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| <div id="content"> |
| <h1>Automatic Reference Counting</h1> |
| |
| <div id="toc"> |
| </div> |
| |
| <div id="meta"> |
| <h1>About this document</h1> |
| |
| <div id="meta.purpose"> |
| <h1>Purpose</h1> |
| |
| <p>The first and primary purpose of this document is to serve as a |
| complete technical specification of Automatic Reference Counting. |
| Given a core Objective-C compiler and runtime, it should be possible |
| to write a compiler and runtime which implements these new |
| semantics.</p> |
| |
| <p>The secondary purpose is to act as a rationale for why ARC was |
| designed in this way. This should remain tightly focused on the |
| technical design and should not stray into marketing speculation.</p> |
| |
| </div> <!-- meta.purpose --> |
| |
| <div id="meta.background"> |
| <h1>Background</h1> |
| |
| <p>This document assumes a basic familiarity with C.</p> |
| |
| <p><span class="term">Blocks</span> are a C language extension for |
| creating anonymous functions. Users interact with and transfer block |
| objects using <span class="term">block pointers</span>, which are |
| represented like a normal pointer. A block may capture values from |
| local variables; when this occurs, memory must be dynamically |
| allocated. The initial allocation is done on the stack, but the |
| runtime provides a <tt>Block_copy</tt> function which, given a block |
| pointer, either copies the underlying block object to the heap, |
| setting its reference count to 1 and returning the new block pointer, |
| or (if the block object is already on the heap) increases its |
| reference count by 1. The paired function is <tt>Block_release</tt>, |
| which decreases the reference count by 1 and destroys the object if |
| the count reaches zero and is on the heap.</p> |
| |
| <p>Objective-C is a set of language extensions, significant enough to |
| be considered a different language. It is a strict superset of C. |
| The extensions can also be imposed on C++, producing a language called |
| Objective-C++. The primary feature is a single-inheritance object |
| system; we briefly describe the modern dialect.</p> |
| |
| <p>Objective-C defines a new type kind, collectively called |
| the <span class="term">object pointer types</span>. This kind has two |
| notable builtin members, <tt>id</tt> and <tt>Class</tt>; <tt>id</tt> |
| is the final supertype of all object pointers. The validity of |
| conversions between object pointer types is not checked at runtime. |
| Users may define <span class="term">classes</span>; each class is a |
| type, and the pointer to that type is an object pointer type. A class |
| may have a superclass; its pointer type is a subtype of its |
| superclass's pointer type. A class has a set |
| of <span class="term">ivars</span>, fields which appear on all |
| instances of that class. For every class <i>T</i> there's an |
| associated metaclass; it has no fields, its superclass is the |
| metaclass of <i>T</i>'s superclass, and its metaclass is a global |
| class. Every class has a global object whose class is the |
| class's metaclass; metaclasses have no associated type, so pointers to |
| this object have type <tt>Class</tt>.</p> |
| |
| <p>A class declaration (<tt>@interface</tt>) declares a set |
| of <span class="term">methods</span>. A method has a return type, a |
| list of argument types, and a <span class="term">selector</span>: a |
| name like <tt>foo:bar:baz:</tt>, where the number of colons |
| corresponds to the number of formal arguments. A method may be an |
| instance method, in which case it can be invoked on objects of the |
| class, or a class method, in which case it can be invoked on objects |
| of the metaclass. A method may be invoked by providing an object |
| (called the <span class="term">receiver</span>) and a list of formal |
| arguments interspersed with the selector, like so:</p> |
| |
| <pre>[receiver foo: fooArg bar: barArg baz: bazArg]</pre> |
| |
| <p>This looks in the dynamic class of the receiver for a method with |
| this name, then in that class's superclass, etc., until it finds |
| something it can execute. The receiver <q>expression</q> may also be |
| the name of a class, in which case the actual receiver is the class |
| object for that class, or (within method definitions) it may |
| be <tt>super</tt>, in which case the lookup algorithm starts with the |
| static superclass instead of the dynamic class. The actual methods |
| dynamically found in a class are not those declared in the |
| <tt>@interface</tt>, but those defined in a separate |
| <tt>@implementation</tt> declaration; however, when compiling a |
| call, typechecking is done based on the methods declared in the |
| <tt>@interface</tt>.</p> |
| |
| <p>Method declarations may also be grouped into |
| <span class="term">protocols</span>, which are not inherently |
