribute__((NSObject))</tt></li> +</ul> + +<p>Other pointer types, such as <tt>int*</tt> and <tt>CFStringRef</tt>, +are not subject to ARC's semantics and restrictions.</p> + +<div class="rationale"> + +<p>Rationale: We are not at liberty to require +all code to be recompiled with ARC; therefore, ARC must interoperate +with Objective-C code which manages retains and releases manually. In +general, there are three requirements in order for a +compiler-supported reference-count system to provide reliable +interoperation:</p> + +<ul> +<li>The type system must reliably identify which objects are to be +managed. An <tt>int*</tt> might be a pointer to a <tt>malloc</tt>'ed +array, or it might be a interior pointer to such an array, or it might +point to some field or local variable. In contrast, values of the +retainable object pointer types are never interior.</li> +<li>The type system must reliably indicate how to +manage objects of a type. This usually means that the type must imply +a procedure for incrementing and decrementing retain counts. +Supporting single-ownership objects requires a lot more explicit +mediation in the language.</li> +<li>There must be reliable conventions for whether and +when <q>ownership</q> is passed between caller and callee, for both +arguments and return values. Objective-C methods follow such a +convention very reliably, at least for system libraries on Mac OS X, +and functions always pass objects at +0. The C-based APIs for Core +Foundation objects, on the other hand, have much more varied transfer +semantics.</li> +</ul> +</div> <!-- rationale --> + +<p>The use of <tt>__attribute__((NSObject))</tt> typedefs is not +recommended. If it's absolutely necessary to use this attribute, be +very explicit about using the typedef, and do not assume that it will +be preserved by language features like <tt>__typeof</tt> and C++ +template argument substitution.</p> + +<div class="rationale"><p>Rationale: any compiler operation which +incidentally strips type <q>sugar</q> from a type will yield a type +without the attribute, which may result in unexpected +behavior.</p></div> + +<div id="objects.retains"> +<h1>Retain count semantics</h1> + +<p>A retainable object pointer is either a <span class="term">null +pointer</span> or a pointer to a valid object. Furthermore, if it has +block pointer type and is not <tt>null</tt> then it must actually be a +pointer to a block object, and if it has <tt>Class</tt> type (possibly +protocol-qualified) then it must actually be a pointer to a class +object. Otherwise ARC does not enforce the Objective-C type system as +long as the implementing methods follow the signature of the static +type. It is undefined behavior if ARC is exposed to an invalid +pointer.</p> + +<p>For ARC's purposes, a valid object is one with <q>well-behaved</q> +retaining operations. Specifically, the object must be laid out such +that the Objective-C message send machinery can successfully send it +the following messages:</p> + +<ul> +<li><tt>retain</tt>, taking no arguments and returning a pointer to +the object.</li> +<li><tt>release</tt>, taking no arguments and returning <tt>void</tt>.</li> +<li><tt>autorelease</tt>, taking no arguments and returning a pointer +to the object.</li> +</ul> + +<p>The behavior of these methods is constrained in the following ways. +The term <span class="term">high-level semantics</span> is an +intentionally vague term; the intent is that programmers must +implement these methods in a way such that the compiler, modifying +code in ways it deems safe according to these constraints, will not +violate their requirements. For example, if the user puts logging +statements in <tt>retain</tt>, they should not be surprised if those +statements are executed more or less often depending on optimization +settings. These constraints are not exhaustive of the optimization +opportunities: values held in local variables are subject to +additional restrictions, described later in this document.</p> + +<p>It is undefined behavior if a computation history featuring a send +of <tt>retain</tt> followed by a send of <tt>release</tt> to the same +object, with no intervening <tt>release</tt> on that object, is not +equivalent under the high-level semantics to a computation +history in which these sends are removed. Note that this implies that +these methods may not raise exceptions.</p> + +<p>It is undefined behavior if a computation history features any use +whatsoever of an object following the completion of a send +of <tt>release</tt> that is not preceded by a send of <tt>retain</tt> +to the same object.</p> + +<p>The behavior of <tt>autorelease</tt> must be equivalent to sending +<tt>release</tt> when one of the autorelease pools currently in scope +is popped. It may not throw an exception.</p> + +<p>When the semantics call for performing one of these operations on a +retainable object pointer, if that pointer is <tt>null</tt> then the +effect is a no-op.</p> + +<p>All of the semantics described in this document are subject to +additional <a href="#optimization">optimization rules</a> which permit +the removal or optimization of operations based on local knowledge of +data flow. The semantics describe the high-level behaviors that the +compiler implements, not an exact sequence of operations that a +program will be compiled into.</p> + +</div> <!-- objects.retains --> + +<div id="objects.operands"> +<h1>Retainable object pointers as operands and arguments</h1> + +<p>In general, ARC does not perform retain or release operations when +simply using a retainable object pointer as an operand within an +expression. This includes:</p> +<ul> +<li>loading a retainable pointer from an object with non-weak +<a href="#ownership">ownership</a>,</li> +<li>passing a retainable pointer as an argument to a function or +method, and</li> +<li>receiving a retainable pointer as the result of a function or +method call.</li> +</ul> + +<div class="rationale"><p>Rationale: while this might seem +uncontroversial, it is actually unsafe when multiple expressions are +evaluated in <q>parallel</q>, as with binary operators and calls, +because (for example) one expression might load from an object while +another writes to it. However, C and C++ already call this undefined +behavior because the evaluations are unsequenced, and ARC simply +exploits that here to avoid needing to retain arguments across a large +number of calls.</p></div> + +<p>The remainder of this section describes exceptions to these rules, +how those exceptions are detected, and what those exceptions imply +semantically.</p> + +<div id="objects.operands.consumed"> +<h1>Consumed parameters</h1> + +<p>A function or method parameter of retainable object pointer type +may be marked as <span class="term">consumed</span>, signifying that +the callee expects to take ownership of a +1 retain count. This is +done by adding the <tt>ns_consumed</tt> attribute to the parameter +declaration, like so:</p> + +<pre>void foo(__attribute((ns_consumed)) id x); +- (void) foo: (id) __attribute((ns_consumed)) x;</pre> + +<p>This attribute is part of the type of the function or method, not +the type of the parameter. It controls only how the argument is +passed and received.</p> + +<p>When passing such an argument, ARC retains the argument prior to +making the call.</p> + +<p>When receiving such an argument, ARC releases the argument at the +end of the function, subject to the usual optimizations for local +values.</p> + +<div class="rationale"><p>Rationale: this formalizes direct transfers +of ownership from a caller to a callee. The most common scenario here +is passing the <tt>self</tt> parameter to <tt>init</tt>, but it is +useful to generalize. Typically, local optimization will remove any +extra retains and releases: on the caller side the retain will be +merged with a +1 source, and on the callee side the release will be +rolled into the initialization of the parameter.</p></div> + +<p>The implicit <tt>self</tt> parameter of a method may be marked as +consumed by adding <tt>__attribute__((ns_consumes_self))</tt> to the +method declaration. Methods in the <tt>init</tt> +<a href="#family">family</a> are treated as if they were implicitly +marked with this attribute.</p> + +<p>It is undefined behavior if an Objective-C message send to a method +with <tt>ns_consumed</tt> parameters (other than self) is made with a +null receiver. It is undefined behavior if the method to which an +Objective-C message send statically resolves to has a different set +of <tt>ns_consumed</tt> parameters than the method it dynamically +resolves to. It is undefined behavior if a block or function call is +made through a static type with a different set of <tt>ns_consumed</tt> +parameters than the implementation of the called block or function.</p> + +<div class="rationale"><p>Rationale: consumed parameters with null +receiver are a guaranteed leak. Mismatches with consumed parameters +will cause over-retains or over-releases, depending on the direction. +The rule about function calls is really just an application of the +existing C/C++ rule about calling functions through an incompatible +function type, but it's useful to state it explicitly.</p></div> + +</div> <!-- objects.operands.consumed --> + +<div id="objects.operands.retained-returns"> +<h1>Retained return values</h1> + +<p>A function or method which returns a retainable object pointer type +may be marked as returning a retained value, signifying that the +caller expects to take ownership of a +1 retain count. This is done +by adding the <tt>ns_returns_retained</tt> attribute to the function or +method declaration, like so:</p> + +<pre>id foo(void) __attribute((ns_returns_retained)); +- (id) foo __attribute((ns_returns_retained));</pre> + +<p>This attribute is part of the type of the function or method.</p> + +<p>When returning from such a function or method, ARC retains the +value at the point of evaluation of the return statement, before +leaving all local scopes.</p> + +<p>When receiving a return result from such a function or method, ARC +releases the value at the end of the full-expression it is contained +within, subject to the usual optimizations for local values.</p> + +<div class="rationale"><p>Rationale: this formalizes direct transfers of +ownership from a callee to a caller. The most common scenario this +models is the retained return from <tt>init</tt>, <tt>alloc</tt>, +<tt>new</tt>, and <tt>copy</tt> methods, but there are other cases in +the frameworks. After optimization there are typically no extra +retains and releases required.</p></div> + +<p>Methods in +the <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>, <tt>mutableCopy</tt>, +and <tt>new</tt> <a href="#family">families</a> are implicitly marked +<tt>__attribute__((ns_returns_retained))</tt>. This may be suppressed +by explicitly marking the +method <tt>__attribute__((ns_returns_not_retained))</tt>.