1.. FIXME: move to the stylesheet or Sphinx plugin 2 3.. raw:: html 4 5 <style> 6 .arc-term { font-style: italic; font-weight: bold; } 7 .revision { font-style: italic; } 8 .when-revised { font-weight: bold; font-style: normal; } 9 10 /* 11 * Automatic numbering is described in this article: 12 * http://dev.opera.com/articles/view/automatic-numbering-with-css-counters/ 13 */ 14 /* 15 * Automatic numbering for the TOC. 16 * This is wrong from the semantics point of view, since it is an ordered 17 * list, but uses "ul" tag. 18 */ 19 div#contents.contents.local ul { 20 counter-reset: toc-section; 21 list-style-type: none; 22 } 23 div#contents.contents.local ul li { 24 counter-increment: toc-section; 25 background: none; // Remove bullets 26 } 27 div#contents.contents.local ul li a.reference:before { 28 content: counters(toc-section, ".") " "; 29 } 30 31 /* Automatic numbering for the body. */ 32 body { 33 counter-reset: section subsection subsubsection; 34 } 35 .section h2 { 36 counter-reset: subsection subsubsection; 37 counter-increment: section; 38 } 39 .section h2 a.toc-backref:before { 40 content: counter(section) " "; 41 } 42 .section h3 { 43 counter-reset: subsubsection; 44 counter-increment: subsection; 45 } 46 .section h3 a.toc-backref:before { 47 content: counter(section) "." counter(subsection) " "; 48 } 49 .section h4 { 50 counter-increment: subsubsection; 51 } 52 .section h4 a.toc-backref:before { 53 content: counter(section) "." counter(subsection) "." counter(subsubsection) " "; 54 } 55 </style> 56 57.. role:: arc-term 58.. role:: revision 59.. role:: when-revised 60 61============================================== 62Objective-C Automatic Reference Counting (ARC) 63============================================== 64 65.. contents:: 66 :local: 67 68.. _arc.meta: 69 70About this document 71=================== 72 73.. _arc.meta.purpose: 74 75Purpose 76------- 77 78The first and primary purpose of this document is to serve as a complete 79technical specification of Automatic Reference Counting. Given a core 80Objective-C compiler and runtime, it should be possible to write a compiler and 81runtime which implements these new semantics. 82 83The secondary purpose is to act as a rationale for why ARC was designed in this 84way. This should remain tightly focused on the technical design and should not 85stray into marketing speculation. 86 87.. _arc.meta.background: 88 89Background 90---------- 91 92This document assumes a basic familiarity with C. 93 94:arc-term:`Blocks` are a C language extension for creating anonymous functions. 95Users interact with and transfer block objects using :arc-term:`block 96pointers`, which are represented like a normal pointer. A block may capture 97values from local variables; when this occurs, memory must be dynamically 98allocated. The initial allocation is done on the stack, but the runtime 99provides a ``Block_copy`` function which, given a block pointer, either copies 100the underlying block object to the heap, setting its reference count to 1 and 101returning the new block pointer, or (if the block object is already on the 102heap) increases its reference count by 1. The paired function is 103``Block_release``, which decreases the reference count by 1 and destroys the 104object if the count reaches zero and is on the heap. 105 106Objective-C is a set of language extensions, significant enough to be 107considered a different language. It is a strict superset of C. The extensions 108can also be imposed on C++, producing a language called Objective-C++. The 109primary feature is a single-inheritance object system; we briefly describe the 110modern dialect. 111 112Objective-C defines a new type kind, collectively called the :arc-term:`object 113pointer types`. This kind has two notable builtin members, ``id`` and 114``Class``; ``id`` is the final supertype of all object pointers. The validity 115of conversions between object pointer types is not checked at runtime. Users 116may define :arc-term:`classes`; each class is a type, and the pointer to that 117type is an object pointer type. A class may have a superclass; its pointer 118type is a subtype of its superclass's pointer type. A class has a set of 119:arc-term:`ivars`, fields which appear on all instances of that class. For 120every class *T* there's an associated metaclass; it has no fields, its 121superclass is the metaclass of *T*'s superclass, and its metaclass is a global 122class. Every class has a global object whose class is the class's metaclass; 123metaclasses have no associated type, so pointers to this object have type 124``Class``. 125 126A class declaration (``@interface``) declares a set of :arc-term:`methods`. A 127method has a return type, a list of argument types, and a :arc-term:`selector`: 128a name like ``foo:bar:baz:``, where the number of colons corresponds to the 129number of formal arguments. A method may be an instance method, in which case 130it can be invoked on objects of the class, or a class method, in which case it 131can be invoked on objects of the metaclass. A method may be invoked by 132providing an object (called the :arc-term:`receiver`) and a list of formal 133arguments interspersed with the selector, like so: 134 135.. code-block:: objc 136 137 [receiver foo: fooArg bar: barArg baz: bazArg] 138 139This looks in the dynamic class of the receiver for a method with this name, 140then in that class's superclass, etc., until it finds something it can execute. 141The receiver "expression" may also be the name of a class, in which case the 142actual receiver is the class object for that class, or (within method 143definitions) it may be ``super``, in which case the lookup algorithm starts 144with the static superclass instead of the dynamic class. The actual methods 145dynamically found in a class are not those declared in the ``@interface``, but 146those defined in a separate ``@implementation`` declaration; however, when 147compiling a call, typechecking is done based on the methods declared in the 148``@interface``. 149 150Method declarations may also be grouped into :arc-term:`protocols`, which are not 151inherently associated with any class, but which classes may claim to follow. 152Object pointer types may be qualified with additional protocols that the object 153is known to support. 154 155:arc-term:`Class extensions` are collections of ivars and methods, designed to 156allow a class's ``@interface`` to be split across multiple files; however, 157there is still a primary implementation file which must see the 158``@interface``\ s of all class extensions. :arc-term:`Categories` allow 159methods (but not ivars) to be declared *post hoc* on an arbitrary class; the 160methods in the category's ``@implementation`` will be dynamically added to that 161class's method tables which the category is loaded at runtime, replacing those 162methods in case of a collision. 163 164In the standard environment, objects are allocated on the heap, and their 165lifetime is manually managed using a reference count. This is done using two 166instance methods which all classes are expected to implement: ``retain`` 167increases the object's reference count by 1, whereas ``release`` decreases it 168by 1 and calls the instance method ``dealloc`` if the count reaches 0. To 169simplify certain operations, there is also an :arc-term:`autorelease pool`, a 170thread-local list of objects to call ``release`` on later; an object can be 171added to this pool by calling ``autorelease`` on it. 172 173Block pointers may be converted to type ``id``; block objects are laid out in a 174way that makes them compatible with Objective-C objects. There is a builtin 175class that all block objects are considered to be objects of; this class 176implements ``retain`` by adjusting the reference count, not by calling 177``Block_copy``. 178 179.. _arc.meta.evolution: 180 181Evolution 182--------- 183 184ARC is under continual evolution, and this document must be updated as the 185language progresses. 186 187If a change increases the expressiveness of the language, for example by 188lifting a restriction or by adding new syntax, the change will be annotated 189with a revision marker, like so: 190 191 ARC applies to Objective-C pointer types, block pointer types, and 192 :when-revised:`[beginning Apple 8.0, LLVM 3.8]` :revision:`BPTRs declared 193 within` ``extern "BCPL"`` blocks. 194 195For now, it is sensible to version this document by the releases of its sole 196implementation (and its host project), clang. "LLVM X.Y" refers to an 197open-source release of clang from the LLVM project. "Apple X.Y" refers to an 198Apple-provided release of the Apple LLVM Compiler. Other organizations that 199prepare their own, separately-versioned clang releases and wish to maintain 200similar information in this document should send requests to cfe-dev. 201 202If a change decreases the expressiveness of the language, for example by 203imposing a new restriction, this should be taken as an oversight in the 204original specification and something to be avoided in all versions. Such 205changes are generally to be avoided. 206 207.. _arc.general: 208 209General 210======= 211 212Automatic Reference Counting implements automatic memory management for 213Objective-C objects and blocks, freeing the programmer from the need to 214explicitly insert retains and releases. It does not provide a cycle collector; 215users must explicitly manage the lifetime of their objects, breaking cycles 216manually or with weak or unsafe references. 217 218ARC may be explicitly enabled with the compiler flag ``-fobjc-arc``. It may 219also be explicitly disabled with the compiler flag ``-fno-objc-arc``. The last 220of these two flags appearing on the compile line "wins". 221 222If ARC is enabled, ``__has_feature(objc_arc)`` will expand to 1 in the 223preprocessor. For more information about ``__has_feature``, see the 224:ref:`language extensions <langext-__has_feature-__has_extension>` document. 225 226.. _arc.objects: 227 228Retainable object pointers 229========================== 230 231This section describes retainable object pointers, their basic operations, and 232the restrictions imposed on their use under ARC. Note in particular that it 233covers the rules for pointer *values* (patterns of bits indicating the location 234of a pointed-to object), not pointer *objects* (locations in memory which store 235pointer values). The rules for objects are covered in the next section. 236 237A :arc-term:`retainable object pointer` (or "retainable pointer") is a value of 238a :arc-term:`retainable object pointer type` ("retainable type"). There are 239three kinds of retainable object pointer types: 240 241* block pointers (formed by applying the caret (``^``) declarator sigil to a 242 function type) 243* Objective-C object pointers (``id``, ``Class``, ``NSFoo*``, etc.) 244* typedefs marked with ``__attribute__((NSObject))`` 245 246Other pointer types, such as ``int*`` and ``CFStringRef``, are not subject to 247ARC's semantics and restrictions. 248 249.. admonition:: Rationale 250 251 We are not at liberty to require all code to be recompiled with ARC; 252 therefore, ARC must interoperate with Objective-C code which manages retains 253 and releases manually. In general, there are three requirements in order for 254 a compiler-supported reference-count system to provide reliable 255 interoperation: 256 257 * The type system must reliably identify which objects are to be managed. An 258 ``int*`` might be a pointer to a ``malloc``'ed array, or it might be an 259 interior pointer to such an array, or it might point to some field or local 260 variable. In contrast, values of the retainable object pointer types are 261 never interior. 262 263 * The type system must reliably indicate how to manage objects of a type. 264 This usually means that the type must imply a procedure for incrementing 265 and decrementing retain counts. Supporting single-ownership objects 266 requires a lot more explicit mediation in the language. 267 268 * There must be reliable conventions for whether and when "ownership" is 269 passed between caller and callee, for both arguments and return values. 270 Objective-C methods follow such a convention very reliably, at least for 271 system libraries on Mac OS X, and functions always pass objects at +0. The 272 C-based APIs for Core Foundation objects, on the other hand, have much more 273 varied transfer semantics. 274 275The use of ``__attribute__((NSObject))`` typedefs is not recommended. If it's 276absolutely necessary to use this attribute, be very explicit about using the 277typedef, and do not assume that it will be preserved by language features like 278``__typeof`` and C++ template argument substitution. 279 280.. admonition:: Rationale 281 282 Any compiler operation which incidentally strips type "sugar" from a type 283 will yield a type without the attribute, which may result in unexpected 284 behavior. 285 286.. _arc.objects.retains: 287 288Retain count semantics 289---------------------- 290 291A retainable object pointer is either a :arc-term:`null pointer` or a pointer 292to a valid object. Furthermore, if it has block pointer type and is not 293``null`` then it must actually be a pointer to a block object, and if it has 294``Class`` type (possibly protocol-qualified) then it must actually be a pointer 295to a class object. Otherwise ARC does not enforce the Objective-C type system 296as long as the implementing methods follow the signature of the static type. 297It is undefined behavior if ARC is exposed to an invalid pointer. 