Mercurial > hg > CbC > CbC_gcc
annotate gcc/doc/objc.texi @ 131:84e7813d76e9
gcc-8.2
| author | mir3636 |
|---|---|
| date | Thu, 25 Oct 2018 07:37:49 +0900 |
| parents | 04ced10e8804 |
| children | 1830386684a0 |
| rev | line source |
|---|---|
| 131 | 1 @c Copyright (C) 1988-2018 Free Software Foundation, Inc. |
| 0 | 2 @c This is part of the GCC manual. |
| 3 @c For copying conditions, see the file gcc.texi. | |
| 4 | |
| 5 @node Objective-C | |
| 6 @comment node-name, next, previous, up | |
| 7 | |
| 111 | 8 @chapter GNU Objective-C Features |
| 0 | 9 |
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10 This document is meant to describe some of the GNU Objective-C |
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11 features. It is not intended to teach you Objective-C. There are |
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12 several resources on the Internet that present the language. |
| 0 | 13 |
| 14 @menu | |
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15 * GNU Objective-C runtime API:: |
| 0 | 16 * Executing code before main:: |
| 17 * Type encoding:: | |
| 18 * Garbage Collection:: | |
| 19 * Constant string objects:: | |
| 20 * compatibility_alias:: | |
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21 * Exceptions:: |
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22 * Synchronization:: |
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23 * Fast enumeration:: |
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24 * Messaging with the GNU Objective-C runtime:: |
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25 @end menu |
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26 |
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27 @c ========================================================================= |
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28 @node GNU Objective-C runtime API |
| 111 | 29 @section GNU Objective-C Runtime API |
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30 |
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31 This section is specific for the GNU Objective-C runtime. If you are |
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32 using a different runtime, you can skip it. |
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33 |
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34 The GNU Objective-C runtime provides an API that allows you to |
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35 interact with the Objective-C runtime system, querying the live |
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36 runtime structures and even manipulating them. This allows you for |
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37 example to inspect and navigate classes, methods and protocols; to |
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38 define new classes or new methods, and even to modify existing classes |
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39 or protocols. |
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40 |
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41 If you are using a ``Foundation'' library such as GNUstep-Base, this |
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42 library will provide you with a rich set of functionality to do most |
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43 of the inspection tasks, and you probably will only need direct access |
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44 to the GNU Objective-C runtime API to define new classes or methods. |
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45 |
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46 @menu |
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47 * Modern GNU Objective-C runtime API:: |
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48 * Traditional GNU Objective-C runtime API:: |
| 0 | 49 @end menu |
| 50 | |
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51 @c ========================================================================= |
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52 @node Modern GNU Objective-C runtime API |
| 111 | 53 @subsection Modern GNU Objective-C Runtime API |
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54 |
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55 The GNU Objective-C runtime provides an API which is similar to the |
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56 one provided by the ``Objective-C 2.0'' Apple/NeXT Objective-C |
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57 runtime. The API is documented in the public header files of the GNU |
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58 Objective-C runtime: |
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59 |
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60 @itemize @bullet |
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61 |
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62 @item |
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63 @file{objc/objc.h}: this is the basic Objective-C header file, |
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64 defining the basic Objective-C types such as @code{id}, @code{Class} |
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65 and @code{BOOL}. You have to include this header to do almost |
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66 anything with Objective-C. |
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67 |
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68 @item |
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69 @file{objc/runtime.h}: this header declares most of the public runtime |
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70 API functions allowing you to inspect and manipulate the Objective-C |
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71 runtime data structures. These functions are fairly standardized |
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72 across Objective-C runtimes and are almost identical to the Apple/NeXT |
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73 Objective-C runtime ones. It does not declare functions in some |
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74 specialized areas (constructing and forwarding message invocations, |
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75 threading) which are in the other headers below. You have to include |
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76 @file{objc/objc.h} and @file{objc/runtime.h} to use any of the |
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77 functions, such as @code{class_getName()}, declared in |
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78 @file{objc/runtime.h}. |
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79 |
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80 @item |
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81 @file{objc/message.h}: this header declares public functions used to |