</p> + +<p>It is undefined behavior if the method to which an Objective-C +message send statically resolves has different retain semantics on its +result from the method it dynamically resolves to. It is undefined +behavior if a block or function call is made through a static type +with different retain semantics on its result from the implementation +of the called block or function.</p> + +<div class="rationale"><p>Rationale: Mismatches with returned results +will cause over-retains or over-releases, depending on the direction. +Again, the rule about function calls is really just an application of +the existing C/C++ rule about calling functions through an +incompatible function type.</p></div> + +</div> <!-- objects.operands.retained-returns --> + +<div id="objects.operands.other-returns"> +<h1>Unretained return values</h1> + +<p>A method or function which returns a retainable object type but +does not return a retained value must ensure that the object is +still valid across the return boundary.</p> + +<p>When returning from such a function or method, ARC retains the +value at the point of evaluation of the return statement, then leaves +all local scopes, and then balances out the retain while ensuring that +the value lives across the call boundary. In the worst case, this may +involve an <tt>autorelease</tt>, but callers must not assume that the +value is actually in the autorelease pool.</p> + +<p>ARC performs no extra mandatory work on the caller side, although +it may elect to do something to shorten the lifetime of the returned +value.</p> + +<div class="rationale"><p>Rationale: it is common in non-ARC code to not +return an autoreleased value; therefore the convention does not force +either path. It is convenient to not be required to do unnecessary +retains and autoreleases; this permits optimizations such as eliding +retain/autoreleases when it can be shown that the original pointer +will still be valid at the point of return.</p></div> + +<p>A method or function may be marked +with <tt>__attribute__((ns_returns_autoreleased))</tt> to indicate +that it returns a pointer which is guaranteed to be valid at least as +long as the innermost autorelease pool. There are no additional +semantics enforced in the definition of such a method; it merely +enables optimizations in callers.</p> + +</div> <!-- objects.operands.other-returns --> + +<div id="objects.operands.casts"> +<h1>Bridged casts</h1> + +<p>A <span class="term">bridged cast</span> is a C-style cast +annotated with one of three keywords:</p> + +<ul> +<li><tt>(__bridge T) op</tt> casts the operand to the destination +type <tt>T</tt>. If <tt>T</tt> is a retainable object pointer type, +then <tt>op</tt> must have a non-retainable pointer type. +If <tt>T</tt> is a non-retainable pointer type, then <tt>op</tt> must +have a retainable object pointer type. Otherwise the cast is +ill-formed. There is no transfer of ownership, and ARC inserts +no retain operations.</li> + +<li><tt>(__bridge_retained T) op</tt> casts the operand, which must +have retainable object pointer type, to the destination type, which +must be a non-retainable pointer type. ARC retains the value, subject +to the usual optimizations on local values, and the recipient is +responsible for balancing that +1.</li> + +<li><tt>(__bridge_transfer T) op</tt> casts the operand, which must +have non-retainable pointer type, to the destination type, which must +be a retainable object pointer type. ARC will release the value at +the end of the enclosing full-expression, subject to the usual +optimizations on local values.</li> +</ul> + +<p>These casts are required in order to transfer objects in and out of +ARC control; see the rationale in the section +on <a href="#objects.restrictions.conversion">conversion of retainable +object pointers</a>.</p> + +<p>Using a <tt>__bridge_retained</tt> or <tt>__bridge_transfer</tt> +cast purely to convince ARC to emit an unbalanced retain or release, +respectively, is poor form.</p> + +</div> <!-- objects.operands.casts --> + +</div> <!-- objects.operands --> + +<div id="objects.restrictions"> +<h1>Restrictions</h1> + +<div id="objects.restrictions.conversion"> +<h1>Conversion of retainable object pointers</h1> + +<p>In general, a program which attempts to implicitly or explicitly +convert a value of retainable object pointer type to any +non-retainable type, or vice-versa, is ill-formed. For example, an +Objective-C object pointer shall not be converted to <tt>void*</tt>. +As an exception, cast to <tt>intptr_t</tt> is allowed because such +casts are not transferring ownership. The <a href="#objects.operands.casts">bridged +casts</a> may be used to perform these conversions where +necessary.</p> + +<div class="rationale"><p>Rationale: we cannot ensure the correct +management of the lifetime of objects if they may be freely passed +around as unmanaged types. The bridged casts are provided so that the +programmer may explicitly describe whether the cast transfers control +into or out of ARC.</p></div> + +<p>However, the following exceptions apply.</p> + +</div> <!-- objects.restrictions.conversion --> + +<div id="objects.restrictions.conversion-exception-known"> +<h1>Conversion to retainable object pointer type of + expressions with known semantics</h1> + +<p><span class="revision"><span class="whenRevised">[beginning Apple + 4.0, LLVM 3.1]</span> These exceptions have been greatly expanded; + they previously applied only to a much-reduced subset which is + difficult to categorize but which included null pointers, message + sends (under the given rules), and the various global constants.</span></p> + +<p>An unbridged conversion to a retainable object pointer type from a +type other than a retainable object pointer type is ill-formed, as +discussed above, unless the operand of the cast has a syntactic form +which is known retained, known unretained, or known +retain-agnostic.</p> + +<p>An expression is <span class="term">known retain-agnostic</span> if +it is:</p> +<ul> +<li>an Objective-C string literal,</li> +<li>a load from a <tt>const</tt> system global variable of +<a href="#misc.c-retainable">C retainable pointer type</a>, or</li> +<li>a null pointer constant.</li> +</ul> + +<p>An expression is <span class="term">known unretained</span> if it +is an rvalue of <a href="#misc.c-retainable">C retainable +pointer type</a> and it is:</p> +<ul> +<li>a direct call to a function, and either that function has the + <tt>cf_returns_not_retained</tt> attribute or it is an + <a href="#misc.c-retainable.audit">audited</a> function that does not + have the <tt>cf_returns_retained</tt> attribute and does not follow + the create/copy naming convention,</li> +<li>a message send, and the declared method either has + the <tt>cf_returns_not_retained</tt> attribute or it has neither + the <tt>cf_returns_retained</tt> attribute nor a + <a href="#family">selector family</a> that implies a retained + result.</li> +</ul> + +<p>An expression is <span class="term">known retained</span> if it is +an rvalue of <a href="#misc.c-retainable">C retainable pointer type</a> +and it is:</p> +<ul> +<li>a message send, and the declared method either has the + <tt>cf_returns_retained</tt> attribute, or it does not have + the <tt>cf_returns_not_retained</tt> attribute but it does have a + <a href="#family">selector family</a> that implies a retained + result.</li> +</ul> + +<p>Furthermore:</p> +<ul> +<li>a comma expression is classified according to its right-hand side,</li> +<li>a statement expression is classified according to its result +expression, if it has one,</li> +<li>an lvalue-to-rvalue conversion applied to an Objective-C property +lvalue is classified according to the underlying message send, and</li> +<li>a conditional operator is classified according to its second and +third operands, if they agree in classification, or else the other +if one is known retain-agnostic.</li> +</ul> + +<p>If the cast operand is known retained, the conversion is treated as +a <tt>__bridge_transfer</tt> cast. If the cast operand is known +unretained or known retain-agnostic, the conversion is treated as +a <tt>__bridge</tt> cast.</p> + +<div class="rationale"><p>Rationale: Bridging casts are annoying. +Absent the ability to completely automate the management of CF +objects, however, we are left with relatively poor attempts to reduce +the need for a glut of explicit bridges. Hence these rules.</p> + +<p>We've so far consciously refrained from implicitly turning retained +CF results from function calls into <tt>__bridge_transfer</tt> casts. +The worry is that some code patterns — for example, creating a +CF value, assigning it to an ObjC-typed local, and then +calling <tt>CFRelease</tt> when done — are a bit too likely to +be accidentally accepted, leading to mysterious behavior.</p></div> + +</div> <!-- objects.restrictions.conversion-exception-known --> + +<div id="objects.restrictions.conversion-exception-contextual"> +<h1>Conversion from retainable object pointer type in certain contexts</h1> + +<p><span class="revision"><span class="whenRevised">[beginning Apple + 4.0, LLVM 3.1]</span></span></p> + +<p>If an expression of retainable object pointer type is explicitly +cast to a <a href="#misc.c-retainable">C retainable pointer type</a>, +the program is ill-formed as discussed above unless the result is +immediately used:</p> + +<ul> +<li>to initialize a parameter in an Objective-C message send where the +parameter is not marked with the <tt>cf_consumed</tt> attribute, or</li> +<li>to initialize a parameter in a direct call to +an <a href="#misc.c-retainable.audit">audited</a> function where the +parameter is not marked with the <tt>cf_consumed</tt> attribute.</li> +</ul> + +<div class="rationale"><p>Rationale: Consumed parameters are left out +because ARC would naturally balance them with a retain, which was +judged too treacherous. This is in part because several of the most +common consuming functions are in the <tt>Release</tt> family, and it +would be quite unfortunate for explicit releases to be silently +balanced out in this way.</p></div> + +</div> <!-- objects.restrictions.conversion-exception-contextual --> + +</div> <!-- objects.restrictions --> + +</div> <!-- objects --> + +<div id="ownership"> +<h1>Ownership qualification</h1> + +<p>This section describes the behavior of <em>objects</em> of +retainable object pointer type; that is, locations in memory which +store retainable object pointers.</p> + +<p>A type is a <span class="term">retainable object owner type</span> +if it is a retainable object pointer type or an array type whose +element type is a retainable object owner type.