298 299For ARC's purposes, a valid object is one with "well-behaved" retaining 300operations. Specifically, the object must be laid out such that the 301Objective-C message send machinery can successfully send it the following 302messages: 303 304* ``retain``, taking no arguments and returning a pointer to the object. 305* ``release``, taking no arguments and returning ``void``. 306* ``autorelease``, taking no arguments and returning a pointer to the object. 307 308The behavior of these methods is constrained in the following ways. The term 309:arc-term:`high-level semantics` is an intentionally vague term; the intent is 310that programmers must implement these methods in a way such that the compiler, 311modifying code in ways it deems safe according to these constraints, will not 312violate their requirements. For example, if the user puts logging statements 313in ``retain``, they should not be surprised if those statements are executed 314more or less often depending on optimization settings. These constraints are 315not exhaustive of the optimization opportunities: values held in local 316variables are subject to additional restrictions, described later in this 317document. 318 319It is undefined behavior if a computation history featuring a send of 320``retain`` followed by a send of ``release`` to the same object, with no 321intervening ``release`` on that object, is not equivalent under the high-level 322semantics to a computation history in which these sends are removed. Note that 323this implies that these methods may not raise exceptions. 324 325It is undefined behavior if a computation history features any use whatsoever 326of an object following the completion of a send of ``release`` that is not 327preceded by a send of ``retain`` to the same object. 328 329The behavior of ``autorelease`` must be equivalent to sending ``release`` when 330one of the autorelease pools currently in scope is popped. It may not throw an 331exception. 332 333When the semantics call for performing one of these operations on a retainable 334object pointer, if that pointer is ``null`` then the effect is a no-op. 335 336All of the semantics described in this document are subject to additional 337:ref:`optimization rules <arc.optimization>` which permit the removal or 338optimization of operations based on local knowledge of data flow. The 339semantics describe the high-level behaviors that the compiler implements, not 340an exact sequence of operations that a program will be compiled into. 341 342.. _arc.objects.operands: 343 344Retainable object pointers as operands and arguments 345---------------------------------------------------- 346 347In general, ARC does not perform retain or release operations when simply using 348a retainable object pointer as an operand within an expression. This includes: 349 350* loading a retainable pointer from an object with non-weak :ref:`ownership 351 <arc.ownership>`, 352* passing a retainable pointer as an argument to a function or method, and 353* receiving a retainable pointer as the result of a function or method call. 354 355.. admonition:: Rationale 356 357 While this might seem uncontroversial, it is actually unsafe when multiple 358 expressions are evaluated in "parallel", as with binary operators and calls, 359 because (for example) one expression might load from an object while another 360 writes to it. However, C and C++ already call this undefined behavior 361 because the evaluations are unsequenced, and ARC simply exploits that here to 362 avoid needing to retain arguments across a large number of calls. 363 364The remainder of this section describes exceptions to these rules, how those 365exceptions are detected, and what those exceptions imply semantically. 366 367.. _arc.objects.operands.consumed: 368 369Consumed parameters 370^^^^^^^^^^^^^^^^^^^ 371 372A function or method parameter of retainable object pointer type may be marked 373as :arc-term:`consumed`, signifying that the callee expects to take ownership 374of a +1 retain count. This is done by adding the ``ns_consumed`` attribute to 375the parameter declaration, like so: 376 377.. code-block:: objc 378 379 void foo(__attribute((ns_consumed)) id x); 380 - (void) foo: (id) __attribute((ns_consumed)) x; 381 382This attribute is part of the type of the function or method, not the type of 383the parameter. It controls only how the argument is passed and received. 384 385When passing such an argument, ARC retains the argument prior to making the 386call. 387 388When receiving such an argument, ARC releases the argument at the end of the 389function, subject to the usual optimizations for local values. 390 391.. admonition:: Rationale 392 393 This formalizes direct transfers of ownership from a caller to a callee. The 394 most common scenario here is passing the ``self`` parameter to ``init``, but 395 it is useful to generalize. Typically, local optimization will remove any 396 extra retains and releases: on the caller side the retain will be merged with 397 a +1 source, and on the callee side the release will be rolled into the 398 initialization of the parameter. 399 400The implicit ``self`` parameter of a method may be marked as consumed by adding 401``__attribute__((ns_consumes_self))`` to the method declaration. Methods in 402the ``init`` :ref:`family <arc.method-families>` are treated as if they were 403implicitly marked with this attribute. 404 405It is undefined behavior if an Objective-C message send to a method with 406``ns_consumed`` parameters (other than self) is made with a null receiver. It 407is undefined behavior if the method to which an Objective-C message send 408statically resolves to has a different set of ``ns_consumed`` parameters than 409the method it dynamically resolves to. It is undefined behavior if a block or 410function call is made through a static type with a different set of 411``ns_consumed`` parameters than the implementation of the called block or 412function. 413 414.. admonition:: Rationale 415 416 Consumed parameters with null receiver are a guaranteed leak. Mismatches 417 with consumed parameters will cause over-retains or over-releases, depending 418 on the direction. The rule about function calls is really just an 419 application of the existing C/C++ rule about calling functions through an 420 incompatible function type, but it's useful to state it explicitly. 421 422.. _arc.object.operands.retained-return-values: 423 424Retained return values 425^^^^^^^^^^^^^^^^^^^^^^ 426 427A function or method which returns a retainable object pointer type may be 428marked as returning a retained value, signifying that the caller expects to take 429ownership of a +1 retain count. This is done by adding the 430``ns_returns_retained`` attribute to the function or method declaration, like 431so: 432 433.. code-block:: objc 434 435 id foo(void) __attribute((ns_returns_retained)); 436 - (id) foo __attribute((ns_returns_retained)); 437 438This attribute is part of the type of the function or method. 439 440When returning from such a function or method, ARC retains the value at the 441point of evaluation of the return statement, before leaving all local scopes. 442 443When receiving a return result from such a function or method, ARC releases the 444value at the end of the full-expression it is contained within, subject to the 445usual optimizations for local values. 446 447.. admonition:: Rationale 448 449 This formalizes direct transfers of ownership from a callee to a caller. The 450 most common scenario this models is the retained return from ``init``, 451 ``alloc``, ``new``, and ``copy`` methods, but there are other cases in the 452 frameworks. After optimization there are typically no extra retains and 453 releases required. 454 455Methods in the ``alloc``, ``copy``, ``init``, ``mutableCopy``, and ``new`` 456:ref:`families <arc.method-families>` are implicitly marked 457``__attribute__((ns_returns_retained))``. This may be suppressed by explicitly 458marking the method ``__attribute__((ns_returns_not_retained))``. 459 460It is undefined behavior if the method to which an Objective-C message send 461statically resolves has different retain semantics on its result from the 462method it dynamically resolves to. It is undefined behavior if a block or 463function call is made through a static type with different retain semantics on 464its result from the implementation of the called block or function. 465 466.. admonition:: Rationale 467 468 Mismatches with returned results will cause over-retains or over-releases, 469 depending on the direction. Again, the rule about function calls is really 470 just an application of the existing C/C++ rule about calling functions 471 through an incompatible function type. 472 473.. _arc.objects.operands.unretained-returns: 474 475Unretained return values 476^^^^^^^^^^^^^^^^^^^^^^^^ 477 478A method or function which returns a retainable object type but does not return 479a retained value must ensure that the object is still valid across the return 480boundary. 481 482When returning from such a function or method, ARC retains the value at the 483point of evaluation of the return statement, then leaves all local scopes, and 484then balances out the retain while ensuring that the value lives across the 485call boundary. In the worst case, this may involve an ``autorelease``, but 486callers must not assume that the value is actually in the autorelease pool. 487 488ARC performs no extra mandatory work on the caller side, although it may elect 489to do something to shorten the lifetime of the returned value. 490 491.. admonition:: Rationale 492 493 It is common in non-ARC code to not return an autoreleased value; therefore 494 the convention does not force either path. It is convenient to not be 495 required to do unnecessary retains and autoreleases; this permits 496 optimizations such as eliding retain/autoreleases when it can be shown that 497 the original pointer will still be valid at the point of return. 498 499A method or function may be marked with 500``__attribute__((ns_returns_autoreleased))`` to indicate that it returns a 501pointer which is guaranteed to be valid at least as long as the innermost 502autorelease pool. There are no additional semantics enforced in the definition 503of such a method; it merely enables optimizations in callers. 504 505.. _arc.objects.operands.casts: 506 507Bridged casts 508^^^^^^^^^^^^^ 509 510A :arc-term:`bridged cast` is a C-style cast annotated with one of three 511keywords: 512 513* ``(__bridge T) op`` casts the operand to the destination type ``T``. If 514 ``T`` is a retainable object pointer type, then ``op`` must have a 515 non-retainable pointer type. If ``T`` is a non-retainable pointer type, 516 then ``op`` must have a retainable object pointer type. Otherwise the cast 517 is ill-formed. There is no transfer of ownership, and ARC inserts no retain 518 operations. 519* ``(__bridge_retained T) op`` casts the operand, which must have retainable 520 object pointer type, to the destination type, which must be a non-retainable 521 pointer type. ARC retains the value, subject to the usual optimizations on 522 local values, and the recipient is responsible for balancing that +1. 523* ``(__bridge_transfer T) op`` casts the operand, which must have 524 non-retainable pointer type, to the destination type, which must be a 525 retainable object pointer type. ARC will release the value at the end of 526 the enclosing full-expression, subject to the usual optimizations on local 527 values. 528 529These casts are required in order to transfer objects in and out of ARC 530control; see the rationale in the section on :ref:`conversion of retainable 531object pointers <arc.objects.restrictions.conversion>`. 532 533Using a ``__bridge_retained`` or ``__bridge_transfer`` cast purely to convince 534ARC to emit an unbalanced retain or release, respectively, is poor form. 535 536.. _arc.objects.restrictions: 537 538Restrictions 539------------ 540 541.. _arc.objects.restrictions.conversion: 542 543Conversion of retainable object pointers 544^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 545 546In general, a program which attempts to implicitly or explicitly convert a 547value of retainable object pointer type to any non-retainable type, or 548vice-versa, is ill-formed. For example, an Objective-C object pointer shall 549not be converted to ``void*``. As an exception, cast to ``intptr_t`` is 550allowed because such casts are not transferring ownership. The :ref:`bridged 551casts <arc.objects.operands.casts>` may be used to perform these conversions 552where necessary. 553 554.. admonition:: Rationale 555 556 We cannot ensure the correct management of the lifetime of objects if they 557 may be freely passed around as unmanaged types. The bridged casts are 558 provided so that the programmer may explicitly describe whether the cast 559 transfers control into or out of ARC. 560 561However, the following exceptions apply. 562 563.. _arc.objects.restrictions.conversion.with.known.semantics: 564 565Conversion to retainable object pointer type of expressions with known semantics 566^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 567 568:when-revised:`[beginning Apple 4.0, LLVM 3.1]` 569:revision:`These exceptions have been greatly expanded; they previously applied 570only to a much-reduced subset which is difficult to categorize but which 571included null pointers, message sends (under the given rules), and the various 572global constants.