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82 construct, deconstruct and forward message invocations. Because |
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83 messaging is done in quite a different way on different runtimes, |
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84 functions in this header are specific to the GNU Objective-C runtime |
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85 implementation. |
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86 |
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87 @item |
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88 @file{objc/objc-exception.h}: this header declares some public |
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89 functions related to Objective-C exceptions. For example functions in |
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90 this header allow you to throw an Objective-C exception from plain |
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91 C/C++ code. |
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92 |
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93 @item |
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94 @file{objc/objc-sync.h}: this header declares some public functions |
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95 related to the Objective-C @code{@@synchronized()} syntax, allowing |
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96 you to emulate an Objective-C @code{@@synchronized()} block in plain |
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97 C/C++ code. |
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98 |
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99 @item |
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100 @file{objc/thr.h}: this header declares a public runtime API threading |
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101 layer that is only provided by the GNU Objective-C runtime. It |
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102 declares functions such as @code{objc_mutex_lock()}, which provide a |
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103 platform-independent set of threading functions. |
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104 |
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105 @end itemize |
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106 |
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107 The header files contain detailed documentation for each function in |
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108 the GNU Objective-C runtime API. |
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109 |
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110 @c ========================================================================= |
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111 @node Traditional GNU Objective-C runtime API |
| 111 | 112 @subsection Traditional GNU Objective-C Runtime API |
|
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113 |
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114 The GNU Objective-C runtime used to provide a different API, which we |
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115 call the ``traditional'' GNU Objective-C runtime API. Functions |
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116 belonging to this API are easy to recognize because they use a |
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117 different naming convention, such as @code{class_get_super_class()} |
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118 (traditional API) instead of @code{class_getSuperclass()} (modern |
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119 API). Software using this API includes the file |
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120 @file{objc/objc-api.h} where it is declared. |
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121 |
| 111 | 122 Starting with GCC 4.7.0, the traditional GNU runtime API is no longer |
| 123 available. | |
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124 |
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125 @c ========================================================================= |
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126 @node Executing code before main |
| 111 | 127 @section @code{+load}: Executing Code before @code{main} |
| 0 | 128 |
|
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129 This section is specific for the GNU Objective-C runtime. If you are |
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130 using a different runtime, you can skip it. |
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131 |
| 0 | 132 The GNU Objective-C runtime provides a way that allows you to execute |
| 133 code before the execution of the program enters the @code{main} | |
| 134 function. The code is executed on a per-class and a per-category basis, | |
| 135 through a special class method @code{+load}. | |
| 136 | |
| 137 This facility is very useful if you want to initialize global variables | |
| 138 which can be accessed by the program directly, without sending a message | |
| 139 to the class first. The usual way to initialize global variables, in the | |
| 140 @code{+initialize} method, might not be useful because | |
| 141 @code{+initialize} is only called when the first message is sent to a | |
| 142 class object, which in some cases could be too late. | |
| 143 | |
| 144 Suppose for example you have a @code{FileStream} class that declares | |
| 145 @code{Stdin}, @code{Stdout} and @code{Stderr} as global variables, like | |
| 146 below: | |
| 147 | |
| 148 @smallexample | |
| 149 | |
| 150 FileStream *Stdin = nil; | |
| 151 FileStream *Stdout = nil; | |
| 152 FileStream *Stderr = nil; | |
| 153 | |
| 154 @@implementation FileStream | |
| 155 | |
| 156 + (void)initialize | |
| 157 @{ | |
| 158 Stdin = [[FileStream new] initWithFd:0]; | |
| 159 Stdout = [[FileStream new] initWithFd:1]; | |
| 160 Stderr = [[FileStream new] initWithFd:2]; | |
| 161 @} | |
| 162 | |
| 163 /* @r{Other methods here} */ | |
| 164 @@end | |
| 165 | |
| 166 @end smallexample | |
| 167 | |
| 168 In this example, the initialization of @code{Stdin}, @code{Stdout} and | |
| 169 @code{Stderr} in @code{+initialize} occurs too late. The programmer can | |
| 170 send a message to one of these objects before the variables are actually | |
| 171 initialized, thus sending messages to the @code{nil} object. The | |
| 172 @code{+initialize} method which actually initializes the global | |
| 173 variables is not invoked until the first message is sent to the class | |
| 174 object. The solution would require these variables to be initialized | |
| 175 just before entering @code{main}. | |
| 176 | |
| 177 The correct solution of the above problem is to use the @code{+load} | |