| associated with any class, but which classes may claim to follow. |
| Object pointer types may be qualified with additional protocols that |
| the object is known to support.</p> |
| |
| <p><span class="term">Class extensions</span> are collections of ivars |
| and methods, designed to allow a class's <tt>@interface</tt> to be |
| split across multiple files; however, there is still a primary |
| implementation file which must see the <tt>@interface</tt>s of all |
| class extensions. |
| <span class="term">Categories</span> allow methods (but not ivars) to |
| be declared <i>post hoc</i> on an arbitrary class; the methods in the |
| category's <tt>@implementation</tt> will be dynamically added to that |
| class's method tables which the category is loaded at runtime, |
| replacing those methods in case of a collision.</p> |
| |
| <p>In the standard environment, objects are allocated on the heap, and |
| their lifetime is manually managed using a reference count. This is |
| done using two instance methods which all classes are expected to |
| implement: <tt>retain</tt> increases the object's reference count by |
| 1, whereas <tt>release</tt> decreases it by 1 and calls the instance |
| method <tt>dealloc</tt> if the count reaches 0. To simplify certain |
| operations, there is also an <span class="term">autorelease |
| pool</span>, a thread-local list of objects to call <tt>release</tt> |
| on later; an object can be added to this pool by |
| calling <tt>autorelease</tt> on it.</p> |
| |
| <p>Block pointers may be converted to type <tt>id</tt>; block objects |
| are laid out in a way that makes them compatible with Objective-C |
| objects. There is a builtin class that all block objects are |
| considered to be objects of; this class implements <tt>retain</tt> by |
| adjusting the reference count, not by calling <tt>Block_copy</tt>.</p> |
| |
| </div> <!-- meta.background --> |
| |
| <div 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 an interior pointer to such an array, or it might |
| point to some field or local variable. In contrast, values of the |
| retainable object pointer types are never interior.</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> |
| |
| <p>Applying <tt>__attribute__((NSObject))</tt> to a property not of |
| retainable object pointer type has the same behavior it does outside |
| of ARC: it requires the property type to be some sort of pointer and |
| permits the use of modifiers other than <tt>assign</tt>. These |
| modifiers only affect the synthesized getter and setter; direct |
| accesses to the ivar (even if synthesized) still have primitive |
| semantics, and the value in the ivar will not be automatically |
| released during deallocation.</p> |
| |
| </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> |
| |
| <p>The instance variables for an ARC-compiled class will be destroyed |
| at some point after control enters the <tt>dealloc</tt> method for the |
| root class of the class. The ordering of the destruction of instance |
| variables is unspecified, both within a single class and between |
| subclasses and superclasses.</p> |
| |
| <div class="rationale"><p>Rationale: the traditional, non-ARC pattern |
| for destroying instance variables is to destroy them immediately |
| before calling <tt>[super dealloc]</tt>. Unfortunately, message |
| sends from the superclass are quite capable of reaching methods in the |
| subclass, and those methods may well read or write to those instance |
| variables. Making such message sends from dealloc is generally |
| discouraged, since the subclass may well rely on other invariants that |
| were broken during <tt>dealloc</tt>, but it's not so inescapably |
| dangerous that we felt comfortable calling it undefined behavior. |
| Therefore we chose to delay destroying the instance variables to a |
| point at which message sends are clearly disallowed: the point at |
| which the root class's deallocation routines take over.</p> |
| |
| <p>In most code, the difference is not observable. It can, however, |
| be observed if an instance variable holds a strong reference to an |
| object whose deallocation will trigger a side-effect which must be |
| carefully ordered with respect to the destruction of the super class. |
| Such code violates the design principle that semantically important |
| behavior should be explicit. A simple fix is to clear the instance |
| variable manually during <tt>dealloc</tt>; a more holistic solution is |
| to move semantically important side-effects out of |
| <tt>dealloc</tt> and into a separate teardown phase which can rely on |
| working with well-formed objects.</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 facilitate the mass annotation of interfaces:</p> |
| |
| <pre>#pragma clang arc_cf_code_audited begin |
| ... |
| #pragma clang 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> |