` 573 574An unbridged conversion to a retainable object pointer type from a type other 575than a retainable object pointer type is ill-formed, as discussed above, unless 576the operand of the cast has a syntactic form which is known retained, known 577unretained, or known retain-agnostic. 578 579An expression is :arc-term:`known retain-agnostic` if it is: 580 581* an Objective-C string literal, 582* a load from a ``const`` system global variable of :ref:`C retainable pointer 583 type <arc.misc.c-retainable>`, or 584* a null pointer constant. 585 586An expression is :arc-term:`known unretained` if it is an rvalue of :ref:`C 587retainable pointer type <arc.misc.c-retainable>` and it is: 588 589* a direct call to a function, and either that function has the 590 ``cf_returns_not_retained`` attribute or it is an :ref:`audited 591 <arc.misc.c-retainable.audit>` function that does not have the 592 ``cf_returns_retained`` attribute and does not follow the create/copy naming 593 convention, 594* a message send, and the declared method either has the 595 ``cf_returns_not_retained`` attribute or it has neither the 596 ``cf_returns_retained`` attribute nor a :ref:`selector family 597 <arc.method-families>` that implies a retained result, or 598* :when-revised:`[beginning LLVM 3.6]` :revision:`a load from a` ``const`` 599 :revision:`non-system global variable.` 600 601An expression is :arc-term:`known retained` if it is an rvalue of :ref:`C 602retainable pointer type <arc.misc.c-retainable>` and it is: 603 604* a message send, and the declared method either has the 605 ``cf_returns_retained`` attribute, or it does not have the 606 ``cf_returns_not_retained`` attribute but it does have a :ref:`selector 607 family <arc.method-families>` that implies a retained result. 608 609Furthermore: 610 611* a comma expression is classified according to its right-hand side, 612* a statement expression is classified according to its result expression, if 613 it has one, 614* an lvalue-to-rvalue conversion applied to an Objective-C property lvalue is 615 classified according to the underlying message send, and 616* a conditional operator is classified according to its second and third 617 operands, if they agree in classification, or else the other if one is known 618 retain-agnostic. 619 620If the cast operand is known retained, the conversion is treated as a 621``__bridge_transfer`` cast. If the cast operand is known unretained or known 622retain-agnostic, the conversion is treated as a ``__bridge`` cast. 623 624.. admonition:: Rationale 625 626 Bridging casts are annoying. Absent the ability to completely automate the 627 management of CF objects, however, we are left with relatively poor attempts 628 to reduce the need for a glut of explicit bridges. Hence these rules. 629 630 We've so far consciously refrained from implicitly turning retained CF 631 results from function calls into ``__bridge_transfer`` casts. The worry is 632 that some code patterns --- for example, creating a CF value, assigning it 633 to an ObjC-typed local, and then calling ``CFRelease`` when done --- are a 634 bit too likely to be accidentally accepted, leading to mysterious behavior. 635 636 For loads from ``const`` global variables of :ref:`C retainable pointer type 637 <arc.misc.c-retainable>`, it is reasonable to assume that global system 638 constants were initialitzed with true constants (e.g. string literals), but 639 user constants might have been initialized with something dynamically 640 allocated, using a global initializer. 641 642.. _arc.objects.restrictions.conversion-exception-contextual: 643 644Conversion from retainable object pointer type in certain contexts 645^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 646 647:when-revised:`[beginning Apple 4.0, LLVM 3.1]` 648 649If an expression of retainable object pointer type is explicitly cast to a 650:ref:`C retainable pointer type <arc.misc.c-retainable>`, the program is 651ill-formed as discussed above unless the result is immediately used: 652 653* to initialize a parameter in an Objective-C message send where the parameter 654 is not marked with the ``cf_consumed`` attribute, or 655* to initialize a parameter in a direct call to an 656 :ref:`audited <arc.misc.c-retainable.audit>` function where the parameter is 657 not marked with the ``cf_consumed`` attribute. 658 659.. admonition:: Rationale 660 661 Consumed parameters are left out because ARC would naturally balance them 662 with a retain, which was judged too treacherous. This is in part because 663 several of the most common consuming functions are in the ``Release`` family, 664 and it would be quite unfortunate for explicit releases to be silently 665 balanced out in this way. 666 667.. _arc.ownership: 668 669Ownership qualification 670======================= 671 672This section describes the behavior of *objects* of retainable object pointer 673type; that is, locations in memory which store retainable object pointers. 674 675A type is a :arc-term:`retainable object owner type` if it is a retainable 676object pointer type or an array type whose element type is a retainable object 677owner type. 678 679An :arc-term:`ownership qualifier` is a type qualifier which applies only to 680retainable object owner types. An array type is ownership-qualified according 681to its element type, and adding an ownership qualifier to an array type so 682qualifies its element type. 683 684A program is ill-formed if it attempts to apply an ownership qualifier to a 685type which is already ownership-qualified, even if it is the same qualifier. 686There is a single exception to this rule: an ownership qualifier may be applied 687to a substituted template type parameter, which overrides the ownership 688qualifier provided by the template argument. 689 690When forming a function type, the result type is adjusted so that any 691top-level ownership qualifier is deleted. 692 693Except as described under the :ref:`inference rules <arc.ownership.inference>`, 694a program is ill-formed if it attempts to form a pointer or reference type to a 695retainable object owner type which lacks an ownership qualifier. 696 697.. admonition:: Rationale 698 699 These rules, together with the inference rules, ensure that all objects and 700 lvalues of retainable object pointer type have an ownership qualifier. The 701 ability to override an ownership qualifier during template substitution is 702 required to counteract the :ref:`inference of __strong for template type 703 arguments <arc.ownership.inference.template.arguments>`. Ownership qualifiers 704 on return types are dropped because they serve no purpose there except to 705 cause spurious problems with overloading and templates. 706 707There are four ownership qualifiers: 708 709* ``__autoreleasing`` 710* ``__strong`` 711* ``__unsafe_unretained`` 712* ``__weak`` 713 714A type is :arc-term:`nontrivially ownership-qualified` if it is qualified with 715``__autoreleasing``, ``__strong``, or ``__weak``. 716 717.. _arc.ownership.spelling: 718 719Spelling 720-------- 721 722The names of the ownership qualifiers are reserved for the implementation. A 723program may not assume that they are or are not implemented with macros, or 724what those macros expand to. 725 726An ownership qualifier may be written anywhere that any other type qualifier 727may be written. 728 729If an ownership qualifier appears in the *declaration-specifiers*, the 730following rules apply: 731 732* if the type specifier is a retainable object owner type, the qualifier 733 initially applies to that type; 734 735* otherwise, if the outermost non-array declarator is a pointer 736 or block pointer declarator, the qualifier initially applies to 737 that type; 738 739* otherwise the program is ill-formed. 740 741* If the qualifier is so applied at a position in the declaration 742 where the next-innermost declarator is a function declarator, and 743 there is an block declarator within that function declarator, then 744 the qualifier applies instead to that block declarator and this rule 745 is considered afresh beginning from the new position. 746 747If an ownership qualifier appears on the declarator name, or on the declared 748object, it is applied to the innermost pointer or block-pointer type. 749 750If an ownership qualifier appears anywhere else in a declarator, it applies to 751the type there. 752 753.. admonition:: Rationale 754 755 Ownership qualifiers are like ``const`` and ``volatile`` in the sense 756 that they may sensibly apply at multiple distinct positions within a 757 declarator. However, unlike those qualifiers, there are many 758 situations where they are not meaningful, and so we make an effort 759 to "move" the qualifier to a place where it will be meaningful. The 760 general goal is to allow the programmer to write, say, ``__strong`` 761 before the entire declaration and have it apply in the leftmost 762 sensible place. 763 764.. _arc.ownership.spelling.property: 765 766Property declarations 767^^^^^^^^^^^^^^^^^^^^^ 768 769A property of retainable object pointer type may have ownership. If the 770property's type is ownership-qualified, then the property has that ownership. 771If the property has one of the following modifiers, then the property has the 772corresponding ownership. A property is ill-formed if it has conflicting 773sources of ownership, or if it has redundant ownership modifiers, or if it has 774``__autoreleasing`` ownership. 775 776* ``assign`` implies ``__unsafe_unretained`` ownership. 777* ``copy`` implies ``__strong`` ownership, as well as the usual behavior of 778 copy semantics on the setter. 779* ``retain`` implies ``__strong`` ownership. 780* ``strong`` implies ``__strong`` ownership. 781* ``unsafe_unretained`` implies ``__unsafe_unretained`` ownership. 782* ``weak`` implies ``__weak`` ownership. 783 784With the exception of ``weak``, these modifiers are available in non-ARC 785modes. 786 787A property's specified ownership is preserved in its metadata, but otherwise 788the meaning is purely conventional unless the property is synthesized. If a 789property is synthesized, then the :arc-term:`associated instance variable` is 790the instance variable which is named, possibly implicitly, by the 791``@synthesize`` declaration. If the associated instance variable already 792exists, then its ownership qualification must equal the ownership of the 793property; otherwise, the instance variable is created with that ownership 794qualification. 795 796A property of retainable object pointer type which is synthesized without a 797source of ownership has the ownership of its associated instance variable, if it 798already exists; otherwise, :when-revised:`[beginning Apple 3.1, LLVM 3.1]` 799:revision:`its ownership is implicitly` ``strong``. Prior to this revision, it 800was ill-formed to synthesize such a property. 801 802.. admonition:: Rationale 803 804 Using ``strong`` by default is safe and consistent with the generic ARC rule 805 about :ref:`inferring ownership <arc.ownership.inference.variables>`. It is, 806 unfortunately, inconsistent with the non-ARC rule which states that such 807 properties are implicitly ``assign``. However, that rule is clearly 808 untenable in ARC, since it leads to default-unsafe code. The main merit to 809 banning the properties is to avoid confusion with non-ARC practice, which did 810 not ultimately strike us as sufficient to justify requiring extra syntax and 811 (more importantly) forcing novices to understand ownership rules just to 812 declare a property when the default is so reasonable. Changing the rule away 813 from non-ARC practice was acceptable because we had conservatively banned the 814 synthesis in order to give ourselves exactly this leeway. 815 816Applying ``__attribute__((NSObject))`` to a property not of retainable object 817pointer type has the same behavior it does outside of ARC: it requires the 818property type to be some sort of pointer and permits the use of modifiers other 819than ``assign``. These modifiers only affect the synthesized getter and 820setter; direct accesses to the ivar (even if synthesized) still have primitive 821semantics, and the value in the ivar will not be automatically released during 822deallocation. 823 824.. _arc.ownership.semantics: 825 826Semantics 827--------- 828 829There are five :arc-term:`managed operations` which may be performed on an 830object of retainable object pointer type. Each qualifier specifies different 831semantics for each of these operations. It is still undefined behavior to 832access an object outside of its lifetime. 833 834A load or store with "primitive semantics" has the same semantics as the 835respective operation would have on an ``void*`` lvalue with the same alignment 836and non-ownership qualification. 837 838:arc-term:`Reading` occurs when performing a lvalue-to-rvalue conversion on an 839object lvalue. 840 841* For ``__weak`` objects, the current pointee is retained and then released at 842 the end of the current full-expression. This must execute atomically with 843 respect to assignments and to the final release of the pointee. 844* For all other objects, the lvalue is loaded with primitive semantics. 