| 178 method instead of @code{+initialize}: | |
| 179 | |
| 180 @smallexample | |
| 181 | |
| 182 @@implementation FileStream | |
| 183 | |
| 184 + (void)load | |
| 185 @{ | |
| 186 Stdin = [[FileStream new] initWithFd:0]; | |
| 187 Stdout = [[FileStream new] initWithFd:1]; | |
| 188 Stderr = [[FileStream new] initWithFd:2]; | |
| 189 @} | |
| 190 | |
| 191 /* @r{Other methods here} */ | |
| 192 @@end | |
| 193 | |
| 194 @end smallexample | |
| 195 | |
| 196 The @code{+load} is a method that is not overridden by categories. If a | |
| 197 class and a category of it both implement @code{+load}, both methods are | |
| 198 invoked. This allows some additional initializations to be performed in | |
| 199 a category. | |
| 200 | |
| 201 This mechanism is not intended to be a replacement for @code{+initialize}. | |
| 202 You should be aware of its limitations when you decide to use it | |
| 203 instead of @code{+initialize}. | |
| 204 | |
| 205 @menu | |
| 206 * What you can and what you cannot do in +load:: | |
| 207 @end menu | |
| 208 | |
| 209 | |
|
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210 @node What you can and what you cannot do in +load |
| 111 | 211 @subsection What You Can and Cannot Do in @code{+load} |
| 0 | 212 |
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213 @code{+load} is to be used only as a last resort. Because it is |
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214 executed very early, most of the Objective-C runtime machinery will |
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215 not be ready when @code{+load} is executed; hence @code{+load} works |
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216 best for executing C code that is independent on the Objective-C |
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217 runtime. |
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218 |
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219 The @code{+load} implementation in the GNU runtime guarantees you the |
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220 following things: |
| 0 | 221 |
| 222 @itemize @bullet | |
| 223 | |
| 224 @item | |
| 225 you can write whatever C code you like; | |
| 226 | |
| 227 @item | |
| 228 you can allocate and send messages to objects whose class is implemented | |
| 229 in the same file; | |
| 230 | |
| 231 @item | |
|
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232 the @code{+load} implementation of all super classes of a class are |
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233 executed before the @code{+load} of that class is executed; |
| 0 | 234 |
| 235 @item | |
| 236 the @code{+load} implementation of a class is executed before the | |
| 237 @code{+load} implementation of any category. | |
| 238 | |
| 239 @end itemize | |
| 240 | |
| 241 In particular, the following things, even if they can work in a | |
| 242 particular case, are not guaranteed: | |
| 243 | |
| 244 @itemize @bullet | |
| 245 | |
| 246 @item | |
| 247 allocation of or sending messages to arbitrary objects; | |
| 248 | |
| 249 @item | |
| 250 allocation of or sending messages to objects whose classes have a | |
| 251 category implemented in the same file; | |
| 252 | |
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253 @item |
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254 sending messages to Objective-C constant strings (@code{@@"this is a |
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255 constant string"}); |
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256 |
| 0 | 257 @end itemize |
| 258 | |
| 259 You should make no assumptions about receiving @code{+load} in sibling | |
| 260 classes when you write @code{+load} of a class. The order in which | |
| 261 sibling classes receive @code{+load} is not guaranteed. | |
| 262 | |
| 263 The order in which @code{+load} and @code{+initialize} are called could | |
| 264 be problematic if this matters. If you don't allocate objects inside | |
| 265 @code{+load}, it is guaranteed that @code{+load} is called before | |
| 266 @code{+initialize}. If you create an object inside @code{+load} the | |
| 267 @code{+initialize} method of object's class is invoked even if | |
| 268 @code{+load} was not invoked. Note if you explicitly call @code{+load} | |
| 269 on a class, @code{+initialize} will be called first. To avoid possible | |
| 270 problems try to implement only one of these methods. | |
| 271 | |
| 272 The @code{+load} method is also invoked when a bundle is dynamically | |
| 273 loaded into your running program. This happens automatically without any | |
| 274 intervening operation from you. When you write bundles and you need to | |
| 275 write @code{+load} you can safely create and send messages to objects whose | |
| 276 classes already exist in the running program. The same restrictions as | |
| 277 above apply to classes defined in bundle. | |
| 278 | |
| 279 | |
| 280 | |
|
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281 @node Type encoding |
| 111 | 282 @section Type Encoding |
| 0 | 283 |
|
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284 This is an advanced section. Type encodings are used extensively by |
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285 the compiler and by the runtime, but you generally do not need to know |
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286 about them to use Objective-C. |
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287 |
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288 The Objective-C compiler generates type encodings for all the types. |
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289 These type encodings are used at runtime to find out information about |
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290 selectors and methods and about objects and classes. |
| 0 | 291 |
| 292 The types are encoded in the following way: | |
| 293 | |
| 294 @c @sp 1 | |
| 295 | |
| 296 @multitable @columnfractions .25 .75 | |
| 297 @item @code{_Bool} | |
| 298 @tab @code{B} | |
| 299 @item @code{char} | |
| 300 @tab @code{c} | |
| 301 @item @code{unsigned char} | |
| 302 @tab @code{C} | |
| 303 @item @code{short} | |
| 304 @tab @code{s} | |
| 305 @item @code{unsigned short} | |
| 306 @tab @code{S} | |
| 307 @item @code{int} | |
| 308 @tab @code{i} | |
| 309 @item @code{unsigned int} | |
| 310 @tab @code{I} | |
| 311 @item @code{long} | |
| 312 @tab @code{l} | |
| 313 @item @code{unsigned long} | |
| 314 @tab @code{L} | |
| 315 @item @code{long long} | |
| 316 @tab @code{q} | |
| 317 @item @code{unsigned long long} | |
| 318 @tab @code{Q} | |
| 319 @item @code{float} | |
| 320 @tab @code{f} | |
| 321 @item @code{double} | |
| 322 @tab @code{d} | |
|
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323 @item @code{long double} |
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324 @tab @code{D} |
| 0 | 325 @item @code{void} |
| 326 @tab @code{v} | |