</p> + +<p>An <span class="term">ownership qualifier</span> is a type +qualifier which applies only to retainable object owner types. An array type is +ownership-qualified according to its element type, and adding an ownership +qualifier to an array type so qualifies its element type.</p> + +<p>A program is ill-formed if it attempts to apply an ownership qualifier +to a type which is already ownership-qualified, even if it is the same +qualifier. There is a single exception to this rule: an ownership qualifier +may be applied to a substituted template type parameter, which overrides the +ownership qualifier provided by the template argument.</p> + +<p>Except as described under +the <a href="#ownership.inference">inference rules</a>, a program is +ill-formed if it attempts to form a pointer or reference type to a +retainable object owner type which lacks an ownership qualifier.</p> + +<div class="rationale"><p>Rationale: these rules, together with the +inference rules, ensure that all objects and lvalues of retainable +object pointer type have an ownership qualifier. The ability to override an ownership qualifier during template substitution is required to counteract the <a href="#ownership.inference.template_arguments">inference of <tt>__strong</tt> for template type arguments</a>. </p></div> + +<p>There are four ownership qualifiers:</p> + +<ul> +<li><tt>__autoreleasing</tt></li> +<li><tt>__strong</tt></li> +<li><tt>__unsafe_unretained</tt></li> +<li><tt>__weak</tt></li> +</ul> + +<p>A type is <span class="term">nontrivially ownership-qualified</span> +if it is qualified with <tt>__autoreleasing</tt>, <tt>__strong</tt>, or +<tt>__weak</tt>.</p> + +<div id="ownership.spelling"> +<h1>Spelling</h1> + +<p>The names of the ownership qualifiers are reserved for the +implementation. A program may not assume that they are or are not +implemented with macros, or what those macros expand to.</p> + +<p>An ownership qualifier may be written anywhere that any other type +qualifier may be written.</p> + +<p>If an ownership qualifier appears in +the <i>declaration-specifiers</i>, the following rules apply:</p> + +<ul> +<li>if the type specifier is a retainable object owner type, the +qualifier applies to that type;</li> +<li>if the outermost non-array part of the declarator is a pointer or +block pointer, the qualifier applies to that type;</li> +<li>otherwise the program is ill-formed.</li> +</ul> + +<p>If an ownership qualifier appears on the declarator name, or on the +declared object, it is applied to outermost pointer or block-pointer +type.</p> + +<p>If an ownership qualifier appears anywhere else in a declarator, it +applies to the type there.</p> + +<div id="ownership.spelling.property"> +<h1>Property declarations</h1> + +<p>A property of retainable object pointer type may have ownership. +If the property's type is ownership-qualified, then the property has +that ownership. If the property has one of the following modifiers, +then the property has the corresponding ownership. A property is +ill-formed if it has conflicting sources of ownership, or if it has +redundant ownership modifiers, or if it has <tt>__autoreleasing</tt> +ownership.</p> + +<ul> +<li><tt>assign</tt> implies <tt>__unsafe_unretained</tt> ownership.</li> +<li><tt>copy</tt> implies <tt>__strong</tt> ownership, as well as the + usual behavior of copy semantics on the setter.</li> +<li><tt>retain</tt> implies <tt>__strong</tt> ownership.</li> +<li><tt>strong</tt> implies <tt>__strong</tt> ownership.</li> +<li><tt>unsafe_unretained</tt> implies <tt>__unsafe_unretained</tt> + ownership.</li> +<li><tt>weak</tt> implies <tt>__weak</tt> ownership.</li> +</ul> + +<p>With the exception of <tt>weak</tt>, these modifiers are available +in non-ARC modes.</p> + +<p>A property's specified ownership is preserved in its metadata, but +otherwise the meaning is purely conventional unless the property is +synthesized. If a property is synthesized, then the +<span class="term">associated instance variable</span> is the +instance variable which is named, possibly implicitly, by the +<tt>@synthesize</tt> declaration. If the associated instance variable +already exists, then its ownership qualification must equal the +ownership of the property; otherwise, the instance variable is created +with that ownership qualification.</p> + +<p>A property of retainable object pointer type which is synthesized +without a source of ownership has the ownership of its associated +instance variable, if it already exists; otherwise, +<span class="revision"><span class="whenRevised">[beginning Apple 3.1, +LLVM 3.1]</span> its ownership is implicitly <tt>strong</tt></span>. +Prior to this revision, it was ill-formed to synthesize such a +property.</p> + +<div class="rationale"><p>Rationale: using <tt>strong</tt> by default +is safe and consistent with the generic ARC rule about +<a href="#ownership.inference.variables">inferring ownership</a>. It +is, unfortunately, inconsistent with the non-ARC rule which states +that such properties are implicitly <tt>assign</tt>. However, that +rule is clearly untenable in ARC, since it leads to default-unsafe +code. The main merit to banning the properties is to avoid confusion +with non-ARC practice, which did not ultimately strike us as +sufficient to justify requiring extra syntax and (more importantly) +forcing novices to understand ownership rules just to declare a +property when the default is so reasonable. Changing the rule away +from non-ARC practice was acceptable because we had conservatively +banned the synthesis in order to give ourselves exactly this +leeway.</p></div> + +</div> <!-- ownership.spelling.property --> + +</div> <!-- ownership.spelling --> + +<div id="ownership.semantics"> +<h1>Semantics</h1> + +<p>There are five <span class="term">managed operations</span> which +may be performed on an object of retainable object pointer type. Each +qualifier specifies different semantics for each of these operations. +It is still undefined behavior to access an object outside of its +lifetime.</p> + +<p>A load or store with <q>primitive semantics</q> has the same +semantics as the respective operation would have on an <tt>void*</tt> +lvalue with the same alignment and non-ownership qualification.</p> + +<p><span class="term">Reading</span> occurs when performing a +lvalue-to-rvalue conversion on an object lvalue.</p> + +<ul> +<li>For <tt>__weak</tt> objects, the current pointee is retained and +then released at the end of the current full-expression. This must +execute atomically with respect to assignments and to the final +release of the pointee.</li> +<li>For all other objects, the lvalue is loaded with primitive +semantics.</li> +</ul> + +<p><span class="term">Assignment</span> occurs when evaluating +an assignment operator. The semantics vary based on the qualification:</p> +<ul> +<li>For <tt>__strong</tt> objects, the new pointee is first retained; +second, the lvalue is loaded with primitive semantics; third, the new +pointee is stored into the lvalue with primitive semantics; and +finally, the old pointee is released. This is not performed +atomically; external synchronization must be used to make this safe in +the face of concurrent loads and stores.</li> +<li>For <tt>__weak</tt> objects, the lvalue is updated to point to the +new pointee, unless the new pointee is an object currently undergoing +deallocation, in which case the lvalue is updated to a null pointer. +This must execute atomically with respect to other assignments to the +object, to reads from the object, and to the final release of the new +pointee.</li> +<li>For <tt>__unsafe_unretained</tt> objects, the new pointee is +stored into the lvalue using primitive semantics.</li> +<li>For <tt>__autoreleasing</tt> objects, the new pointee is retained, +autoreleased, and stored into the lvalue using primitive semantics.</li> +</ul> + +<p><span class="term">Initialization</span> occurs when an object's +lifetime begins, which depends on its storage duration. +Initialization proceeds in two stages:</p> +<ol> +<li>First, a null pointer is stored into the lvalue using primitive +semantics. This step is skipped if the object +is <tt>__unsafe_unretained</tt>.</li> +<li>Second, if the object has an initializer, that expression is +evaluated and then assigned into the object using the usual assignment +semantics.</li> +</ol> + +<p><span class="term">Destruction</span> occurs when an object's +lifetime ends. In all cases it is semantically equivalent to +assigning a null pointer to the object, with the proviso that of +course the object cannot be legally read after the object's lifetime +ends.</p> + +<p><span class="term">Moving</span> occurs in specific situations +where an lvalue is <q>moved from</q>, meaning that its current pointee +will be used but the object may be left in a different (but still +valid) state. This arises with <tt>__block</tt> variables and rvalue +references in C++. For <tt>__strong</tt> lvalues, moving is equivalent +to loading the lvalue with primitive semantics, writing a null pointer +to it with primitive semantics, and then releasing the result of the +load at the end of the current full-expression. For all other +lvalues, moving is equivalent to reading the object.</p> + +</div> <!-- ownership.semantics --> + +<div id="ownership.restrictions"> +<h1>Restrictions</h1> + +<div id="ownership.restrictions.weak"> +<h1>Weak-unavailable types</h1> + +<p>It is explicitly permitted for Objective-C classes to not +support <tt>__weak</tt> references. It is undefined behavior to +perform an operation with weak assignment semantics with a pointer to +an Objective-C object whose class does not support <tt>__weak</tt> +references.</p> + +<div class="rationale"><p>Rationale: historically, it has been +possible for a class to provide its own reference-count implementation +by overriding <tt>retain</tt>, <tt>release</tt>, etc. However, weak +references to an object require coordination with its class's +reference-count implementation because, among other things, weak loads +and stores must be atomic with respect to the final release. +Therefore, existing custom reference-count implementations will +generally not support weak references without additional effort. This +is unavoidable without breaking binary compatibility.</p></div> + +<p>A class may indicate that it does not support weak references by +providing the <tt>objc_arc_weak_unavailable</tt> attribute on the +class's interface declaration. A retainable object pointer type +is <span class="term">weak-unavailable</span> if is a pointer to an +(optionally protocol-qualified) Objective-C class <tt>T</tt> +where <tt>T</tt> or one of its superclasses has +the <tt>objc_arc_weak_unavailable</tt> attribute. A program is +ill-formed if it applies the <tt>__weak</tt> ownership qualifier to a +weak-unavailable type or if the value operand of a weak assignment +operation has a weak-unavailable type.