845 846:arc-term:`Assignment` occurs when evaluating an assignment operator. The 847semantics vary based on the qualification: 848 849* For ``__strong`` objects, the new pointee is first retained; second, the 850 lvalue is loaded with primitive semantics; third, the new pointee is stored 851 into the lvalue with primitive semantics; and finally, the old pointee is 852 released. This is not performed atomically; external synchronization must be 853 used to make this safe in the face of concurrent loads and stores. 854* For ``__weak`` objects, the lvalue is updated to point to the new pointee, 855 unless the new pointee is an object currently undergoing deallocation, in 856 which case the lvalue is updated to a null pointer. This must execute 857 atomically with respect to other assignments to the object, to reads from the 858 object, and to the final release of the new pointee. 859* For ``__unsafe_unretained`` objects, the new pointee is stored into the 860 lvalue using primitive semantics. 861* For ``__autoreleasing`` objects, the new pointee is retained, autoreleased, 862 and stored into the lvalue using primitive semantics. 863 864:arc-term:`Initialization` occurs when an object's lifetime begins, which 865depends on its storage duration. Initialization proceeds in two stages: 866 867#. First, a null pointer is stored into the lvalue using primitive semantics. 868 This step is skipped if the object is ``__unsafe_unretained``. 869#. Second, if the object has an initializer, that expression is evaluated and 870 then assigned into the object using the usual assignment semantics. 871 872:arc-term:`Destruction` occurs when an object's lifetime ends. In all cases it 873is semantically equivalent to assigning a null pointer to the object, with the 874proviso that of course the object cannot be legally read after the object's 875lifetime ends. 876 877:arc-term:`Moving` occurs in specific situations where an lvalue is "moved 878from", meaning that its current pointee will be used but the object may be left 879in a different (but still valid) state. This arises with ``__block`` variables 880and rvalue references in C++. For ``__strong`` lvalues, moving is equivalent 881to loading the lvalue with primitive semantics, writing a null pointer to it 882with primitive semantics, and then releasing the result of the load at the end 883of the current full-expression. For all other lvalues, moving is equivalent to 884reading the object. 885 886.. _arc.ownership.restrictions: 887 888Restrictions 889------------ 890 891.. _arc.ownership.restrictions.weak: 892 893Weak-unavailable types 894^^^^^^^^^^^^^^^^^^^^^^ 895 896It is explicitly permitted for Objective-C classes to not support ``__weak`` 897references. It is undefined behavior to perform an operation with weak 898assignment semantics with a pointer to an Objective-C object whose class does 899not support ``__weak`` references. 900 901.. admonition:: Rationale 902 903 Historically, it has been possible for a class to provide its own 904 reference-count implementation by overriding ``retain``, ``release``, etc. 905 However, weak references to an object require coordination with its class's 906 reference-count implementation because, among other things, weak loads and 907 stores must be atomic with respect to the final release. Therefore, existing 908 custom reference-count implementations will generally not support weak 909 references without additional effort. This is unavoidable without breaking 910 binary compatibility. 911 912A class may indicate that it does not support weak references by providing the 913``objc_arc_weak_unavailable`` attribute on the class's interface declaration. A 914retainable object pointer type is **weak-unavailable** if 915is a pointer to an (optionally protocol-qualified) Objective-C class ``T`` where 916``T`` or one of its superclasses has the ``objc_arc_weak_unavailable`` 917attribute. A program is ill-formed if it applies the ``__weak`` ownership 918qualifier to a weak-unavailable type or if the value operand of a weak 919assignment operation has a weak-unavailable type. 920 921.. _arc.ownership.restrictions.autoreleasing: 922 923Storage duration of ``__autoreleasing`` objects 924^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 925 926A program is ill-formed if it declares an ``__autoreleasing`` object of 927non-automatic storage duration. A program is ill-formed if it captures an 928``__autoreleasing`` object in a block or, unless by reference, in a C++11 929lambda. 930 931.. admonition:: Rationale 932 933 Autorelease pools are tied to the current thread and scope by their nature. 934 While it is possible to have temporary objects whose instance variables are 935 filled with autoreleased objects, there is no way that ARC can provide any 936 sort of safety guarantee there. 937 938It is undefined behavior if a non-null pointer is assigned to an 939``__autoreleasing`` object while an autorelease pool is in scope and then that 940object is read after the autorelease pool's scope is left. 941 942.. _arc.ownership.restrictions.conversion.indirect: 943 944Conversion of pointers to ownership-qualified types 945^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 946 947A program is ill-formed if an expression of type ``T*`` is converted, 948explicitly or implicitly, to the type ``U*``, where ``T`` and ``U`` have 949different ownership qualification, unless: 950 951* ``T`` is qualified with ``__strong``, ``__autoreleasing``, or 952 ``__unsafe_unretained``, and ``U`` is qualified with both ``const`` and 953 ``__unsafe_unretained``; or 954* either ``T`` or ``U`` is ``cv void``, where ``cv`` is an optional sequence 955 of non-ownership qualifiers; or 956* the conversion is requested with a ``reinterpret_cast`` in Objective-C++; or 957* the conversion is a well-formed :ref:`pass-by-writeback 958 <arc.ownership.restrictions.pass_by_writeback>`. 959 960The analogous rule applies to ``T&`` and ``U&`` in Objective-C++. 961 962.. admonition:: Rationale 963 964 These rules provide a reasonable level of type-safety for indirect pointers, 965 as long as the underlying memory is not deallocated. The conversion to 966 ``const __unsafe_unretained`` is permitted because the semantics of reads are 967 equivalent across all these ownership semantics, and that's a very useful and 968 common pattern. The interconversion with ``void*`` is useful for allocating 969 memory or otherwise escaping the type system, but use it carefully. 970 ``reinterpret_cast`` is considered to be an obvious enough sign of taking 971 responsibility for any problems. 972 973It is undefined behavior to access an ownership-qualified object through an 974lvalue of a differently-qualified type, except that any non-``__weak`` object 975may be read through an ``__unsafe_unretained`` lvalue. 976 977It is undefined behavior if a managed operation is performed on a ``__strong`` 978or ``__weak`` object without a guarantee that it contains a primitive zero 979bit-pattern, or if the storage for such an object is freed or reused without the 980object being first assigned a null pointer. 981 982.. admonition:: Rationale 983 984 ARC cannot differentiate between an assignment operator which is intended to 985 "initialize" dynamic memory and one which is intended to potentially replace 986 a value. Therefore the object's pointer must be valid before letting ARC at 987 it. Similarly, C and Objective-C do not provide any language hooks for 988 destroying objects held in dynamic memory, so it is the programmer's 989 responsibility to avoid leaks (``__strong`` objects) and consistency errors 990 (``__weak`` objects). 991 992These requirements are followed automatically in Objective-C++ when creating 993objects of retainable object owner type with ``new`` or ``new[]`` and destroying 994them with ``delete``, ``delete[]``, or a pseudo-destructor expression. Note 995that arrays of nontrivially-ownership-qualified type are not ABI compatible with 996non-ARC code because the element type is non-POD: such arrays that are 997``new[]``'d in ARC translation units cannot be ``delete[]``'d in non-ARC 998translation units and vice-versa. 999 1000.. _arc.ownership.restrictions.pass_by_writeback: 1001 1002Passing to an out parameter by writeback 1003^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1004 1005If the argument passed to a parameter of type ``T __autoreleasing *`` has type 1006``U oq *``, where ``oq`` is an ownership qualifier, then the argument is a 1007candidate for :arc-term:`pass-by-writeback`` if: 1008 1009* ``oq`` is ``__strong`` or ``__weak``, and 1010* it would be legal to initialize a ``T __strong *`` with a ``U __strong *``. 1011 1012For purposes of overload resolution, an implicit conversion sequence requiring 1013a pass-by-writeback is always worse than an implicit conversion sequence not 1014requiring a pass-by-writeback. 1015 1016The pass-by-writeback is ill-formed if the argument expression does not have a 1017legal form: 1018 1019* ``&var``, where ``var`` is a scalar variable of automatic storage duration 1020 with retainable object pointer type 1021* a conditional expression where the second and third operands are both legal 1022 forms 1023* a cast whose operand is a legal form 1024* a null pointer constant 1025 1026.. admonition:: Rationale 1027 1028 The restriction in the form of the argument serves two purposes. First, it 1029 makes it impossible to pass the address of an array to the argument, which 1030 serves to protect against an otherwise serious risk of mis-inferring an 1031 "array" argument as an out-parameter. Second, it makes it much less likely 1032 that the user will see confusing aliasing problems due to the implementation, 1033 below, where their store to the writeback temporary is not immediately seen 1034 in the original argument variable. 1035 1036A pass-by-writeback is evaluated as follows: 1037 1038#. The argument is evaluated to yield a pointer ``p`` of type ``U oq *``. 1039#. If ``p`` is a null pointer, then a null pointer is passed as the argument, 1040 and no further work is required for the pass-by-writeback. 1041#. Otherwise, a temporary of type ``T __autoreleasing`` is created and 1042 initialized to a null pointer. 1043#. If the parameter is not an Objective-C method parameter marked ``out``, 1044 then ``*p`` is read, and the result is written into the temporary with 1045 primitive semantics. 1046#. The address of the temporary is passed as the argument to the actual call. 1047#. After the call completes, the temporary is loaded with primitive 1048 semantics, and that value is assigned into ``*p``. 1049 1050.. admonition:: Rationale 1051 1052 This is all admittedly convoluted. In an ideal world, we would see that a 1053 local variable is being passed to an out-parameter and retroactively modify 1054 its type to be ``__autoreleasing`` rather than ``__strong``. This would be 1055 remarkably difficult and not always well-founded under the C type system. 1056 However, it was judged unacceptably invasive to require programmers to write 1057 ``__autoreleasing`` on all the variables they intend to use for 1058 out-parameters. This was the least bad solution. 1059 1060.. _arc.ownership.restrictions.records: 1061 1062Ownership-qualified fields of structs and unions 1063^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1064 1065A program is ill-formed if it declares a member of a C struct or union to have 1066a nontrivially ownership-qualified type. 1067 1068.. admonition:: Rationale 1069 1070 The resulting type would be non-POD in the C++ sense, but C does not give us 1071 very good language tools for managing the lifetime of aggregates, so it is 1072 more convenient to simply forbid them. It is still possible to manage this 1073 with a ``void*`` or an ``__unsafe_unretained`` object. 1074 1075This restriction does not apply in Objective-C++. However, nontrivally 1076ownership-qualified types are considered non-POD: in C++11 terms, they are not 1077trivially default constructible, copy constructible, move constructible, copy 1078assignable, move assignable, or destructible. It is a violation of C++'s One 1079Definition Rule to use a class outside of ARC that, under ARC, would have a 1080nontrivially ownership-qualified member. 1081 1082.. admonition:: Rationale 1083 1084 Unlike in C, we can express all the necessary ARC semantics for 1085 ownership-qualified subobjects as suboperations of the (default) special 1086 member functions for the class. These functions then become non-trivial. 1087 This has the non-obvious result that the class will have a non-trivial copy 1088 constructor and non-trivial destructor; if this would not normally be true 1089 outside of ARC, objects of the type will be passed and returned in an 1090 ABI-incompatible manner. 1091 1092.. _arc.ownership.inference: 1093 1094Ownership inference 1095------------------- 1096 1097.. _arc.ownership.inference.variables: 1098 1099Objects 1100^^^^^^^ 1101 1102If an object is declared with retainable object owner type, but without an 1103explicit ownership qualifier, its type is implicitly adjusted to have 1104``__strong`` qualification. 1105 1106As a special case, if the object's base type is ``Class`` (possibly 1107protocol-qualified), the type is adjusted to have ``__unsafe_unretained`` 1108qualification instead. 