| 327 @item @code{id} | |
| 328 @tab @code{@@} | |
| 329 @item @code{Class} | |
| 330 @tab @code{#} | |
| 331 @item @code{SEL} | |
| 332 @tab @code{:} | |
| 333 @item @code{char*} | |
| 334 @tab @code{*} | |
|
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335 @item @code{enum} |
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336 @tab an @code{enum} is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration |
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337 values. Often the compiler users @code{unsigned int}, which is then encoded as @code{I}. |
| 0 | 338 @item unknown type |
| 339 @tab @code{?} | |
| 340 @item Complex types | |
| 341 @tab @code{j} followed by the inner type. For example @code{_Complex double} is encoded as "jd". | |
| 342 @item bit-fields | |
| 343 @tab @code{b} followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below) | |
| 344 @end multitable | |
| 345 | |
| 346 @c @sp 1 | |
| 347 | |
|
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348 The encoding of bit-fields has changed to allow bit-fields to be |
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349 properly handled by the runtime functions that compute sizes and |
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350 alignments of types that contain bit-fields. The previous encoding |
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351 contained only the size of the bit-field. Using only this information |
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352 it is not possible to reliably compute the size occupied by the |
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353 bit-field. This is very important in the presence of the Boehm's |
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354 garbage collector because the objects are allocated using the typed |
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355 memory facility available in this collector. The typed memory |
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356 allocation requires information about where the pointers are located |
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357 inside the object. |
| 0 | 358 |
| 359 The position in the bit-field is the position, counting in bits, of the | |
| 360 bit closest to the beginning of the structure. | |
| 361 | |
| 362 The non-atomic types are encoded as follows: | |
| 363 | |
| 364 @c @sp 1 | |
| 365 | |
| 366 @multitable @columnfractions .2 .8 | |
| 367 @item pointers | |
| 368 @tab @samp{^} followed by the pointed type. | |
| 369 @item arrays | |
| 370 @tab @samp{[} followed by the number of elements in the array followed by the type of the elements followed by @samp{]} | |
| 371 @item structures | |
| 372 @tab @samp{@{} followed by the name of the structure (or @samp{?} if the structure is unnamed), the @samp{=} sign, the type of the members and by @samp{@}} | |
| 373 @item unions | |
| 374 @tab @samp{(} followed by the name of the structure (or @samp{?} if the union is unnamed), the @samp{=} sign, the type of the members followed by @samp{)} | |
|
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375 @item vectors |
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376 @tab @samp{![} followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by @samp{]} |
| 0 | 377 @end multitable |
| 378 | |
| 379 Here are some types and their encodings, as they are generated by the | |
| 380 compiler on an i386 machine: | |
| 381 | |
| 382 @sp 1 | |
| 383 | |
| 111 | 384 @multitable @columnfractions .60 .40 |
| 0 | 385 @item Objective-C type |
| 386 @tab Compiler encoding | |
| 387 @item | |
| 388 @smallexample | |
| 389 int a[10]; | |
| 390 @end smallexample | |
| 391 @tab @code{[10i]} | |
| 392 @item | |
| 393 @smallexample | |
| 394 struct @{ | |
| 395 int i; | |
| 396 float f[3]; | |
| 397 int a:3; | |
| 398 int b:2; | |
| 399 char c; | |
| 400 @} | |
| 401 @end smallexample | |
| 402 @tab @code{@{?=i[3f]b128i3b131i2c@}} | |
|
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403 @item |
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404 @smallexample |
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405 int a __attribute__ ((vector_size (16))); |
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406 @end smallexample |
| 111 | 407 @tab @code{![16,16i]} (alignment depends on the machine) |
| 0 | 408 @end multitable |
| 409 | |
| 410 @sp 1 | |
| 411 | |
| 412 In addition to the types the compiler also encodes the type | |
| 413 specifiers. The table below describes the encoding of the current | |
| 414 Objective-C type specifiers: | |
| 415 | |
| 416 @sp 1 | |
| 417 | |
| 418 @multitable @columnfractions .25 .75 | |
| 419 @item Specifier | |
| 420 @tab Encoding | |
| 421 @item @code{const} | |
| 422 @tab @code{r} | |
| 423 @item @code{in} | |
| 424 @tab @code{n} | |
| 425 @item @code{inout} | |
| 426 @tab @code{N} | |
| 427 @item @code{out} | |
| 428 @tab @code{o} | |
| 429 @item @code{bycopy} | |
| 430 @tab @code{O} | |
|
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431 @item @code{byref} |
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432 @tab @code{R} |
| 0 | 433 @item @code{oneway} |
| 434 @tab @code{V} | |
| 435 @end multitable | |
| 436 | |
| 437 @sp 1 | |
| 438 | |
| 439 The type specifiers are encoded just before the type. Unlike types | |
| 440 however, the type specifiers are only encoded when they appear in method | |
| 441 argument types. | |
| 442 | |
|
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443 Note how @code{const} interacts with pointers: |
| 0 | 444 |
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445 @sp 1 |
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446 |
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447 @multitable @columnfractions .25 .75 |
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448 @item Objective-C type |
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449 @tab Compiler encoding |
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450 @item |
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451 @smallexample |
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452 const int |
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453 @end smallexample |
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454 @tab @code{ri} |
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455 @item |
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456 @smallexample |
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457 const int* |
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458 @end smallexample |
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459 @tab @code{^ri} |
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460 @item |
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461 @smallexample |
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462 int *const |