</p> +</div> <!-- ownership.restrictions.weak --> + +<div id="ownership.restrictions.autoreleasing"> +<h1>Storage duration of <tt>__autoreleasing</tt> objects</h1> + +<p>A program is ill-formed if it declares an <tt>__autoreleasing</tt> +object of non-automatic storage duration. A program is ill-formed +if it captures an <tt>__autoreleasing</tt> object in a block or, +unless by reference, in a C++11 lambda.</p> + +<div class="rationale"><p>Rationale: autorelease pools are tied to the +current thread and scope by their nature. While it is possible to +have temporary objects whose instance variables are filled with +autoreleased objects, there is no way that ARC can provide any sort of +safety guarantee there.</p></div> + +<p>It is undefined behavior if a non-null pointer is assigned to +an <tt>__autoreleasing</tt> object while an autorelease pool is in +scope and then that object is read after the autorelease pool's scope +is left.</p> + +</div> + +<div id="ownership.restrictions.conversion.indirect"> +<h1>Conversion of pointers to ownership-qualified types</h1> + +<p>A program is ill-formed if an expression of type <tt>T*</tt> is +converted, explicitly or implicitly, to the type <tt>U*</tt>, +where <tt>T</tt> and <tt>U</tt> have different ownership +qualification, unless:</p> +<ul> +<li><tt>T</tt> is qualified with <tt>__strong</tt>, + <tt>__autoreleasing</tt>, or <tt>__unsafe_unretained</tt>, and + <tt>U</tt> is qualified with both <tt>const</tt> and + <tt>__unsafe_unretained</tt>; or</li> +<li>either <tt>T</tt> or <tt>U</tt> is <tt>cv void</tt>, where +<tt>cv</tt> is an optional sequence of non-ownership qualifiers; or</li> +<li>the conversion is requested with a <tt>reinterpret_cast</tt> in + Objective-C++; or</li> +<li>the conversion is a +well-formed <a href="#ownership.restrictions.pass_by_writeback">pass-by-writeback</a>.</li> +</ul> + +<p>The analogous rule applies to <tt>T&</tt> and <tt>U&</tt> in +Objective-C++.</p> + +<div class="rationale"><p>Rationale: these rules provide a reasonable +level of type-safety for indirect pointers, as long as the underlying +memory is not deallocated. The conversion to <tt>const +__unsafe_unretained</tt> is permitted because the semantics of reads +are equivalent across all these ownership semantics, and that's a very +useful and common pattern. The interconversion with <tt>void*</tt> is +useful for allocating memory or otherwise escaping the type system, +but use it carefully. <tt>reinterpret_cast</tt> is considered to be +an obvious enough sign of taking responsibility for any +problems.</p></div> + +<p>It is undefined behavior to access an ownership-qualified object +through an lvalue of a differently-qualified type, except that any +non-<tt>__weak</tt> object may be read through +an <tt>__unsafe_unretained</tt> lvalue.</p> + +<p>It is undefined behavior if a managed operation is performed on +a <tt>__strong</tt> or <tt>__weak</tt> object without a guarantee that +it contains a primitive zero bit-pattern, or if the storage for such +an object is freed or reused without the object being first assigned a +null pointer.</p> + +<div class="rationale"><p>Rationale: ARC cannot differentiate between +an assignment operator which is intended to <q>initialize</q> dynamic +memory and one which is intended to potentially replace a value. +Therefore the object's pointer must be valid before letting ARC at it. +Similarly, C and Objective-C do not provide any language hooks for +destroying objects held in dynamic memory, so it is the programmer's +responsibility to avoid leaks (<tt>__strong</tt> objects) and +consistency errors (<tt>__weak</tt> objects).</p> + +<p>These requirements are followed automatically in Objective-C++ when +creating objects of retainable object owner type with <tt>new</tt> +or <tt>new[]</tt> and destroying them with <tt>delete</tt>, +<tt>delete[]</tt>, or a pseudo-destructor expression. Note that +arrays of nontrivially-ownership-qualified type are not ABI compatible +with non-ARC code because the element type is non-POD: such arrays +that are <tt>new[]</tt>'d in ARC translation units cannot +be <tt>delete[]</tt>'d in non-ARC translation units and +vice-versa.</p></div> + +</div> + +<div id="ownership.restrictions.pass_by_writeback"> +<h1>Passing to an out parameter by writeback</h1> + +<p>If the argument passed to a parameter of type +<tt>T __autoreleasing *</tt> has type <tt>U oq *</tt>, +where <tt>oq</tt> is an ownership qualifier, then the argument is a +candidate for <span class="term">pass-by-writeback</span> if:</p> + +<ul> +<li><tt>oq</tt> is <tt>__strong</tt> or <tt>__weak</tt>, and</li> +<li>it would be legal to initialize a <tt>T __strong *</tt> with +a <tt>U __strong *</tt>.</li> +</ul> + +<p>For purposes of overload resolution, an implicit conversion +sequence requiring a pass-by-writeback is always worse than an +implicit conversion sequence not requiring a pass-by-writeback.</p> + +<p>The pass-by-writeback is ill-formed if the argument expression does +not have a legal form:</p> + +<ul> +<li><tt>&var</tt>, where <tt>var</tt> is a scalar variable of +automatic storage duration with retainable object pointer type</li> +<li>a conditional expression where the second and third operands are +both legal forms</li> +<li>a cast whose operand is a legal form</li> +<li>a null pointer constant</li> +</ul> + +<div class="rationale"><p>Rationale: the restriction in the form of +the argument serves two purposes. First, it makes it impossible to +pass the address of an array to the argument, which serves to protect +against an otherwise serious risk of mis-inferring an <q>array</q> +argument as an out-parameter. Second, it makes it much less likely +that the user will see confusing aliasing problems due to the +implementation, below, where their store to the writeback temporary is +not immediately seen in the original argument variable.</p></div> + +<p>A pass-by-writeback is evaluated as follows:</p> +<ol> +<li>The argument is evaluated to yield a pointer <tt>p</tt> of + type <tt>U oq *</tt>.</li> +<li>If <tt>p</tt> is a null pointer, then a null pointer is passed as + the argument, and no further work is required for the pass-by-writeback.</li> +<li>Otherwise, a temporary of type <tt>T __autoreleasing</tt> is + created and initialized to a null pointer.</li> +<li>If the parameter is not an Objective-C method parameter marked + <tt>out</tt>, then <tt>*p</tt> is read, and the result is written + into the temporary with primitive semantics.</li> +<li>The address of the temporary is passed as the argument to the + actual call.</li> +<li>After the call completes, the temporary is loaded with primitive + semantics, and that value is assigned into <tt>*p</tt>.</li> +</ol> + +<div class="rationale"><p>Rationale: this is all admittedly +convoluted. In an ideal world, we would see that a local variable is +being passed to an out-parameter and retroactively modify its type to +be <tt>__autoreleasing</tt> rather than <tt>__strong</tt>. This would +be remarkably difficult and not always well-founded under the C type +system. However, it was judged unacceptably invasive to require +programmers to write <tt>__autoreleasing</tt> on all the variables +they intend to use for out-parameters. This was the least bad +solution.</p></div> + +</div> + +<div id="ownership.restrictions.records"> +<h1>Ownership-qualified fields of structs and unions</h1> + +<p>A program is ill-formed if it declares a member of a C struct or +union to have a nontrivially ownership-qualified type.</p> + +<div class="rationale"><p>Rationale: the resulting type would be +non-POD in the C++ sense, but C does not give us very good language +tools for managing the lifetime of aggregates, so it is more +convenient to simply forbid them. It is still possible to manage this +with a <tt>void*</tt> or an <tt>__unsafe_unretained</tt> +object.</p></div> + +<p>This restriction does not apply in Objective-C++. However, +nontrivally ownership-qualified types are considered non-POD: in C++11 +terms, they are not trivially default constructible, copy +constructible, move constructible, copy assignable, move assignable, +or destructible. It is a violation of C++'s One Definition Rule to use +a class outside of ARC that, under ARC, would have a nontrivially +ownership-qualified member.</p> + +<div class="rationale"><p>Rationale: unlike in C, we can express all +the necessary ARC semantics for ownership-qualified subobjects as +suboperations of the (default) special member functions for the class. +These functions then become non-trivial. This has the non-obvious +result that the class will have a non-trivial copy constructor and +non-trivial destructor; if this would not normally be true outside of +ARC, objects of the type will be passed and returned in an +ABI-incompatible manner.</p></div> + +</div> + +</div> + +<div id="ownership.inference"> +<h1>Ownership inference</h1> + +<div id="ownership.inference.variables"> +<h1>Objects</h1> + +<p>If an object is declared with retainable object owner type, but +without an explicit ownership qualifier, its type is implicitly +adjusted to have <tt>__strong</tt> qualification.</p> + +<p>As a special case, if the object's base type is <tt>Class</tt> +(possibly protocol-qualified), the type is adjusted to +have <tt>__unsafe_unretained</tt> qualification instead.</p> + +</div> + +<div id="ownership.inference.indirect_parameters"> +<h1>Indirect parameters</h1> + +<p>If a function or method parameter has type <tt>T*</tt>, where +<tt>T</tt> is an ownership-unqualified retainable object pointer type, +then:</p> + +<ul> +<li>if <tt>T</tt> is <tt>const</tt>-qualified or <tt>Class</tt>, then +it is implicitly qualified with <tt>__unsafe_unretained</tt>;</li> +<li>otherwise, it is implicitly qualified +with <tt>__autoreleasing</tt>.</li> +</ul> + +<div class="rationale"><p>Rationale: <tt>__autoreleasing</tt> exists +mostly for this case, the Cocoa convention for out-parameters. Since +a pointer to <tt>const</tt> is obviously not an out-parameter, we +instead use a type more useful for passing arrays. If the user +instead intends to pass in a <em>mutable</em> array, inferring +<tt>__autoreleasing</tt> is the wrong thing to do; this directs some +of the caution in the following rules about writeback.</p></div> + +<p>Such a type written anywhere else would be ill-formed by the +general rule requiring ownership qualifiers.</p> + +<p>This rule does not apply in Objective-C++ if a parameter's type is +dependent in a template pattern and is only <em>instantiated</em> to +a type which would be a pointer to an unqualified retainable object +pointer type. Such code is still ill-formed.