1109 1110.. _arc.ownership.inference.indirect_parameters: 1111 1112Indirect parameters 1113^^^^^^^^^^^^^^^^^^^ 1114 1115If a function or method parameter has type ``T*``, where ``T`` is an 1116ownership-unqualified retainable object pointer type, then: 1117 1118* if ``T`` is ``const``-qualified or ``Class``, then it is implicitly 1119 qualified with ``__unsafe_unretained``; 1120* otherwise, it is implicitly qualified with ``__autoreleasing``. 1121 1122.. admonition:: Rationale 1123 1124 ``__autoreleasing`` exists mostly for this case, the Cocoa convention for 1125 out-parameters. Since a pointer to ``const`` is obviously not an 1126 out-parameter, we instead use a type more useful for passing arrays. If the 1127 user instead intends to pass in a *mutable* array, inferring 1128 ``__autoreleasing`` is the wrong thing to do; this directs some of the 1129 caution in the following rules about writeback. 1130 1131Such a type written anywhere else would be ill-formed by the general rule 1132requiring ownership qualifiers. 1133 1134This rule does not apply in Objective-C++ if a parameter's type is dependent in 1135a template pattern and is only *instantiated* to a type which would be a 1136pointer to an unqualified retainable object pointer type. Such code is still 1137ill-formed. 1138 1139.. admonition:: Rationale 1140 1141 The convention is very unlikely to be intentional in template code. 1142 1143.. _arc.ownership.inference.template.arguments: 1144 1145Template arguments 1146^^^^^^^^^^^^^^^^^^ 1147 1148If a template argument for a template type parameter is an retainable object 1149owner type that does not have an explicit ownership qualifier, it is adjusted 1150to have ``__strong`` qualification. This adjustment occurs regardless of 1151whether the template argument was deduced or explicitly specified. 1152 1153.. admonition:: Rationale 1154 1155 ``__strong`` is a useful default for containers (e.g., ``std::vector<id>``), 1156 which would otherwise require explicit qualification. Moreover, unqualified 1157 retainable object pointer types are unlikely to be useful within templates, 1158 since they generally need to have a qualifier applied to the before being 1159 used. 1160 1161.. _arc.method-families: 1162 1163Method families 1164=============== 1165 1166An Objective-C method may fall into a :arc-term:`method family`, which is a 1167conventional set of behaviors ascribed to it by the Cocoa conventions. 1168 1169A method is in a certain method family if: 1170 1171* it has a ``objc_method_family`` attribute placing it in that family; or if 1172 not that, 1173* it does not have an ``objc_method_family`` attribute placing it in a 1174 different or no family, and 1175* its selector falls into the corresponding selector family, and 1176* its signature obeys the added restrictions of the method family. 1177 1178A selector is in a certain selector family if, ignoring any leading 1179underscores, the first component of the selector either consists entirely of 1180the name of the method family or it begins with that name followed by a 1181character other than a lowercase letter. For example, ``_perform:with:`` and 1182``performWith:`` would fall into the ``perform`` family (if we recognized one), 1183but ``performing:with`` would not. 1184 1185The families and their added restrictions are: 1186 1187* ``alloc`` methods must return a retainable object pointer type. 1188* ``copy`` methods must return a retainable object pointer type. 1189* ``mutableCopy`` methods must return a retainable object pointer type. 1190* ``new`` methods must return a retainable object pointer type. 1191* ``init`` methods must be instance methods and must return an Objective-C 1192 pointer type. Additionally, a program is ill-formed if it declares or 1193 contains a call to an ``init`` method whose return type is neither ``id`` nor 1194 a pointer to a super-class or sub-class of the declaring class (if the method 1195 was declared on a class) or the static receiver type of the call (if it was 1196 declared on a protocol). 1197 1198 .. admonition:: Rationale 1199 1200 There are a fair number of existing methods with ``init``-like selectors 1201 which nonetheless don't follow the ``init`` conventions. Typically these 1202 are either accidental naming collisions or helper methods called during 1203 initialization. Because of the peculiar retain/release behavior of 1204 ``init`` methods, it's very important not to treat these methods as 1205 ``init`` methods if they aren't meant to be. It was felt that implicitly 1206 defining these methods out of the family based on the exact relationship 1207 between the return type and the declaring class would be much too subtle 1208 and fragile. Therefore we identify a small number of legitimate-seeming 1209 return types and call everything else an error. This serves the secondary 1210 purpose of encouraging programmers not to accidentally give methods names 1211 in the ``init`` family. 1212 1213 Note that a method with an ``init``-family selector which returns a 1214 non-Objective-C type (e.g. ``void``) is perfectly well-formed; it simply 1215 isn't in the ``init`` family. 1216 1217A program is ill-formed if a method's declarations, implementations, and 1218overrides do not all have the same method family. 1219 1220.. _arc.family.attribute: 1221 1222Explicit method family control 1223------------------------------ 1224 1225A method may be annotated with the ``objc_method_family`` attribute to 1226precisely control which method family it belongs to. If a method in an 1227``@implementation`` does not have this attribute, but there is a method 1228declared in the corresponding ``@interface`` that does, then the attribute is 1229copied to the declaration in the ``@implementation``. The attribute is 1230available outside of ARC, and may be tested for with the preprocessor query 1231``__has_attribute(objc_method_family)``. 1232 1233The attribute is spelled 1234``__attribute__((objc_method_family(`` *family* ``)))``. If *family* is 1235``none``, the method has no family, even if it would otherwise be considered to 1236have one based on its selector and type. Otherwise, *family* must be one of 1237``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``, in which case the 1238method is considered to belong to the corresponding family regardless of its 1239selector. It is an error if a method that is explicitly added to a family in 1240this way does not meet the requirements of the family other than the selector 1241naming convention. 1242 1243.. admonition:: Rationale 1244 1245 The rules codified in this document describe the standard conventions of 1246 Objective-C. However, as these conventions have not heretofore been enforced 1247 by an unforgiving mechanical system, they are only imperfectly kept, 1248 especially as they haven't always even been precisely defined. While it is 1249 possible to define low-level ownership semantics with attributes like 1250 ``ns_returns_retained``, this attribute allows the user to communicate 1251 semantic intent, which is of use both to ARC (which, e.g., treats calls to 1252 ``init`` specially) and the static analyzer. 1253 1254.. _arc.family.semantics: 1255 1256Semantics of method families 1257---------------------------- 1258 1259A method's membership in a method family may imply non-standard semantics for 1260its parameters and return type. 1261 1262Methods in the ``alloc``, ``copy``, ``mutableCopy``, and ``new`` families --- 1263that is, methods in all the currently-defined families except ``init`` --- 1264implicitly :ref:`return a retained object 1265<arc.object.operands.retained-return-values>` as if they were annotated with 1266the ``ns_returns_retained`` attribute. This can be overridden by annotating 1267the method with either of the ``ns_returns_autoreleased`` or 1268``ns_returns_not_retained`` attributes. 1269 1270Properties also follow same naming rules as methods. This means that those in 1271the ``alloc``, ``copy``, ``mutableCopy``, and ``new`` families provide access 1272to :ref:`retained objects <arc.object.operands.retained-return-values>`. This 1273can be overridden by annotating the property with ``ns_returns_not_retained`` 1274attribute. 1275 1276.. _arc.family.semantics.init: 1277 1278Semantics of ``init`` 1279^^^^^^^^^^^^^^^^^^^^^ 1280 1281Methods in the ``init`` family implicitly :ref:`consume 1282<arc.objects.operands.consumed>` their ``self`` parameter and :ref:`return a 1283retained object <arc.object.operands.retained-return-values>`. Neither of 1284these properties can be altered through attributes. 1285 1286A call to an ``init`` method with a receiver that is either ``self`` (possibly 1287parenthesized or casted) or ``super`` is called a :arc-term:`delegate init 1288call`. It is an error for a delegate init call to be made except from an 1289``init`` method, and excluding blocks within such methods. 1290 1291As an exception to the :ref:`usual rule <arc.misc.self>`, the variable ``self`` 1292is mutable in an ``init`` method and has the usual semantics for a ``__strong`` 1293variable. However, it is undefined behavior and the program is ill-formed, no 1294diagnostic required, if an ``init`` method attempts to use the previous value 1295of ``self`` after the completion of a delegate init call. It is conventional, 1296but not required, for an ``init`` method to return ``self``. 1297 1298It is undefined behavior for a program to cause two or more calls to ``init`` 1299methods on the same object, except that each ``init`` method invocation may 1300perform at most one delegate init call. 1301 1302.. _arc.family.semantics.result_type: 1303 1304Related result types 1305^^^^^^^^^^^^^^^^^^^^ 1306 1307Certain methods are candidates to have :arc-term:`related result types`: 1308 1309* class methods in the ``alloc`` and ``new`` method families 1310* instance methods in the ``init`` family 1311* the instance method ``self`` 1312* outside of ARC, the instance methods ``retain`` and ``autorelease`` 1313 1314If the formal result type of such a method is ``id`` or protocol-qualified 1315``id``, or a type equal to the declaring class or a superclass, then it is said 1316to have a related result type. In this case, when invoked in an explicit 1317message send, it is assumed to return a type related to the type of the 1318receiver: 1319 1320* if it is a class method, and the receiver is a class name ``T``, the message 1321 send expression has type ``T*``; otherwise 1322* if it is an instance method, and the receiver has type ``T``, the message 1323 send expression has type ``T``; otherwise 1324* the message send expression has the normal result type of the method. 1325 1326This is a new rule of the Objective-C language and applies outside of ARC. 1327 1328.. admonition:: Rationale 1329 1330 ARC's automatic code emission is more prone than most code to signature 1331 errors, i.e. errors where a call was emitted against one method signature, 1332 but the implementing method has an incompatible signature. Having more 1333 precise type information helps drastically lower this risk, as well as 1334 catching a number of latent bugs. 1335 1336.. _arc.optimization: 1337 1338Optimization 1339============ 1340 1341Within this section, the word :arc-term:`function` will be used to 1342refer to any structured unit of code, be it a C function, an 1343Objective-C method, or a block. 1344 1345This specification describes ARC as performing specific ``retain`` and 1346``release`` operations on retainable object pointers at specific 1347points during the execution of a program. These operations make up a 1348non-contiguous subsequence of the computation history of the program. 1349The portion of this sequence for a particular retainable object 1350pointer for which a specific function execution is directly 1351responsible is the :arc-term:`formal local retain history` of the 1352object pointer. The corresponding actual sequence executed is the 1353`dynamic local retain history`. 1354 1355However, under certain circumstances, ARC is permitted to re-order and 1356eliminate operations in a manner which may alter the overall 1357computation history beyond what is permitted by the general "as if" 1358rule of C/C++ and the :ref:`restrictions <arc.objects.retains>` on 1359the implementation of ``retain`` and ``release``. 1360 1361.. admonition:: Rationale 1362 1363 Specifically, ARC is sometimes permitted to optimize ``release`` 1364 operations in ways which might cause an object to be deallocated 1365 before it would otherwise be. Without this, it would be almost 1366 impossible to eliminate any ``retain``/``release`` pairs. For 1367 example, consider the following code: 1368 1369 .. code-block:: objc 1370 1371 id x = _ivar; 1372 [x foo]; 1373 1374 If we were not permitted in any event to shorten the lifetime of the 1375 object in ``x``, then we would not be able to eliminate this retain 1376 and release unless we could prove that the message send could not 1377 modify ``_ivar`` (or deallocate ``self``). Since message sends are 1378 opaque to the optimizer, this is not possible, and so ARC's hands 1379 would be almost completely tied. 1380 1381ARC makes no guarantees about the execution of a computation history 1382which contains undefined behavior. In particular, ARC makes no 1383guarantees in the presence of race conditions. 1384 1385ARC may assume that any retainable object pointers it receives or 1386generates are instantaneously valid from that point until a point 1387which, by the concurrency model of the host language, happens-after 1388the generation of the pointer and happens-before a release of that 1389object (possibly via an aliasing pointer or indirectly due to 1390destruction of a different object). 