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463 @end smallexample |
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464 @tab @code{r^i} |
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465 @end multitable |
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466 |
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467 @sp 1 |
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468 |
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469 @code{const int*} is a pointer to a @code{const int}, and so is |
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470 encoded as @code{^ri}. @code{int* const}, instead, is a @code{const} |
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471 pointer to an @code{int}, and so is encoded as @code{r^i}. |
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472 |
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473 Finally, there is a complication when encoding @code{const char *} |
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474 versus @code{char * const}. Because @code{char *} is encoded as |
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475 @code{*} and not as @code{^c}, there is no way to express the fact |
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476 that @code{r} applies to the pointer or to the pointee. |
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477 |
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478 Hence, it is assumed as a convention that @code{r*} means @code{const |
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479 char *} (since it is what is most often meant), and there is no way to |
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480 encode @code{char *const}. @code{char *const} would simply be encoded |
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481 as @code{*}, and the @code{const} is lost. |
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482 |
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483 @menu |
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484 * Legacy type encoding:: |
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485 * @@encode:: |
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486 * Method signatures:: |
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487 @end menu |
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488 |
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489 @node Legacy type encoding |
| 111 | 490 @subsection Legacy Type Encoding |
|
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491 |
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492 Unfortunately, historically GCC used to have a number of bugs in its |
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493 encoding code. The NeXT runtime expects GCC to emit type encodings in |
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494 this historical format (compatible with GCC-3.3), so when using the |
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495 NeXT runtime, GCC will introduce on purpose a number of incorrect |
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496 encodings: |
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497 |
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498 @itemize @bullet |
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499 |
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500 @item |
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501 the read-only qualifier of the pointee gets emitted before the '^'. |
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502 The read-only qualifier of the pointer itself gets ignored, unless it |
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503 is a typedef. Also, the 'r' is only emitted for the outermost type. |
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504 |
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505 @item |
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506 32-bit longs are encoded as 'l' or 'L', but not always. For typedefs, |
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507 the compiler uses 'i' or 'I' instead if encoding a struct field or a |
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508 pointer. |
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509 |
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510 @item |
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511 @code{enum}s are always encoded as 'i' (int) even if they are actually |
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512 unsigned or long. |
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513 |
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514 @end itemize |
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515 |
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516 In addition to that, the NeXT runtime uses a different encoding for |
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517 bitfields. It encodes them as @code{b} followed by the size, without |
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518 a bit offset or the underlying field type. |
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519 |
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520 @node @@encode |
| 111 | 521 @subsection @code{@@encode} |
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522 |
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523 GNU Objective-C supports the @code{@@encode} syntax that allows you to |
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524 create a type encoding from a C/Objective-C type. For example, |
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525 @code{@@encode(int)} is compiled by the compiler into @code{"i"}. |
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526 |
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527 @code{@@encode} does not support type qualifiers other than |
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528 @code{const}. For example, @code{@@encode(const char*)} is valid and |
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529 is compiled into @code{"r*"}, while @code{@@encode(bycopy char *)} is |
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530 invalid and will cause a compilation error. |
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531 |
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532 @node Method signatures |
| 111 | 533 @subsection Method Signatures |
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534 |
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535 This section documents the encoding of method types, which is rarely |
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536 needed to use Objective-C. You should skip it at a first reading; the |
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537 runtime provides functions that will work on methods and can walk |
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538 through the list of parameters and interpret them for you. These |