</p> + +<div class="rationale"><p>Rationale: the convention is very unlikely +to be intentional in template code.</p></div> + +</div> <!-- ownership.inference.indirect_parameters --> + +<div id="ownership.inference.template_arguments"> +<h1>Template arguments</h1> + +<p>If a template argument for a template type parameter is an +retainable object owner type that does not have an explicit ownership +qualifier, it is adjusted to have <tt>__strong</tt> +qualification. This adjustment occurs regardless of whether the +template argument was deduced or explicitly specified. </p> + +<div class="rationale"><p>Rationale: <tt>__strong</tt> is a useful default for containers (e.g., <tt>std::vector<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> + +</div> <!-- ownership.inference.template_arguments --> +</div> <!-- ownership.inference --> +</div> <!-- ownership --> + + +<div id="family"> +<h1>Method families</h1> + +<p>An Objective-C method may fall into a <span class="term">method +family</span>, which is a conventional set of behaviors ascribed to it +by the Cocoa conventions.</p> + +<p>A method is in a certain method family if:</p> +<ul> +<li>it has a <tt>objc_method_family</tt> attribute placing it in that + family; or if not that,</li> +<li>it does not have an <tt>objc_method_family</tt> attribute placing + it in a different or no family, and</li> +<li>its selector falls into the corresponding selector family, and</li> +<li>its signature obeys the added restrictions of the method family.</li> +</ul> + +<p>A selector is in a certain selector family if, ignoring any leading +underscores, the first component of the selector either consists +entirely of the name of the method family or it begins with that name +followed by a character other than a lowercase letter. For +example, <tt>_perform:with:</tt> and <tt>performWith:</tt> would fall +into the <tt>perform</tt> family (if we recognized one), +but <tt>performing:with</tt> would not.</p> + +<p>The families and their added restrictions are:</p> + +<ul> +<li><tt>alloc</tt> methods must return a retainable object pointer type.</li> +<li><tt>copy</tt> methods must return a retainable object pointer type.</li> +<li><tt>mutableCopy</tt> methods must return a retainable object pointer type.</li> +<li><tt>new</tt> methods must return a retainable object pointer type.</li> +<li><tt>init</tt> methods must be instance methods and must return an +Objective-C pointer type. Additionally, a program is ill-formed if it +declares or contains a call to an <tt>init</tt> method whose return +type is neither <tt>id</tt> nor a pointer to a super-class or +sub-class of the declaring class (if the method was declared on +a class) or the static receiver type of the call (if it was declared +on a protocol). + +<div class="rationale"><p>Rationale: there are a fair number of existing +methods with <tt>init</tt>-like selectors which nonetheless don't +follow the <tt>init</tt> conventions. Typically these are either +accidental naming collisions or helper methods called during +initialization. Because of the peculiar retain/release behavior +of <tt>init</tt> methods, it's very important not to treat these +methods as <tt>init</tt> methods if they aren't meant to be. It was +felt that implicitly defining these methods out of the family based on +the exact relationship between the return type and the declaring class +would be much too subtle and fragile. Therefore we identify a small +number of legitimate-seeming return types and call everything else an +error. This serves the secondary purpose of encouraging programmers +not to accidentally give methods names in the <tt>init</tt> family.</p> + +<p>Note that a method with an <tt>init</tt>-family selector which +returns a non-Objective-C type (e.g. <tt>void</tt>) is perfectly +well-formed; it simply isn't in the <tt>init</tt> family.</p></div> +</li> +</ul> + +<p>A program is ill-formed if a method's declarations, +implementations, and overrides do not all have the same method +family.</p> + +<div id="family.attribute"> +<h1>Explicit method family control</h1> + +<p>A method may be annotated with the <tt>objc_method_family</tt> +attribute to precisely control which method family it belongs to. If +a method in an <tt>@implementation</tt> does not have this attribute, +but there is a method declared in the corresponding <tt>@interface</tt> +that does, then the attribute is copied to the declaration in the +<tt>@implementation</tt>. The attribute is available outside of ARC, +and may be tested for with the preprocessor query +<tt>__has_attribute(objc_method_family)</tt>.</p> + +<p>The attribute is spelled +<tt>__attribute__((objc_method_family(<i>family</i>)))</tt>. +If <i>family</i> is <tt>none</tt>, the method has no family, even if +it would otherwise be considered to have one based on its selector and +type. Otherwise, <i>family</i> must be one +of <tt>alloc</tt>, <tt>copy</tt>, <tt>init</tt>, +<tt>mutableCopy</tt>, or <tt>new</tt>, in which case the method is +considered to belong to the corresponding family regardless of its +selector. It is an error if a method that is explicitly added to a +family in this way does not meet the requirements of the family other +than the selector naming convention.</p> + +<div class="rationale"><p>Rationale: the rules codified in this document +describe the standard conventions of Objective-C. However, as these +conventions have not heretofore been enforced by an unforgiving +mechanical system, they are only imperfectly kept, especially as they +haven't always even been precisely defined. While it is possible to +define low-level ownership semantics with attributes like +<tt>ns_returns_retained</tt>, this attribute allows the user to +communicate semantic intent, which is of use both to ARC (which, e.g., +treats calls to <tt>init</tt> specially) and the static analyzer.</p></div> +</div> + +<div id="family.semantics"> +<h1>Semantics of method families</h1> + +<p>A method's membership in a method family may imply non-standard +semantics for its parameters and return type.</p> + +<p>Methods in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>, +and <tt>new</tt> families — that is, methods in all the +currently-defined families except <tt>init</tt> — implicitly +<a href="#objects.operands.retained_returns">return a retained +object</a> as if they were annotated with +the <tt>ns_returns_retained</tt> attribute. This can be overridden by +annotating the method with either of +the <tt>ns_returns_autoreleased</tt> or +<tt>ns_returns_not_retained</tt> attributes.</p> + +<p>Properties also follow same naming rules as methods. This means that +those in the <tt>alloc</tt>, <tt>copy</tt>, <tt>mutableCopy</tt>, +and <tt>new</tt> families provide access to +<a href="#objects.operands.retained_returns">retained objects</a>. +This can be overridden by annotating the property with +<tt>ns_returns_not_retained</tt> attribute.</p> + +<div id="family.semantics.init"> +<h1>Semantics of <tt>init</tt></h1> +<p>Methods in the <tt>init</tt> family implicitly +<a href="#objects.operands.consumed">consume</a> their <tt>self</tt> +parameter and <a href="#objects.operands.retained_returns">return a +retained object</a>. Neither of these properties can be altered +through attributes.</p> + +<p>A call to an <tt>init</tt> method with a receiver that is either +<tt>self</tt> (possibly parenthesized or casted) or <tt>super</tt> is +called a <span class="term">delegate init call</span>. It is an error +for a delegate init call to be made except from an <tt>init</tt> +method, and excluding blocks within such methods.</p> + +<p>As an exception to the <a href="misc.self">usual rule</a>, the +variable <tt>self</tt> is mutable in an <tt>init</tt> method and has +the usual semantics for a <tt>__strong</tt> variable. However, it is +undefined behavior and the program is ill-formed, no diagnostic +required, if an <tt>init</tt> method attempts to use the previous +value of <tt>self</tt> after the completion of a delegate init call. +It is conventional, but not required, for an <tt>init</tt> method to +return <tt>self</tt>.</p> + +<p>It is undefined behavior for a program to cause two or more calls +to <tt>init</tt> methods on the same object, except that +each <tt>init</tt> method invocation may perform at most one delegate +init call.</p> + +</div> <!-- family.semantics.init --> + +<div id="family.semantics.result_type"> +<h1>Related result types</h1> + +<p>Certain methods are candidates to have <span class="term">related +result types</span>:</p> +<ul> +<li>class methods in the <tt>alloc</tt> and <tt>new</tt> method families</li> +<li>instance methods in the <tt>init</tt> family</li> +<li>the instance method <tt>self</tt></li> +<li>outside of ARC, the instance methods <tt>retain</tt> and <tt>autorelease</tt></li> +</ul> + +<p>If the formal result type of such a method is <tt>id</tt> or +protocol-qualified <tt>id</tt>, or a type equal to the declaring class +or a superclass, then it is said to have a related result type. In +this case, when invoked in an explicit message send, it is assumed to +return a type related to the type of the receiver:</p> + +<ul> +<li>if it is a class method, and the receiver is a class +name <tt>T</tt>, the message send expression has type <tt>T*</tt>; +otherwise</li> +<li>if it is an instance method, and the receiver has type <tt>T</tt>, +the message send expression has type <tt>T</tt>; otherwise</li> +<li>the message send expression has the normal result type of the +method.</li> +</ul> + +<p>This is a new rule of the Objective-C language and applies outside +of ARC.</p> + +<div class="rationale"><p>Rationale: ARC's automatic code emission is +more prone than most code to signature errors, i.e. errors where a +call was emitted against one method signature, but the implementing +method has an incompatible signature. Having more precise type +information helps drastically lower this risk, as well as catching +a number of latent bugs.</p></div> + +</div> <!-- family.semantics.result_type --> +</div> <!-- family.semantics --> +</div> <!-- family --> + +<div id="optimization"> +<h1>Optimization</h1> + +<p>ARC applies aggressive rules for the optimization of local +behavior. These rules are based around a core assumption of +<span class="term">local balancing</span>: that other code will +perform retains and releases as necessary (and only as necessary) for +its own safety, and so the optimizer does not need to consider global +properties of the retain and release sequence. For example, if a +retain and release immediately bracket a call, the optimizer can +delete the retain and release on the assumption that the called +function will not do a constant number of unmotivated releases +followed by a constant number of <q>balancing</q> retains, such that +the local retain/release pair is the only thing preventing the called +function from ending up with a dangling reference.