1391 1392.. admonition:: Rationale 1393 1394 There is very little point in trying to guarantee correctness in the 1395 presence of race conditions. ARC does not have a stack-scanning 1396 garbage collector, and guaranteeing the atomicity of every load and 1397 store operation would be prohibitive and preclude a vast amount of 1398 optimization. 1399 1400ARC may assume that non-ARC code engages in sensible balancing 1401behavior and does not rely on exact or minimum retain count values 1402except as guaranteed by ``__strong`` object invariants or +1 transfer 1403conventions. For example, if an object is provably double-retained 1404and double-released, ARC may eliminate the inner retain and release; 1405it does not need to guard against code which performs an unbalanced 1406release followed by a "balancing" retain. 1407 1408.. _arc.optimization.liveness: 1409 1410Object liveness 1411--------------- 1412 1413ARC may not allow a retainable object ``X`` to be deallocated at a 1414time ``T`` in a computation history if: 1415 1416* ``X`` is the value stored in a ``__strong`` object ``S`` with 1417 :ref:`precise lifetime semantics <arc.optimization.precise>`, or 1418 1419* ``X`` is the value stored in a ``__strong`` object ``S`` with 1420 imprecise lifetime semantics and, at some point after ``T`` but 1421 before the next store to ``S``, the computation history features a 1422 load from ``S`` and in some way depends on the value loaded, or 1423 1424* ``X`` is a value described as being released at the end of the 1425 current full-expression and, at some point after ``T`` but before 1426 the end of the full-expression, the computation history depends 1427 on that value. 1428 1429.. admonition:: Rationale 1430 1431 The intent of the second rule is to say that objects held in normal 1432 ``__strong`` local variables may be released as soon as the value in 1433 the variable is no longer being used: either the variable stops 1434 being used completely or a new value is stored in the variable. 1435 1436 The intent of the third rule is to say that return values may be 1437 released after they've been used. 1438 1439A computation history depends on a pointer value ``P`` if it: 1440 1441* performs a pointer comparison with ``P``, 1442* loads from ``P``, 1443* stores to ``P``, 1444* depends on a pointer value ``Q`` derived via pointer arithmetic 1445 from ``P`` (including an instance-variable or field access), or 1446* depends on a pointer value ``Q`` loaded from ``P``. 1447 1448Dependency applies only to values derived directly or indirectly from 1449a particular expression result and does not occur merely because a 1450separate pointer value dynamically aliases ``P``. Furthermore, this 1451dependency is not carried by values that are stored to objects. 1452 1453.. admonition:: Rationale 1454 1455 The restrictions on dependency are intended to make this analysis 1456 feasible by an optimizer with only incomplete information about a 1457 program. Essentially, dependence is carried to "obvious" uses of a 1458 pointer. Merely passing a pointer argument to a function does not 1459 itself cause dependence, but since generally the optimizer will not 1460 be able to prove that the function doesn't depend on that parameter, 1461 it will be forced to conservatively assume it does. 1462 1463 Dependency propagates to values loaded from a pointer because those 1464 values might be invalidated by deallocating the object. For 1465 example, given the code ``__strong id x = p->ivar;``, ARC must not 1466 move the release of ``p`` to between the load of ``p->ivar`` and the 1467 retain of that value for storing into ``x``. 1468 1469 Dependency does not propagate through stores of dependent pointer 1470 values because doing so would allow dependency to outlive the 1471 full-expression which produced the original value. For example, the 1472 address of an instance variable could be written to some global 1473 location and then freely accessed during the lifetime of the local, 1474 or a function could return an inner pointer of an object and store 1475 it to a local. These cases would be potentially impossible to 1476 reason about and so would basically prevent any optimizations based 1477 on imprecise lifetime. There are also uncommon enough to make it 1478 reasonable to require the precise-lifetime annotation if someone 1479 really wants to rely on them. 1480 1481 Dependency does propagate through return values of pointer type. 1482 The compelling source of need for this rule is a property accessor 1483 which returns an un-autoreleased result; the calling function must 1484 have the chance to operate on the value, e.g. to retain it, before 1485 ARC releases the original pointer. Note again, however, that 1486 dependence does not survive a store, so ARC does not guarantee the 1487 continued validity of the return value past the end of the 1488 full-expression. 1489 1490.. _arc.optimization.object_lifetime: 1491 1492No object lifetime extension 1493---------------------------- 1494 1495If, in the formal computation history of the program, an object ``X`` 1496has been deallocated by the time of an observable side-effect, then 1497ARC must cause ``X`` to be deallocated by no later than the occurrence 1498of that side-effect, except as influenced by the re-ordering of the 1499destruction of objects. 1500 1501.. admonition:: Rationale 1502 1503 This rule is intended to prohibit ARC from observably extending the 1504 lifetime of a retainable object, other than as specified in this 1505 document. Together with the rule limiting the transformation of 1506 releases, this rule requires ARC to eliminate retains and release 1507 only in pairs. 1508 1509 ARC's power to reorder the destruction of objects is critical to its 1510 ability to do any optimization, for essentially the same reason that 1511 it must retain the power to decrease the lifetime of an object. 1512 Unfortunately, while it's generally poor style for the destruction 1513 of objects to have arbitrary side-effects, it's certainly possible. 1514 Hence the caveat. 1515 1516.. _arc.optimization.precise: 1517 1518Precise lifetime semantics 1519-------------------------- 1520 1521In general, ARC maintains an invariant that a retainable object pointer held in 1522a ``__strong`` object will be retained for the full formal lifetime of the 1523object. Objects subject to this invariant have :arc-term:`precise lifetime 1524semantics`. 1525 1526By default, local variables of automatic storage duration do not have precise 1527lifetime semantics. Such objects are simply strong references which hold 1528values of retainable object pointer type, and these values are still fully 1529subject to the optimizations on values under local control. 1530 1531.. admonition:: Rationale 1532 1533 Applying these precise-lifetime semantics strictly would be prohibitive. 1534 Many useful optimizations that might theoretically decrease the lifetime of 1535 an object would be rendered impossible. Essentially, it promises too much. 1536 1537A local variable of retainable object owner type and automatic storage duration 1538may be annotated with the ``objc_precise_lifetime`` attribute to indicate that 1539it should be considered to be an object with precise lifetime semantics. 1540 1541.. admonition:: Rationale 1542 1543 Nonetheless, it is sometimes useful to be able to force an object to be 1544 released at a precise time, even if that object does not appear to be used. 1545 This is likely to be uncommon enough that the syntactic weight of explicitly 1546 requesting these semantics will not be burdensome, and may even make the code 1547 clearer. 1548 1549.. _arc.misc: 1550 1551Miscellaneous 1552============= 1553 1554.. _arc.misc.special_methods: 1555 1556Special methods 1557--------------- 1558 1559.. _arc.misc.special_methods.retain: 1560 1561Memory management methods 1562^^^^^^^^^^^^^^^^^^^^^^^^^ 1563 1564A program is ill-formed if it contains a method definition, message send, or 1565``@selector`` expression for any of the following selectors: 1566 1567* ``autorelease`` 1568* ``release`` 1569* ``retain`` 1570* ``retainCount`` 1571 1572.. admonition:: Rationale 1573 1574 ``retainCount`` is banned because ARC robs it of consistent semantics. The 1575 others were banned after weighing three options for how to deal with message 1576 sends: 1577 1578 **Honoring** them would work out very poorly if a programmer naively or 1579 accidentally tried to incorporate code written for manual retain/release code 1580 into an ARC program. At best, such code would do twice as much work as 1581 necessary; quite frequently, however, ARC and the explicit code would both 1582 try to balance the same retain, leading to crashes. The cost is losing the 1583 ability to perform "unrooted" retains, i.e. retains not logically 1584 corresponding to a strong reference in the object graph. 1585 1586 **Ignoring** them would badly violate user expectations about their code. 1587 While it *would* make it easier to develop code simultaneously for ARC and 1588 non-ARC, there is very little reason to do so except for certain library 1589 developers. ARC and non-ARC translation units share an execution model and 1590 can seamlessly interoperate. Within a translation unit, a developer who 1591 faithfully maintains their code in non-ARC mode is suffering all the 1592 restrictions of ARC for zero benefit, while a developer who isn't testing the 1593 non-ARC mode is likely to be unpleasantly surprised if they try to go back to 1594 it. 1595 1596 **Banning** them has the disadvantage of making it very awkward to migrate 1597 existing code to ARC. The best answer to that, given a number of other 1598 changes and restrictions in ARC, is to provide a specialized tool to assist 1599 users in that migration. 1600 1601 Implementing these methods was banned because they are too integral to the 1602 semantics of ARC; many tricks which worked tolerably under manual reference 1603 counting will misbehave if ARC performs an ephemeral extra retain or two. If 1604 absolutely required, it is still possible to implement them in non-ARC code, 1605 for example in a category; the implementations must obey the :ref:`semantics 1606 <arc.objects.retains>` laid out elsewhere in this document. 1607 1608.. _arc.misc.special_methods.dealloc: 1609 1610``dealloc`` 1611^^^^^^^^^^^ 1612 1613A program is ill-formed if it contains a message send or ``@selector`` 1614expression for the selector ``dealloc``. 1615 1616.. admonition:: Rationale 1617 1618 There are no legitimate reasons to call ``dealloc`` directly. 1619 1620A class may provide a method definition for an instance method named 1621``dealloc``. This method will be called after the final ``release`` of the 1622object but before it is deallocated or any of its instance variables are 1623destroyed. The superclass's implementation of ``dealloc`` will be called 1624automatically when the method returns. 1625 1626.. admonition:: Rationale 1627 1628 Even though ARC destroys instance variables automatically, there are still 1629 legitimate reasons to write a ``dealloc`` method, such as freeing 1630 non-retainable resources. Failing to call ``[super dealloc]`` in such a 1631 method is nearly always a bug. Sometimes, the object is simply trying to 1632 prevent itself from being destroyed, but ``dealloc`` is really far too late 1633 for the object to be raising such objections. Somewhat more legitimately, an 1634 object may have been pool-allocated and should not be deallocated with 1635 ``free``; for now, this can only be supported with a ``dealloc`` 1636 implementation outside of ARC. Such an implementation must be very careful 1637 to do all the other work that ``NSObject``'s ``dealloc`` would, which is 1638 outside the scope of this document to describe. 1639 1640The instance variables for an ARC-compiled class will be destroyed at some 1641point after control enters the ``dealloc`` method for the root class of the 1642class. The ordering of the destruction of instance variables is unspecified, 1643both within a single class and between subclasses and superclasses. 1644 1645.. admonition:: Rationale 1646 1647 The traditional, non-ARC pattern for destroying instance variables is to 1648 destroy them immediately before calling ``[super dealloc]``. Unfortunately, 1649 message sends from the superclass are quite capable of reaching methods in 1650 the subclass, and those methods may well read or write to those instance 1651 variables. Making such message sends from dealloc is generally discouraged, 1652 since the subclass may well rely on other invariants that were broken during 1653 ``dealloc``, but it's not so inescapably dangerous that we felt comfortable 1654 calling it undefined behavior. Therefore we chose to delay destroying the 1655 instance variables to a point at which message sends are clearly disallowed: 1656 the point at which the root class's deallocation routines take over. 1657 1658 In most code, the difference is not observable. It can, however, be observed 1659 if an instance variable holds a strong reference to an object whose 1660 deallocation will trigger a side-effect which must be carefully ordered with 1661 respect to the destruction of the super class. Such code violates the design 1662 principle that semantically important behavior should be explicit. A simple 1663 fix is to clear the instance variable manually during ``dealloc``; a more 1664 holistic solution is to move semantically important side-effects out of 1665 ``dealloc`` and into a separate teardown phase which can rely on working with 1666 well-formed objects. 