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539 functions are part of the public ``API'' and are the preferred way to |
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540 interact with method signatures from user code. |
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541 |
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542 But if you need to debug a problem with method signatures and need to |
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543 know how they are implemented (i.e., the ``ABI''), read on. |
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544 |
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545 Methods have their ``signature'' encoded and made available to the |
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546 runtime. The ``signature'' encodes all the information required to |
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547 dynamically build invocations of the method at runtime: return type |
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548 and arguments. |
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549 |
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550 The ``signature'' is a null-terminated string, composed of the following: |
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551 |
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552 @itemize @bullet |
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553 |
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554 @item |
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555 The return type, including type qualifiers. For example, a method |
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556 returning @code{int} would have @code{i} here. |
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557 |
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558 @item |
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559 The total size (in bytes) required to pass all the parameters. This |
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560 includes the two hidden parameters (the object @code{self} and the |
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561 method selector @code{_cmd}). |
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562 |
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563 @item |
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564 Each argument, with the type encoding, followed by the offset (in |
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565 bytes) of the argument in the list of parameters. |
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566 |
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567 @end itemize |
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568 |
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569 For example, a method with no arguments and returning @code{int} would |
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570 have the signature @code{i8@@0:4} if the size of a pointer is 4. The |
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571 signature is interpreted as follows: the @code{i} is the return type |
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572 (an @code{int}), the @code{8} is the total size of the parameters in |
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573 bytes (two pointers each of size 4), the @code{@@0} is the first |
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574 parameter (an object at byte offset @code{0}) and @code{:4} is the |
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575 second parameter (a @code{SEL} at byte offset @code{4}). |
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576 |
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577 You can easily find more examples by running the ``strings'' program |
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578 on an Objective-C object file compiled by GCC. You'll see a lot of |
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579 strings that look very much like @code{i8@@0:4}. They are signatures |
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580 of Objective-C methods. |
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581 |
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582 |
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583 @node Garbage Collection |
| 0 | 584 @section Garbage Collection |
| 585 | |
|
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586 This section is specific for the GNU Objective-C runtime. If you are |
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587 using a different runtime, you can skip it. |
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588 |
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589 Support for garbage collection with the GNU runtime has been added by |
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590 using a powerful conservative garbage collector, known as the |
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591 Boehm-Demers-Weiser conservative garbage collector. |
| 0 | 592 |
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593 To enable the support for it you have to configure the compiler using |
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594 an additional argument, @w{@option{--enable-objc-gc}}. This will |
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595 build the boehm-gc library, and build an additional runtime library |
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596 which has several enhancements to support the garbage collector. The |
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597 new library has a new name, @file{libobjc_gc.a} to not conflict with |
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598 the non-garbage-collected library. |
| 0 | 599 |
| 600 When the garbage collector is used, the objects are allocated using the | |
| 601 so-called typed memory allocation mechanism available in the | |
| 602 Boehm-Demers-Weiser collector. This mode requires precise information on | |
| 603 where pointers are located inside objects. This information is computed | |
| 604 once per class, immediately after the class has been initialized. | |
| 605 | |
| 606 There is a new runtime function @code{class_ivar_set_gcinvisible()} | |
| 607 which can be used to declare a so-called @dfn{weak pointer} | |
| 608 reference. Such a pointer is basically hidden for the garbage collector; | |
| 609 this can be useful in certain situations, especially when you want to | |
| 610 keep track of the allocated objects, yet allow them to be | |
| 611 collected. This kind of pointers can only be members of objects, you | |
| 612 cannot declare a global pointer as a weak reference. Every type which is | |
| 613 a pointer type can be declared a weak pointer, including @code{id}, | |
| 614 @code{Class} and @code{SEL}. | |
| 615 | |