</p> + +<p>The optimizer assumes that when a new value enters local control, +e.g. from a load of a non-local object or as the result of a function +call, it is instaneously valid. Subsequently, a retain and release of +a value are necessary on a computation path only if there is a use of +that value before the release and after any operation which might +cause a release of the value (including indirectly or non-locally), +and only if the value is not demonstrably already retained.</p> + +<p>The complete optimization rules are quite complicated, but it would +still be useful to document them here.</p> + +<div id="optimization.precise"> +<h1>Precise lifetime semantics</h1> + +<p>In general, ARC maintains an invariant that a retainable object +pointer held in a <tt>__strong</tt> object will be retained for the +full formal lifetime of the object. Objects subject to this invariant +have <span class="term">precise lifetime semantics</span>.</p> + +<p>By default, local variables of automatic storage duration do not +have precise lifetime semantics. Such objects are simply strong +references which hold values of retainable object pointer type, and +these values are still fully subject to the optimizations on values +under local control.</p> + +<div class="rationale"><p>Rationale: applying these precise-lifetime +semantics strictly would be prohibitive. Many useful optimizations +that might theoretically decrease the lifetime of an object would be +rendered impossible. Essentially, it promises too much.</p></div> + +<p>A local variable of retainable object owner type and automatic +storage duration may be annotated with the <tt>objc_precise_lifetime</tt> +attribute to indicate that it should be considered to be an object +with precise lifetime semantics.</p> + +<div class="rationale"><p>Rationale: nonetheless, it is sometimes +useful to be able to force an object to be released at a precise time, +even if that object does not appear to be used. This is likely to be +uncommon enough that the syntactic weight of explicitly requesting +these semantics will not be burdensome, and may even make the code +clearer.</p></div> + +</div> <!-- optimization.precise --> + +</div> <!-- optimization --> + +<div id="misc"> +<h1>Miscellaneous</h1> + +<div id="misc.special_methods"> +<h1>Special methods</h1> + +<div id="misc.special_methods.retain"> +<h1>Memory management methods</h1> + +<p>A program is ill-formed if it contains a method definition, message +send, or <tt>@selector</tt> expression for any of the following +selectors:</p> +<ul> +<li><tt>autorelease</tt></li> +<li><tt>release</tt></li> +<li><tt>retain</tt></li> +<li><tt>retainCount</tt></li> +</ul> + +<div class="rationale"><p>Rationale: <tt>retainCount</tt> is banned +because ARC robs it of consistent semantics. The others were banned +after weighing three options for how to deal with message sends:</p> + +<p><b>Honoring</b> them would work out very poorly if a programmer +naively or accidentally tried to incorporate code written for manual +retain/release code into an ARC program. At best, such code would do +twice as much work as necessary; quite frequently, however, ARC and +the explicit code would both try to balance the same retain, leading +to crashes. The cost is losing the ability to perform <q>unrooted</q> +retains, i.e. retains not logically corresponding to a strong +reference in the object graph.</p> + +<p><b>Ignoring</b> them would badly violate user expectations about their +code. While it <em>would</em> make it easier to develop code simultaneously +for ARC and non-ARC, there is very little reason to do so except for +certain library developers. ARC and non-ARC translation units share +an execution model and can seamlessly interoperate. Within a +translation unit, a developer who faithfully maintains their code in +non-ARC mode is suffering all the restrictions of ARC for zero +benefit, while a developer who isn't testing the non-ARC mode is +likely to be unpleasantly surprised if they try to go back to it.</p> + +<p><b>Banning</b> them has the disadvantage of making it very awkward +to migrate existing code to ARC. The best answer to that, given a +number of other changes and restrictions in ARC, is to provide a +specialized tool to assist users in that migration.</p> + +<p>Implementing these methods was banned because they are too integral +to the semantics of ARC; many tricks which worked tolerably under +manual reference counting will misbehave if ARC performs an ephemeral +extra retain or two. If absolutely required, it is still possible to +implement them in non-ARC code, for example in a category; the +implementations must obey the <a href="#objects.retains">semantics</a> +laid out elsewhere in this document.</p> + +</div> +</div> <!-- misc.special_methods.retain --> + +<div id="misc.special_methods.dealloc"> +<h1><tt>dealloc</tt></h1> + +<p>A program is ill-formed if it contains a message send +or <tt>@selector</tt> expression for the selector <tt>dealloc</tt>.</p> + +<div class="rationale"><p>Rationale: there are no legitimate reasons +to call <tt>dealloc</tt> directly.</p></div> + +<p>A class may provide a method definition for an instance method +named <tt>dealloc</tt>. This method will be called after the final +<tt>release</tt> of the object but before it is deallocated or any of +its instance variables are destroyed. The superclass's implementation +of <tt>dealloc</tt> will be called automatically when the method +returns.</p> + +<div class="rationale"><p>Rationale: even though ARC destroys instance +variables automatically, there are still legitimate reasons to write +a <tt>dealloc</tt> method, such as freeing non-retainable resources. +Failing to call <tt>[super dealloc]</tt> in such a method is nearly +always a bug. Sometimes, the object is simply trying to prevent +itself from being destroyed, but <tt>dealloc</tt> is really far too +late for the object to be raising such objections. Somewhat more +legitimately, an object may have been pool-allocated and should not be +deallocated with <tt>free</tt>; for now, this can only be supported +with a <tt>dealloc</tt> implementation outside of ARC. Such an +implementation must be very careful to do all the other work +that <tt>NSObject</tt>'s <tt>dealloc</tt> would, which is outside the +scope of this document to describe.</p></div> + +</div> + +</div> <!-- misc.special_methods --> + +<div id="autoreleasepool"> +<h1><tt>@autoreleasepool</tt></h1> + +<p>To simplify the use of autorelease pools, and to bring them under +the control of the compiler, a new kind of statement is available in +Objective-C. It is written <tt>@autoreleasepool</tt> followed by +a <i>compound-statement</i>, i.e. by a new scope delimited by curly +braces. Upon entry to this block, the current state of the +autorelease pool is captured. When the block is exited normally, +whether by fallthrough or directed control flow (such +as <tt>return</tt> or <tt>break</tt>), the autorelease pool is +restored to the saved state, releasing all the objects in it. When +the block is exited with an exception, the pool is not drained.</p> + +<p><tt>@autoreleasepool</tt> may be used in non-ARC translation units, +with equivalent semantics.</p> + +<p>A program is ill-formed if it refers to the +<tt>NSAutoreleasePool</tt> class.</p> + +<div class="rationale"><p>Rationale: autorelease pools are clearly +important for the compiler to reason about, but it is far too much to +expect the compiler to accurately reason about control dependencies +between two calls. It is also very easy to accidentally forget to +drain an autorelease pool when using the manual API, and this can +significantly inflate the process's high-water-mark. The introduction +of a new scope is unfortunate but basically required for sane +interaction with the rest of the language. Not draining the pool +during an unwind is apparently required by the Objective-C exceptions +implementation.</p></div> + +</div> <!-- autoreleasepool --> + +<div id="misc.self"> +<h1><tt>self</tt></h1> + +<p>The <tt>self</tt> parameter variable of an Objective-C method is +never actually retained by the implementation. It is undefined +behavior, or at least dangerous, to cause an object to be deallocated +during a message send to that object.</p> + +<p>To make this safe, for Objective-C instance methods <tt>self</tt> is +implicitly <tt>const</tt> unless the method is in the <a +href="#family.semantics.init"><tt>init</tt> family</a>. Further, <tt>self</tt> +is <b>always</b> implicitly <tt>const</tt> within a class method.</p> + +<div class="rationale"><p>Rationale: the cost of +retaining <tt>self</tt> in all methods was found to be prohibitive, as +it tends to be live across calls, preventing the optimizer from +proving that the retain and release are unnecessary — for good +reason, as it's quite possible in theory to cause an object to be +deallocated during its execution without this retain and release. +Since it's extremely uncommon to actually do so, even unintentionally, +and since there's no natural way for the programmer to remove this +retain/release pair otherwise (as there is for other parameters by, +say, making the variable <tt>__unsafe_unretained</tt>), we chose to +make this optimizing assumption and shift some amount of risk to the +user.</p></div> + +</div> <!-- misc.self --> + +<div id="misc.enumeration"> +<h1>Fast enumeration iteration variables</h1> + +<p>If a variable is declared in the condition of an Objective-C fast +enumeration loop, and the variable has no explicit ownership +qualifier, then it is qualified with <tt>const __strong</tt> and +objects encountered during the enumeration are not actually +retained.</p> + +<div class="rationale"><p>Rationale: this is an optimization made +possible because fast enumeration loops promise to keep the objects +retained during enumeration, and the collection itself cannot be +synchronously modified. It can be overridden by explicitly qualifying +the variable with <tt>__strong</tt>, which will make the variable +mutable again and cause the loop to retain the objects it +encounters.</p></div> + +</div> <!-- misc.enumeration --> + +<div id="misc.blocks"> +<h1>Blocks</h1> + +<p>The implicit <tt>const</tt> capture variables created when +evaluating a block literal expression have the same ownership +semantics as the local variables they capture. The capture is +performed by reading from the captured variable and initializing the +capture variable with that value; the capture variable is destroyed +when the block literal is, i.e. at the end of the enclosing scope.