1667 1668.. _arc.misc.autoreleasepool: 1669 1670``@autoreleasepool`` 1671-------------------- 1672 1673To simplify the use of autorelease pools, and to bring them under the control 1674of the compiler, a new kind of statement is available in Objective-C. It is 1675written ``@autoreleasepool`` followed by a *compound-statement*, i.e. by a new 1676scope delimited by curly braces. Upon entry to this block, the current state 1677of the autorelease pool is captured. When the block is exited normally, 1678whether by fallthrough or directed control flow (such as ``return`` or 1679``break``), the autorelease pool is restored to the saved state, releasing all 1680the objects in it. When the block is exited with an exception, the pool is not 1681drained. 1682 1683``@autoreleasepool`` may be used in non-ARC translation units, with equivalent 1684semantics. 1685 1686A program is ill-formed if it refers to the ``NSAutoreleasePool`` class. 1687 1688.. admonition:: Rationale 1689 1690 Autorelease pools are clearly important for the compiler to reason about, but 1691 it is far too much to expect the compiler to accurately reason about control 1692 dependencies between two calls. It is also very easy to accidentally forget 1693 to drain an autorelease pool when using the manual API, and this can 1694 significantly inflate the process's high-water-mark. The introduction of a 1695 new scope is unfortunate but basically required for sane interaction with the 1696 rest of the language. Not draining the pool during an unwind is apparently 1697 required by the Objective-C exceptions implementation. 1698 1699.. _arc.misc.self: 1700 1701``self`` 1702-------- 1703 1704The ``self`` parameter variable of an Objective-C method is never actually 1705retained by the implementation. It is undefined behavior, or at least 1706dangerous, to cause an object to be deallocated during a message send to that 1707object. 1708 1709To make this safe, for Objective-C instance methods ``self`` is implicitly 1710``const`` unless the method is in the :ref:`init family 1711<arc.family.semantics.init>`. Further, ``self`` is **always** implicitly 1712``const`` within a class method. 1713 1714.. admonition:: Rationale 1715 1716 The cost of retaining ``self`` in all methods was found to be prohibitive, as 1717 it tends to be live across calls, preventing the optimizer from proving that 1718 the retain and release are unnecessary --- for good reason, as it's quite 1719 possible in theory to cause an object to be deallocated during its execution 1720 without this retain and release. Since it's extremely uncommon to actually 1721 do so, even unintentionally, and since there's no natural way for the 1722 programmer to remove this retain/release pair otherwise (as there is for 1723 other parameters by, say, making the variable ``__unsafe_unretained``), we 1724 chose to make this optimizing assumption and shift some amount of risk to the 1725 user. 1726 1727.. _arc.misc.enumeration: 1728 1729Fast enumeration iteration variables 1730------------------------------------ 1731 1732If a variable is declared in the condition of an Objective-C fast enumeration 1733loop, and the variable has no explicit ownership qualifier, then it is 1734qualified with ``const __strong`` and objects encountered during the 1735enumeration are not actually retained. 1736 1737.. admonition:: Rationale 1738 1739 This is an optimization made possible because fast enumeration loops promise 1740 to keep the objects retained during enumeration, and the collection itself 1741 cannot be synchronously modified. It can be overridden by explicitly 1742 qualifying the variable with ``__strong``, which will make the variable 1743 mutable again and cause the loop to retain the objects it encounters. 1744 1745.. _arc.misc.blocks: 1746 1747Blocks 1748------ 1749 1750The implicit ``const`` capture variables created when evaluating a block 1751literal expression have the same ownership semantics as the local variables 1752they capture. The capture is performed by reading from the captured variable 1753and initializing the capture variable with that value; the capture variable is 1754destroyed when the block literal is, i.e. at the end of the enclosing scope. 1755 1756The :ref:`inference <arc.ownership.inference>` rules apply equally to 1757``__block`` variables, which is a shift in semantics from non-ARC, where 1758``__block`` variables did not implicitly retain during capture. 1759 1760``__block`` variables of retainable object owner type are moved off the stack 1761by initializing the heap copy with the result of moving from the stack copy. 1762 1763With the exception of retains done as part of initializing a ``__strong`` 1764parameter variable or reading a ``__weak`` variable, whenever these semantics 1765call for retaining a value of block-pointer type, it has the effect of a 1766``Block_copy``. The optimizer may remove such copies when it sees that the 1767result is used only as an argument to a call. 1768 1769.. _arc.misc.exceptions: 1770 1771Exceptions 1772---------- 1773 1774By default in Objective C, ARC is not exception-safe for normal releases: 1775 1776* It does not end the lifetime of ``__strong`` variables when their scopes are 1777 abnormally terminated by an exception. 1778* It does not perform releases which would occur at the end of a 1779 full-expression if that full-expression throws an exception. 1780 1781A program may be compiled with the option ``-fobjc-arc-exceptions`` in order to 1782enable these, or with the option ``-fno-objc-arc-exceptions`` to explicitly 1783disable them, with the last such argument "winning". 1784 1785.. admonition:: Rationale 1786 1787 The standard Cocoa convention is that exceptions signal programmer error and 1788 are not intended to be recovered from. Making code exceptions-safe by 1789 default would impose severe runtime and code size penalties on code that 1790 typically does not actually care about exceptions safety. Therefore, 1791 ARC-generated code leaks by default on exceptions, which is just fine if the 1792 process is going to be immediately terminated anyway. Programs which do care 1793 about recovering from exceptions should enable the option. 1794 1795In Objective-C++, ``-fobjc-arc-exceptions`` is enabled by default. 1796 1797.. admonition:: Rationale 1798 1799 C++ already introduces pervasive exceptions-cleanup code of the sort that ARC 1800 introduces. C++ programmers who have not already disabled exceptions are 1801 much more likely to actual require exception-safety. 1802 1803ARC does end the lifetimes of ``__weak`` objects when an exception terminates 1804their scope unless exceptions are disabled in the compiler. 1805 1806.. admonition:: Rationale 1807 1808 The consequence of a local ``__weak`` object not being destroyed is very 1809 likely to be corruption of the Objective-C runtime, so we want to be safer 1810 here. Of course, potentially massive leaks are about as likely to take down 1811 the process as this corruption is if the program does try to recover from 1812 exceptions. 1813 1814.. _arc.misc.interior: 1815 1816Interior pointers 1817----------------- 1818 1819An Objective-C method returning a non-retainable pointer may be annotated with 1820the ``objc_returns_inner_pointer`` attribute to indicate that it returns a 1821handle to the internal data of an object, and that this reference will be 1822invalidated if the object is destroyed. When such a message is sent to an 1823object, the object's lifetime will be extended until at least the earliest of: 1824 1825* the last use of the returned pointer, or any pointer derived from it, in the 1826 calling function or 1827* the autorelease pool is restored to a previous state. 1828 1829.. admonition:: Rationale 1830 1831 Rationale: not all memory and resources are managed with reference counts; it 1832 is common for objects to manage private resources in their own, private way. 1833 Typically these resources are completely encapsulated within the object, but 1834 some classes offer their users direct access for efficiency. If ARC is not 1835 aware of methods that return such "interior" pointers, its optimizations can 1836 cause the owning object to be reclaimed too soon. This attribute informs ARC 1837 that it must tread lightly. 1838 1839 The extension rules are somewhat intentionally vague. The autorelease pool 1840 limit is there to permit a simple implementation to simply retain and 1841 autorelease the receiver. The other limit permits some amount of 1842 optimization. The phrase "derived from" is intended to encompass the results 1843 both of pointer transformations, such as casts and arithmetic, and of loading 1844 from such derived pointers; furthermore, it applies whether or not such 1845 derivations are applied directly in the calling code or by other utility code 1846 (for example, the C library routine ``strchr``). However, the implementation 1847 never need account for uses after a return from the code which calls the 1848 method returning an interior pointer. 1849 1850As an exception, no extension is required if the receiver is loaded directly 1851from a ``__strong`` object with :ref:`precise lifetime semantics 1852<arc.optimization.precise>`. 1853 1854.. admonition:: Rationale 1855 1856 Implicit autoreleases carry the risk of significantly inflating memory use, 1857 so it's important to provide users a way of avoiding these autoreleases. 1858 Tying this to precise lifetime semantics is ideal, as for local variables 1859 this requires a very explicit annotation, which allows ARC to trust the user 1860 with good cheer. 1861 1862.. _arc.misc.c-retainable: 1863 1864C retainable pointer types 1865-------------------------- 1866 1867A type is a :arc-term:`C retainable pointer type` if it is a pointer to 1868(possibly qualified) ``void`` or a pointer to a (possibly qualifier) ``struct`` 1869or ``class`` type. 1870 1871.. admonition:: Rationale 1872 1873 ARC does not manage pointers of CoreFoundation type (or any of the related 1874 families of retainable C pointers which interoperate with Objective-C for 1875 retain/release operation). In fact, ARC does not even know how to 1876 distinguish these types from arbitrary C pointer types. The intent of this 1877 concept is to filter out some obviously non-object types while leaving a hook 1878 for later tightening if a means of exhaustively marking CF types is made 1879 available. 1880 1881.. _arc.misc.c-retainable.audit: 1882 1883Auditing of C retainable pointer interfaces 1884^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1885 1886:when-revised:`[beginning Apple 4.0, LLVM 3.1]` 1887 1888A C function may be marked with the ``cf_audited_transfer`` attribute to 1889express that, except as otherwise marked with attributes, it obeys the 1890parameter (consuming vs. non-consuming) and return (retained vs. non-retained) 1891conventions for a C function of its name, namely: 1892 1893* A parameter of C retainable pointer type is assumed to not be consumed 1894 unless it is marked with the ``cf_consumed`` attribute, and 1895* A result of C retainable pointer type is assumed to not be returned retained 1896 unless the function is either marked ``cf_returns_retained`` or it follows 1897 the create/copy naming convention and is not marked 1898 ``cf_returns_not_retained``. 1899 1900A function obeys the :arc-term:`create/copy` naming convention if its name 1901contains as a substring: 1902 1903* either "Create" or "Copy" not followed by a lowercase letter, or 1904* either "create" or "copy" not followed by a lowercase letter and 1905 not preceded by any letter, whether uppercase or lowercase. 1906 1907A second attribute, ``cf_unknown_transfer``, signifies that a function's 1908transfer semantics cannot be accurately captured using any of these 1909annotations. A program is ill-formed if it annotates the same function with 1910both ``cf_audited_transfer`` and ``cf_unknown_transfer``. 1911 1912A pragma is provided to facilitate the mass annotation of interfaces: 1913 1914.. code-block:: objc 1915 1916 #pragma clang arc_cf_code_audited begin 1917 ... 1918 #pragma clang arc_cf_code_audited end 1919 1920All C functions declared within the extent of this pragma are treated as if 1921annotated with the ``cf_audited_transfer`` attribute unless they otherwise have 1922the ``cf_unknown_transfer`` attribute. The pragma is accepted in all language 1923modes. A program is ill-formed if it attempts to change files, whether by 1924including a file or ending the current file, within the extent of this pragma. 1925 1926It is possible to test for all the features in this section with 1927``__has_feature(arc_cf_code_audited)``. 1928 1929.. admonition:: Rationale 1930 1931 A significant inconvenience in ARC programming is the necessity of 1932 interacting with APIs based around C retainable pointers. These features are 1933 designed to make it relatively easy for API authors to quickly review and 1934 annotate their interfaces, in turn improving the fidelity of tools such as 1935 the static analyzer and ARC. The single-file restriction on the pragma is 1936 designed to eliminate the risk of accidentally annotating some other header's 1937 interfaces. 