| 616 Here is an example of how to use this feature. Suppose you want to | |
| 617 implement a class whose instances hold a weak pointer reference; the | |
| 618 following class does this: | |
| 619 | |
| 620 @smallexample | |
| 621 | |
| 622 @@interface WeakPointer : Object | |
| 623 @{ | |
| 624 const void* weakPointer; | |
| 625 @} | |
| 626 | |
| 627 - initWithPointer:(const void*)p; | |
| 628 - (const void*)weakPointer; | |
| 629 @@end | |
| 630 | |
| 631 | |
| 632 @@implementation WeakPointer | |
| 633 | |
| 634 + (void)initialize | |
| 635 @{ | |
| 111 | 636 if (self == objc_lookUpClass ("WeakPointer")) |
| 637 class_ivar_set_gcinvisible (self, "weakPointer", YES); | |
| 0 | 638 @} |
| 639 | |
| 640 - initWithPointer:(const void*)p | |
| 641 @{ | |
| 642 weakPointer = p; | |
| 643 return self; | |
| 644 @} | |
| 645 | |
| 646 - (const void*)weakPointer | |
| 647 @{ | |
| 648 return weakPointer; | |
| 649 @} | |
| 650 | |
| 651 @@end | |
| 652 | |
| 653 @end smallexample | |
| 654 | |
| 655 Weak pointers are supported through a new type character specifier | |
| 656 represented by the @samp{!} character. The | |
| 657 @code{class_ivar_set_gcinvisible()} function adds or removes this | |
| 658 specifier to the string type description of the instance variable named | |
| 659 as argument. | |
| 660 | |
| 661 @c ========================================================================= | |
| 662 @node Constant string objects | |
| 111 | 663 @section Constant String Objects |
| 0 | 664 |
| 665 GNU Objective-C provides constant string objects that are generated | |
| 666 directly by the compiler. You declare a constant string object by | |
| 667 prefixing a C constant string with the character @samp{@@}: | |
| 668 | |
| 669 @smallexample | |
| 670 id myString = @@"this is a constant string object"; | |
| 671 @end smallexample | |
| 672 | |
| 673 The constant string objects are by default instances of the | |
| 674 @code{NXConstantString} class which is provided by the GNU Objective-C | |
| 675 runtime. To get the definition of this class you must include the | |
| 676 @file{objc/NXConstStr.h} header file. | |
| 677 | |
| 678 User defined libraries may want to implement their own constant string | |
| 679 class. To be able to support them, the GNU Objective-C compiler provides | |
| 680 a new command line options @option{-fconstant-string-class=@var{class-name}}. | |
| 681 The provided class should adhere to a strict structure, the same | |
| 682 as @code{NXConstantString}'s structure: | |
| 683 | |
| 684 @smallexample | |
| 685 | |
| 686 @@interface MyConstantStringClass | |
| 687 @{ | |
| 688 Class isa; | |
| 689 char *c_string; | |
| 690 unsigned int len; | |
| 691 @} | |
| 692 @@end | |
| 693 | |
| 694 @end smallexample | |
| 695 | |
| 696 @code{NXConstantString} inherits from @code{Object}; user class | |
| 697 libraries may choose to inherit the customized constant string class | |
| 698 from a different class than @code{Object}. There is no requirement in | |
| 699 the methods the constant string class has to implement, but the final | |
| 700 ivar layout of the class must be the compatible with the given | |
| 701 structure. | |
| 702 | |
| 703 When the compiler creates the statically allocated constant string | |
| 704 object, the @code{c_string} field will be filled by the compiler with | |
| 705 the string; the @code{length} field will be filled by the compiler with | |
| 706 the string length; the @code{isa} pointer will be filled with | |
| 707 @code{NULL} by the compiler, and it will later be fixed up automatically | |
| 708 at runtime by the GNU Objective-C runtime library to point to the class | |
| 709 which was set by the @option{-fconstant-string-class} option when the | |
| 710 object file is loaded (if you wonder how it works behind the scenes, the | |
| 711 name of the class to use, and the list of static objects to fixup, are | |
| 712 stored by the compiler in the object file in a place where the GNU | |
| 713 runtime library will find them at runtime). | |
| 714 | |
| 715 As a result, when a file is compiled with the | |
| 716 @option{-fconstant-string-class} option, all the constant string objects | |
| 717 will be instances of the class specified as argument to this option. It | |
| 718 is possible to have multiple compilation units referring to different | |
| 719 constant string classes, neither the compiler nor the linker impose any | |
| 720 restrictions in doing this. | |
| 721 | |
| 722 @c ========================================================================= | |
| 723 @node compatibility_alias | |
| 111 | 724 @section @code{compatibility_alias} |
| 0 | 725 |
| 726 The keyword @code{@@compatibility_alias} allows you to define a class name | |
| 727 as equivalent to another class name. For example: | |
| 728 | |
| 729 @smallexample | |
| 730 @@compatibility_alias WOApplication GSWApplication; | |
| 731 @end smallexample | |
| 732 | |
| 733 tells the compiler that each time it encounters @code{WOApplication} as | |
| 734 a class name, it should replace it with @code{GSWApplication} (that is, | |
| 735 @code{WOApplication} is just an alias for @code{GSWApplication}). | |
| 736 | |
| 737 There are some constraints on how this can be used--- | |
| 738 | |
| 739 @itemize @bullet | |
| 740 | |
| 741 @item @code{WOApplication} (the alias) must not be an existing class; | |
| 742 | |
| 743 @item @code{GSWApplication} (the real class) must be an existing class. | |
| 744 | |
| 745 @end itemize | |
|
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746 |
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747 @c ========================================================================= |
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748 @node Exceptions |
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749 @section Exceptions |
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750 |
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751 GNU Objective-C provides exception support built into the language, as |
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752 in the following example: |
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753 |
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754 @smallexample |
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755 @@try @{ |
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756 @dots{} |
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757 @@throw expr; |
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758 @dots{} |
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759 @} |
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760 @@catch (AnObjCClass *exc) @{ |
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761 @dots{} |
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762 @@throw expr; |
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763 @dots{} |