</p> + +<p>The <a href="#ownership.inference">inference</a> rules apply +equally to <tt>__block</tt> variables, which is a shift in semantics +from non-ARC, where <tt>__block</tt> variables did not implicitly +retain during capture.</p> + +<p><tt>__block</tt> variables of retainable object owner type are +moved off the stack by initializing the heap copy with the result of +moving from the stack copy.</p> + +<p>With the exception of retains done as part of initializing +a <tt>__strong</tt> parameter variable or reading a <tt>__weak</tt> +variable, whenever these semantics call for retaining a value of +block-pointer type, it has the effect of a <tt>Block_copy</tt>. The +optimizer may remove such copies when it sees that the result is +used only as an argument to a call.</p> + +</div> <!-- misc.blocks --> + +<div id="misc.exceptions"> +<h1>Exceptions</h1> + +<p>By default in Objective C, ARC is not exception-safe for normal +releases:</p> +<ul> +<li>It does not end the lifetime of <tt>__strong</tt> variables when +their scopes are abnormally terminated by an exception.</li> +<li>It does not perform releases which would occur at the end of +a full-expression if that full-expression throws an exception.</li> +</ul> + +<p>A program may be compiled with the option +<tt>-fobjc-arc-exceptions</tt> in order to enable these, or with the +option <tt>-fno-objc-arc-exceptions</tt> to explicitly disable them, +with the last such argument <q>winning</q>.</p> + +<div class="rationale"><p>Rationale: the standard Cocoa convention is +that exceptions signal programmer error and are not intended to be +recovered from. Making code exceptions-safe by default would impose +severe runtime and code size penalties on code that typically does not +actually care about exceptions safety. Therefore, ARC-generated code +leaks by default on exceptions, which is just fine if the process is +going to be immediately terminated anyway. Programs which do care +about recovering from exceptions should enable the option.</p></div> + +<p>In Objective-C++, <tt>-fobjc-arc-exceptions</tt> is enabled by +default.</p> + +<div class="rationale"><p>Rationale: C++ already introduces pervasive +exceptions-cleanup code of the sort that ARC introduces. C++ +programmers who have not already disabled exceptions are much more +likely to actual require exception-safety.</p></div> + +<p>ARC does end the lifetimes of <tt>__weak</tt> objects when an +exception terminates their scope unless exceptions are disabled in the +compiler.</p> + +<div class="rationale"><p>Rationale: the consequence of a +local <tt>__weak</tt> object not being destroyed is very likely to be +corruption of the Objective-C runtime, so we want to be safer here. +Of course, potentially massive leaks are about as likely to take down +the process as this corruption is if the program does try to recover +from exceptions.</p></div> + +</div> <!-- misc.exceptions --> + +<div id="misc.interior"> +<h1>Interior pointers</h1> + +<p>An Objective-C method returning a non-retainable pointer may be +annotated with the <tt>objc_returns_inner_pointer</tt> attribute to +indicate that it returns a handle to the internal data of an object, +and that this reference will be invalidated if the object is +destroyed. When such a message is sent to an object, the object's +lifetime will be extended until at least the earliest of:</p> + +<ul> +<li>the last use of the returned pointer, or any pointer derived from +it, in the calling function or</li> +<li>the autorelease pool is restored to a previous state.</li> +</ul> + +<div class="rationale"><p>Rationale: not all memory and resources are +managed with reference counts; it is common for objects to manage +private resources in their own, private way. Typically these +resources are completely encapsulated within the object, but some +classes offer their users direct access for efficiency. If ARC is not +aware of methods that return such <q>interior</q> pointers, its +optimizations can cause the owning object to be reclaimed too soon. +This attribute informs ARC that it must tread lightly.</p> + +<p>The extension rules are somewhat intentionally vague. The +autorelease pool limit is there to permit a simple implementation to +simply retain and autorelease the receiver. The other limit permits +some amount of optimization. The phrase <q>derived from</q> is +intended to encompass the results both of pointer transformations, +such as casts and arithmetic, and of loading from such derived +pointers; furthermore, it applies whether or not such derivations are +applied directly in the calling code or by other utility code (for +example, the C library routine <tt>strchr</tt>). However, the +implementation never need account for uses after a return from the +code which calls the method returning an interior pointer.</p></div> + +<p>As an exception, no extension is required if the receiver is loaded +directly from a <tt>__strong</tt> object +with <a href="#optimization.precise">precise lifetime semantics</a>.</p> + +<div class="rationale"><p>Rationale: implicit autoreleases carry the +risk of significantly inflating memory use, so it's important to +provide users a way of avoiding these autoreleases. Tying this to +precise lifetime semantics is ideal, as for local variables this +requires a very explicit annotation, which allows ARC to trust the +user with good cheer.</p></div> + +</div> <!-- misc.interior --> + +<div id="misc.c-retainable"> +<h1>C retainable pointer types</h1> + +<p>A type is a <span class="term">C retainable pointer type</span> +if it is a pointer to (possibly qualified) <tt>void</tt> or a +pointer to a (possibly qualifier) <tt>struct</tt> or <tt>class</tt> +type.</p> + +<div class="rationale"><p>Rationale: ARC does not manage pointers of +CoreFoundation type (or any of the related families of retainable C +pointers which interoperate with Objective-C for retain/release +operation). In fact, ARC does not even know how to distinguish these +types from arbitrary C pointer types. The intent of this concept is +to filter out some obviously non-object types while leaving a hook for +later tightening if a means of exhaustively marking CF types is made +available.</p></div> + +<div id="misc.c-retainable.audit"> +<h1>Auditing of C retainable pointer interfaces</h1> + +<p><span class="revision"><span class="whenRevised">[beginning Apple 4.0, LLVM 3.1]</span></span></p> + +<p>A C function may be marked with the <tt>cf_audited_transfer</tt> +attribute to express that, except as otherwise marked with attributes, +it obeys the parameter (consuming vs. non-consuming) and return +(retained vs. non-retained) conventions for a C function of its name, +namely:</p> + +<ul> +<li>A parameter of C retainable pointer type is assumed to not be +consumed unless it is marked with the <tt>cf_consumed</tt> attribute, and</li> +<li>A result of C retainable pointer type is assumed to not be +returned retained unless the function is either +marked <tt>cf_returns_retained</tt> or it follows +the create/copy naming convention and is not +marked <tt>cf_returns_not_retained</tt>.</li> +</ul> + +<p>A function obeys the <span class="term">create/copy</span> naming +convention if its name contains as a substring:</p> +<ul> +<li>either <q>Create</q> or <q>Copy</q> not followed by a lowercase letter, or</li> +<li>either <q>create</q> or <q>copy</q> not followed by a lowercase +letter and not preceded by any letter, whether uppercase or lowercase.</li> +</ul> + +<p>A second attribute, <tt>cf_unknown_transfer</tt>, signifies that a +function's transfer semantics cannot be accurately captured using any +of these annotations. A program is ill-formed if it annotates the +same function with both <tt>cf_audited_transfer</tt> +and <tt>cf_unknown_transfer</tt>.</p> + +<p>A pragma is provided to faciliate the mass annotation of interfaces:</p> + +<pre>#pragma arc_cf_code_audited begin +... +#pragma arc_cf_code_audited end</pre> + +<p>All C functions declared within the extent of this pragma are +treated as if annotated with the <tt>cf_audited_transfer</tt> +attribute unless they otherwise have the <tt>cf_unknown_transfer</tt> +attribute. The pragma is accepted in all language modes. A program +is ill-formed if it attempts to change files, whether by including a +file or ending the current file, within the extent of this pragma.</p> + +<p>It is possible to test for all the features in this section with +<tt>__has_feature(arc_cf_code_audited)</tt>.</p> + +<div class="rationale"><p>Rationale: A significant inconvenience in +ARC programming is the necessity of interacting with APIs based around +C retainable pointers. These features are designed to make it +relatively easy for API authors to quickly review and annotate their +interfaces, in turn improving the fidelity of tools such as the static +analyzer and ARC. The single-file restriction on the pragma is +designed to eliminate the risk of accidentally annotating some other +header's interfaces.</p></div> + +</div> <!-- misc.c-retainable.audit --> + +</div> <!-- misc.c-retainable --> + +</div> <!-- misc --> + +<div id="runtime"> +<h1>Runtime support</h1> + +<p>This section describes the interaction between the ARC runtime and +the code generated by the ARC compiler. This is not part of the ARC +language specification; instead, it is effectively a language-specific +ABI supplement, akin to the <q>Itanium</q> generic ABI for C++.</p> + +<p>Ownership qualification does not alter the storage requirements for +objects, except that it is undefined behavior if a <tt>__weak</tt> +object is inadequately aligned for an object of type <tt>id</tt>. The +other qualifiers may be used on explicitly under-aligned memory.</p> + +<p>The runtime tracks <tt>__weak</tt> objects which holds non-null +values. It is undefined behavior to direct modify a <tt>__weak</tt> +object which is being tracked by the runtime except through an +<a href="#runtime.objc_storeWeak"><tt>objc_storeWeak</tt></a>, +<a href="#runtime.objc_destroyWeak"><tt>objc_destroyWeak</tt></a>, +or <a href="#runtime.objc_moveWeak"><tt>objc_moveWeak</tt></a> +call.</p> + +<p>The runtime must provide a number of new entrypoints which the +compiler may emit, which are described in the remainder of this +section.</p> + +<div class="rationale"><p>Rationale: Several of these functions are +semantically equivalent to a message send; we emit calls to C +functions instead because:</p> +<ul> +<li>the machine code to do so is significantly smaller,</li> +<li>it is much easier to recognize the C functions in the ARC optimizer, and</li> +<li>a sufficient sophisticated runtime may be able to avoid the +message send in common cases.