1938 1939.. _arc.runtime: 1940 1941Runtime support 1942=============== 1943 1944This section describes the interaction between the ARC runtime and the code 1945generated by the ARC compiler. This is not part of the ARC language 1946specification; instead, it is effectively a language-specific ABI supplement, 1947akin to the "Itanium" generic ABI for C++. 1948 1949Ownership qualification does not alter the storage requirements for objects, 1950except that it is undefined behavior if a ``__weak`` object is inadequately 1951aligned for an object of type ``id``. The other qualifiers may be used on 1952explicitly under-aligned memory. 1953 1954The runtime tracks ``__weak`` objects which holds non-null values. It is 1955undefined behavior to direct modify a ``__weak`` object which is being tracked 1956by the runtime except through an 1957:ref:`objc_storeWeak <arc.runtime.objc_storeWeak>`, 1958:ref:`objc_destroyWeak <arc.runtime.objc_destroyWeak>`, or 1959:ref:`objc_moveWeak <arc.runtime.objc_moveWeak>` call. 1960 1961The runtime must provide a number of new entrypoints which the compiler may 1962emit, which are described in the remainder of this section. 1963 1964.. admonition:: Rationale 1965 1966 Several of these functions are semantically equivalent to a message send; we 1967 emit calls to C functions instead because: 1968 1969 * the machine code to do so is significantly smaller, 1970 * it is much easier to recognize the C functions in the ARC optimizer, and 1971 * a sufficient sophisticated runtime may be able to avoid the message send in 1972 common cases. 1973 1974 Several other of these functions are "fused" operations which can be 1975 described entirely in terms of other operations. We use the fused operations 1976 primarily as a code-size optimization, although in some cases there is also a 1977 real potential for avoiding redundant operations in the runtime. 1978 1979.. _arc.runtime.objc_autorelease: 1980 1981``id objc_autorelease(id value);`` 1982---------------------------------- 1983 1984*Precondition:* ``value`` is null or a pointer to a valid object. 1985 1986If ``value`` is null, this call has no effect. Otherwise, it adds the object 1987to the innermost autorelease pool exactly as if the object had been sent the 1988``autorelease`` message. 1989 1990Always returns ``value``. 1991 1992.. _arc.runtime.objc_autoreleasePoolPop: 1993 1994``void objc_autoreleasePoolPop(void *pool);`` 1995--------------------------------------------- 1996 1997*Precondition:* ``pool`` is the result of a previous call to 1998:ref:`objc_autoreleasePoolPush <arc.runtime.objc_autoreleasePoolPush>` on the 1999current thread, where neither ``pool`` nor any enclosing pool have previously 2000been popped. 2001 2002Releases all the objects added to the given autorelease pool and any 2003autorelease pools it encloses, then sets the current autorelease pool to the 2004pool directly enclosing ``pool``. 2005 2006.. _arc.runtime.objc_autoreleasePoolPush: 2007 2008``void *objc_autoreleasePoolPush(void);`` 2009----------------------------------------- 2010 2011Creates a new autorelease pool that is enclosed by the current pool, makes that 2012the current pool, and returns an opaque "handle" to it. 2013 2014.. admonition:: Rationale 2015 2016 While the interface is described as an explicit hierarchy of pools, the rules 2017 allow the implementation to just keep a stack of objects, using the stack 2018 depth as the opaque pool handle. 2019 2020.. _arc.runtime.objc_autoreleaseReturnValue: 2021 2022``id objc_autoreleaseReturnValue(id value);`` 2023--------------------------------------------- 2024 2025*Precondition:* ``value`` is null or a pointer to a valid object. 2026 2027If ``value`` is null, this call has no effect. Otherwise, it makes a best 2028effort to hand off ownership of a retain count on the object to a call to 2029:ref:`objc_retainAutoreleasedReturnValue 2030<arc.runtime.objc_retainAutoreleasedReturnValue>` for the same object in an 2031enclosing call frame. If this is not possible, the object is autoreleased as 2032above. 2033 2034Always returns ``value``. 2035 2036.. _arc.runtime.objc_copyWeak: 2037 2038``void objc_copyWeak(id *dest, id *src);`` 2039------------------------------------------ 2040 2041*Precondition:* ``src`` is a valid pointer which either contains a null pointer 2042or has been registered as a ``__weak`` object. ``dest`` is a valid pointer 2043which has not been registered as a ``__weak`` object. 2044 2045``dest`` is initialized to be equivalent to ``src``, potentially registering it 2046with the runtime. Equivalent to the following code: 2047 2048.. code-block:: objc 2049 2050 void objc_copyWeak(id *dest, id *src) { 2051 objc_release(objc_initWeak(dest, objc_loadWeakRetained(src))); 2052 } 2053 2054Must be atomic with respect to calls to ``objc_storeWeak`` on ``src``. 2055 2056.. _arc.runtime.objc_destroyWeak: 2057 2058``void objc_destroyWeak(id *object);`` 2059-------------------------------------- 2060 2061*Precondition:* ``object`` is a valid pointer which either contains a null 2062pointer or has been registered as a ``__weak`` object. 2063 2064``object`` is unregistered as a weak object, if it ever was. The current value 2065of ``object`` is left unspecified; otherwise, equivalent to the following code: 2066 2067.. code-block:: objc 2068 2069 void objc_destroyWeak(id *object) { 2070 objc_storeWeak(object, nil); 2071 } 2072 2073Does not need to be atomic with respect to calls to ``objc_storeWeak`` on 2074``object``. 2075 2076.. _arc.runtime.objc_initWeak: 2077 2078``id objc_initWeak(id *object, id value);`` 2079------------------------------------------- 2080 2081*Precondition:* ``object`` is a valid pointer which has not been registered as 2082a ``__weak`` object. ``value`` is null or a pointer to a valid object. 2083 2084If ``value`` is a null pointer or the object to which it points has begun 2085deallocation, ``object`` is zero-initialized. Otherwise, ``object`` is 2086registered as a ``__weak`` object pointing to ``value``. Equivalent to the 2087following code: 2088 2089.. code-block:: objc 2090 2091 id objc_initWeak(id *object, id value) { 2092 *object = nil; 2093 return objc_storeWeak(object, value); 2094 } 2095 2096Returns the value of ``object`` after the call. 2097 2098Does not need to be atomic with respect to calls to ``objc_storeWeak`` on 2099``object``. 2100 2101.. _arc.runtime.objc_loadWeak: 2102 2103``id objc_loadWeak(id *object);`` 2104--------------------------------- 2105 2106*Precondition:* ``object`` is a valid pointer which either contains a null 2107pointer or has been registered as a ``__weak`` object. 2108 2109If ``object`` is registered as a ``__weak`` object, and the last value stored 2110into ``object`` has not yet been deallocated or begun deallocation, retains and 2111autoreleases that value and returns it. Otherwise returns null. Equivalent to 2112the following code: 2113 2114.. code-block:: objc 2115 2116 id objc_loadWeak(id *object) { 2117 return objc_autorelease(objc_loadWeakRetained(object)); 2118 } 2119 2120Must be atomic with respect to calls to ``objc_storeWeak`` on ``object``. 2121 2122.. admonition:: Rationale 2123 2124 Loading weak references would be inherently prone to race conditions without 2125 the retain. 2126 2127.. _arc.runtime.objc_loadWeakRetained: 2128 2129``id objc_loadWeakRetained(id *object);`` 2130----------------------------------------- 2131 2132*Precondition:* ``object`` is a valid pointer which either contains a null 2133pointer or has been registered as a ``__weak`` object. 2134 2135If ``object`` is registered as a ``__weak`` object, and the last value stored 2136into ``object`` has not yet been deallocated or begun deallocation, retains 2137that value and returns it. Otherwise returns null. 2138 2139Must be atomic with respect to calls to ``objc_storeWeak`` on ``object``. 2140 2141.. _arc.runtime.objc_moveWeak: 2142 2143``void objc_moveWeak(id *dest, id *src);`` 2144------------------------------------------ 2145 2146*Precondition:* ``src`` is a valid pointer which either contains a null pointer 2147or has been registered as a ``__weak`` object. ``dest`` is a valid pointer 2148which has not been registered as a ``__weak`` object. 2149 2150``dest`` is initialized to be equivalent to ``src``, potentially registering it 2151with the runtime. ``src`` may then be left in its original state, in which 2152case this call is equivalent to :ref:`objc_copyWeak 2153<arc.runtime.objc_copyWeak>`, or it may be left as null. 2154 2155Must be atomic with respect to calls to ``objc_storeWeak`` on ``src``. 2156 2157.. _arc.runtime.objc_release: 2158 2159``void objc_release(id value);`` 2160-------------------------------- 2161 2162*Precondition:* ``value`` is null or a pointer to a valid object. 2163 2164If ``value`` is null, this call has no effect. Otherwise, it performs a 2165release operation exactly as if the object had been sent the ``release`` 2166message. 2167 2168.. _arc.runtime.objc_retain: 2169 2170``id objc_retain(id value);`` 2171----------------------------- 2172 2173*Precondition:* ``value`` is null or a pointer to a valid object. 2174 2175If ``value`` is null, this call has no effect. Otherwise, it performs a retain 2176operation exactly as if the object had been sent the ``retain`` message. 2177 2178Always returns ``value``. 2179 2180.. _arc.runtime.objc_retainAutorelease: 2181 2182``id objc_retainAutorelease(id value);`` 2183---------------------------------------- 2184 2185*Precondition:* ``value`` is null or a pointer to a valid object. 2186 2187If ``value`` is null, this call has no effect. Otherwise, it performs a retain 2188operation followed by an autorelease operation. Equivalent to the following 2189code: 2190 2191.. code-block:: objc 2192 2193 id objc_retainAutorelease(id value) { 2194 return objc_autorelease(objc_retain(value)); 2195 } 2196 2197Always returns ``value``. 2198 2199.. _arc.runtime.objc_retainAutoreleaseReturnValue: 2200 2201``id objc_retainAutoreleaseReturnValue(id value);`` 2202--------------------------------------------------- 2203 2204*Precondition:* ``value`` is null or a pointer to a valid object. 2205 2206If ``value`` is null, this call has no effect. Otherwise, it performs a retain 2207operation followed by the operation described in 2208:ref:`objc_autoreleaseReturnValue <arc.runtime.objc_autoreleaseReturnValue>`. 2209Equivalent to the following code: 2210 2211.. code-block:: objc 2212 2213 id objc_retainAutoreleaseReturnValue(id value) { 2214 return objc_autoreleaseReturnValue(objc_retain(value)); 2215 } 2216 2217Always returns ``value``. 2218 2219.. _arc.runtime.objc_retainAutoreleasedReturnValue: 2220 2221``id objc_retainAutoreleasedReturnValue(id value);`` 2222---------------------------------------------------- 2223 2224*Precondition:* ``value`` is null or a pointer to a valid object. 2225 2226If ``value`` is null, this call has no effect. Otherwise, it attempts to 2227accept a hand off of a retain count from a call to 2228:ref:`objc_autoreleaseReturnValue <arc.runtime.objc_autoreleaseReturnValue>` on 2229``value`` in a recently-called function or something it calls. If that fails, 2230it performs a retain operation exactly like :ref:`objc_retain 2231<arc.runtime.objc_retain>`. 2232 2233Always returns ``value``. 2234 2235.. _arc.runtime.objc_retainBlock: 2236 2237``id objc_retainBlock(id value);`` 2238---------------------------------- 2239 2240*Precondition:* ``value`` is null or a pointer to a valid block object. 2241 2242If ``value`` is null, this call has no effect. Otherwise, if the block pointed 2243to by ``value`` is still on the stack, it is copied to the heap and the address 2244of the copy is returned. Otherwise a retain operation is performed on the 2245block exactly as if it had been sent the ``retain`` message. 2246 2247.. _arc.runtime.objc_storeStrong: 2248 2249``id objc_storeStrong(id *object, id value);`` 2250---------------------------------------------- 2251 2252*Precondition:* ``object`` is a valid pointer to a ``__strong`` object which is 2253adequately aligned for a pointer. ``value`` is null or a pointer to a valid 2254object. 2255 2256Performs the complete sequence for assigning to a ``__strong`` object of 2257non-block type [*]_. Equivalent to the following code: 2258 2259.. code-block:: objc 2260 2261 id objc_storeStrong(id *object, id value) { 2262 value = [value retain]; 2263 id oldValue = *object; 2264 *object = value; 2265 [oldValue release]; 2266 return value; 2267 } 2268 2269Always returns ``value``. 2270 2271.. [*] This does not imply that a ``__strong`` object of block type is an 2272 invalid argument to this function. Rather it implies that an ``objc_retain`` 2273 and not an ``objc_retainBlock`` operation will be emitted if the argument is 2274 a block. 2275 2276.. _arc.runtime.objc_storeWeak: 2277 2278``id objc_storeWeak(id *object, id value);`` 2279-------------------------------------------- 2280 2281*Precondition:* ``object`` is a valid pointer which either contains a null 2282pointer or has been registered as a ``__weak`` object. ``value`` is null or a 2283pointer to a valid object. 2284 2285If ``value`` is a null pointer or the object to which it points has begun 2286deallocation, ``object`` is assigned null and unregistered as a ``__weak`` 2287object. Otherwise, ``object`` is registered as a ``__weak`` object or has its 2288registration updated to point to ``value``. 2289 2290Returns the value of ``object`` after the call. 2291 2292