</li> +</ul> + +<p>Several other of these functions are <q>fused</q> operations which +can be described entirely in terms of other operations. We use the +fused operations primarily as a code-size optimization, although in +some cases there is also a real potential for avoiding redundant +operations in the runtime.</p> + +</div> + +<div id="runtime.objc_autorelease"> +<h1><tt>id objc_autorelease(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +adds the object to the innermost autorelease pool exactly as if the +object had been sent the <tt>autorelease</tt> message.</p> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_autorelease --> + +<div id="runtime.objc_autoreleasePoolPop"> +<h1><tt>void objc_autoreleasePoolPop(void *pool);</tt></h1> +<p><i>Precondition:</i> <tt>pool</tt> is the result of a previous call to +<a href="runtime.objc_autoreleasePoolPush"><tt>objc_autoreleasePoolPush</tt></a> +on the current thread, where neither <tt>pool</tt> nor any enclosing +pool have previously been popped.</p> +<p>Releases all the objects added to the given autorelease pool and +any autorelease pools it encloses, then sets the current autorelease +pool to the pool directly enclosing <tt>pool</tt>.</p> +</div> <!-- runtime.objc_autoreleasePoolPop --> + +<div id="runtime.objc_autoreleasePoolPush"> +<h1><tt>void *objc_autoreleasePoolPush(void);</tt></h1> +<p>Creates a new autorelease pool that is enclosed by the current +pool, makes that the current pool, and returns an opaque <q>handle</q> +to it.</p> + +<div class="rationale"><p>Rationale: while the interface is described +as an explicit hierarchy of pools, the rules allow the implementation +to just keep a stack of objects, using the stack depth as the opaque +pool handle.</p></div> + +</div> <!-- runtime.objc_autoreleasePoolPush --> + +<div id="runtime.objc_autoreleaseReturnValue"> +<h1><tt>id objc_autoreleaseReturnValue(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +makes a best effort to hand off ownership of a retain count on the +object to a call +to <a href="runtime.objc_retainAutoreleasedReturnValue"><tt>objc_retainAutoreleasedReturnValue</tt></a> +for the same object in an enclosing call frame. If this is not +possible, the object is autoreleased as above.</p> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_autoreleaseReturnValue --> + +<div id="runtime.objc_copyWeak"> +<h1><tt>void objc_copyWeak(id *dest, id *src);</tt></h1> +<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either +contains a null pointer or has been registered as a <tt>__weak</tt> +object. <tt>dest</tt> is a valid pointer which has not been +registered as a <tt>__weak</tt> object.</p> +<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>, +potentially registering it with the runtime. Equivalent to the +following code:</p> +<pre>void objc_copyWeak(id *dest, id *src) { + objc_release(objc_initWeak(dest, objc_loadWeakRetained(src))); +}</pre> +<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt> +on <tt>src</tt>.</p> +</div> <!-- runtime.objc_copyWeak --> + +<div id="runtime.objc_destroyWeak"> +<h1><tt>void objc_destroyWeak(id *object);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which +either contains a null pointer or has been registered as +a <tt>__weak</tt> object.</p> +<p><tt>object</tt> is unregistered as a weak object, if it ever was. +The current value of <tt>object</tt> is left unspecified; otherwise, +equivalent to the following code:</p> +<pre>void objc_destroyWeak(id *object) { + objc_storeWeak(object, nil); +}</pre> +<p>Does not need to be atomic with respect to calls +to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p> +</div> <!-- runtime.objc_destroyWeak --> + +<div id="runtime.objc_initWeak"> +<h1><tt>id objc_initWeak(id *object, id value);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which has +not been registered as a <tt>__weak</tt> object. <tt>value</tt> is +null or a pointer to a valid object.</p> +<p>If <tt>value</tt> is a null pointer or the object to which it +points has begun deallocation, <tt>object</tt> is zero-initialized. +Otherwise, <tt>object</tt> is registered as a <tt>__weak</tt> object +pointing to <tt>value</tt>. Equivalent to the following code:</p> +<pre>id objc_initWeak(id *object, id value) { + *object = nil; + return objc_storeWeak(object, value); +}</pre> +<p>Returns the value of <tt>object</tt> after the call.</p> +<p>Does not need to be atomic with respect to calls +to <tt>objc_storeWeak</tt> on <tt>object</tt>.</p> +</div> <!-- runtime.objc_initWeak --> + +<div id="runtime.objc_loadWeak"> +<h1><tt>id objc_loadWeak(id *object);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which +either contains a null pointer or has been registered as +a <tt>__weak</tt> object.</p> +<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and +the last value stored into <tt>object</tt> has not yet been +deallocated or begun deallocation, retains and autoreleases that value +and returns it. Otherwise returns null. Equivalent to the following +code:</p> +<pre>id objc_loadWeak(id *object) { + return objc_autorelease(objc_loadWeakRetained(object)); +}</pre> +<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt> +on <tt>object</tt>.</p> +<div class="rationale">Rationale: loading weak references would be +inherently prone to race conditions without the retain.</div> +</div> <!-- runtime.objc_loadWeak --> + +<div id="runtime.objc_loadWeakRetained"> +<h1><tt>id objc_loadWeakRetained(id *object);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which +either contains a null pointer or has been registered as +a <tt>__weak</tt> object.</p> +<p>If <tt>object</tt> is registered as a <tt>__weak</tt> object, and +the last value stored into <tt>object</tt> has not yet been +deallocated or begun deallocation, retains that value and returns it. +Otherwise returns null.</p> +<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt> +on <tt>object</tt>.</p> +</div> <!-- runtime.objc_loadWeakRetained --> + +<div id="runtime.objc_moveWeak"> +<h1><tt>void objc_moveWeak(id *dest, id *src);</tt></h1> +<p><i>Precondition:</i> <tt>src</tt> is a valid pointer which either +contains a null pointer or has been registered as a <tt>__weak</tt> +object. <tt>dest</tt> is a valid pointer which has not been +registered as a <tt>__weak</tt> object.</p> +<p><tt>dest</tt> is initialized to be equivalent to <tt>src</tt>, +potentially registering it with the runtime. <tt>src</tt> may then be +left in its original state, in which case this call is equivalent +to <a href="#runtime.objc_copyWeak"><tt>objc_copyWeak</tt></a>, or it +may be left as null.</p> +<p>Must be atomic with respect to calls to <tt>objc_storeWeak</tt> +on <tt>src</tt>.</p> +</div> <!-- runtime.objc_moveWeak --> + +<div id="runtime.objc_release"> +<h1><tt>void objc_release(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +performs a release operation exactly as if the object had been sent +the <tt>release</tt> message.</p> +</div> <!-- runtime.objc_release --> + +<div id="runtime.objc_retain"> +<h1><tt>id objc_retain(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +performs a retain operation exactly as if the object had been sent +the <tt>retain</tt> message.</p> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_retain --> + +<div id="runtime.objc_retainAutorelease"> +<h1><tt>id objc_retainAutorelease(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +performs a retain operation followed by an autorelease operation. +Equivalent to the following code:</p> +<pre>id objc_retainAutorelease(id value) { + return objc_autorelease(objc_retain(value)); +}</pre> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_retainAutorelease --> + +<div id="runtime.objc_retainAutoreleaseReturnValue"> +<h1><tt>id objc_retainAutoreleaseReturnValue(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +performs a retain operation followed by the operation described in +<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a>. +Equivalent to the following code:</p> +<pre>id objc_retainAutoreleaseReturnValue(id value) { + return objc_autoreleaseReturnValue(objc_retain(value)); +}</pre> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_retainAutoreleaseReturnValue --> + +<div id="runtime.objc_retainAutoreleasedReturnValue"> +<h1><tt>id objc_retainAutoreleasedReturnValue(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, it +attempts to accept a hand off of a retain count from a call to +<a href="#runtime.objc_autoreleaseReturnValue"><tt>objc_autoreleaseReturnValue</tt></a> +on <tt>value</tt> in a recently-called function or something it +calls. If that fails, it performs a retain operation exactly +like <a href="#runtime.objc_retain"><tt>objc_retain</tt></a>.</p> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_retainAutoreleasedReturnValue --> + +<div id="runtime.objc_retainBlock"> +<h1><tt>id objc_retainBlock(id value);</tt></h1> +<p><i>Precondition:</i> <tt>value</tt> is null or a pointer to a +valid block object.</p> +<p>If <tt>value</tt> is null, this call has no effect. Otherwise, if +the block pointed to by <tt>value</tt> is still on the stack, it is +copied to the heap and the address of the copy is returned. Otherwise +a retain operation is performed on the block exactly as if it had been +sent the <tt>retain</tt> message.</p> +</div> <!-- runtime.objc_retainBlock --> + +<div id="runtime.objc_storeStrong"> +<h1><tt>id objc_storeStrong(id *object, id value);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer to +a <tt>__strong</tt> object which is adequately aligned for a +pointer. <tt>value</tt> is null or a pointer to a valid object.</p> +<p>Performs the complete sequence for assigning to a <tt>__strong</tt> +object of non-block type. Equivalent to the following code:</p> +<pre>id objc_storeStrong(id *object, id value) { + value = [value retain]; + id oldValue = *object; + *object = value; + [oldValue release]; + return value; +}</pre> +<p>Always returns <tt>value</tt>.</p> +</div> <!-- runtime.objc_storeStrong --> + +<div id="runtime.objc_storeWeak"> +<h1><tt>id objc_storeWeak(id *object, id value);</tt></h1> +<p><i>Precondition:</i> <tt>object</tt> is a valid pointer which +either contains a null pointer or has been registered as +a <tt>__weak</tt> object. <tt>value</tt> is null or a pointer to a +valid object.</p> +<p>If <tt>value</tt> is a null pointer or the object to which it +points has begun deallocation, <tt>object</tt> is assigned null +and unregistered as a <tt>__weak</tt> object. Otherwise, +<tt>object</tt> is registered as a <tt>__weak</tt> object or has its +registration updated to point to <tt>value</tt>.</p> +<p>Returns the value of <tt>object</tt> after the call.</p> +</div> <!-- runtime.objc_storeWeak --> + +</div> <!-- runtime --> +</div> <!-- root --> +</body> +</html> |