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764 @@throw; |
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765 @dots{} |
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766 @} |
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767 @@catch (AnotherClass *exc) @{ |
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768 @dots{} |
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769 @} |
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770 @@catch (id allOthers) @{ |
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771 @dots{} |
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772 @} |
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773 @@finally @{ |
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774 @dots{} |
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775 @@throw expr; |
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776 @dots{} |
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777 @} |
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778 @end smallexample |
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779 |
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780 The @code{@@throw} statement may appear anywhere in an Objective-C or |
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781 Objective-C++ program; when used inside of a @code{@@catch} block, the |
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782 @code{@@throw} may appear without an argument (as shown above), in |
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783 which case the object caught by the @code{@@catch} will be rethrown. |
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784 |
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785 Note that only (pointers to) Objective-C objects may be thrown and |
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786 caught using this scheme. When an object is thrown, it will be caught |
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787 by the nearest @code{@@catch} clause capable of handling objects of |
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788 that type, analogously to how @code{catch} blocks work in C++ and |
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789 Java. A @code{@@catch(id @dots{})} clause (as shown above) may also |
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790 be provided to catch any and all Objective-C exceptions not caught by |
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791 previous @code{@@catch} clauses (if any). |
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792 |
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793 The @code{@@finally} clause, if present, will be executed upon exit |
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794 from the immediately preceding @code{@@try @dots{} @@catch} section. |
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795 This will happen regardless of whether any exceptions are thrown, |
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796 caught or rethrown inside the @code{@@try @dots{} @@catch} section, |
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797 analogously to the behavior of the @code{finally} clause in Java. |
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798 |
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799 There are several caveats to using the new exception mechanism: |
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800 |
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801 @itemize @bullet |
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802 @item |
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803 The @option{-fobjc-exceptions} command line option must be used when |
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804 compiling Objective-C files that use exceptions. |
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805 |
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806 @item |
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807 With the GNU runtime, exceptions are always implemented as ``native'' |
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808 exceptions and it is recommended that the @option{-fexceptions} and |
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809 @option{-shared-libgcc} options are used when linking. |
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810 |
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811 @item |
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812 With the NeXT runtime, although currently designed to be binary |
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813 compatible with @code{NS_HANDLER}-style idioms provided by the |
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814 @code{NSException} class, the new exceptions can only be used on Mac |
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815 OS X 10.3 (Panther) and later systems, due to additional functionality |
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816 needed in the NeXT Objective-C runtime. |
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817 |
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818 @item |
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819 As mentioned above, the new exceptions do not support handling |
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820 types other than Objective-C objects. Furthermore, when used from |
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821 Objective-C++, the Objective-C exception model does not interoperate with C++ |
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822 exceptions at this time. This means you cannot @code{@@throw} an exception |
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823 from Objective-C and @code{catch} it in C++, or vice versa |
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824 (i.e., @code{throw @dots{} @@catch}). |
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825 @end itemize |
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826 |
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827 @c ========================================================================= |
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828 @node Synchronization |
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829 @section Synchronization |
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830 |
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831 GNU Objective-C provides support for synchronized blocks: |
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832 |
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833 @smallexample |
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834 @@synchronized (ObjCClass *guard) @{ |
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835 @dots{} |
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836 @} |
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837 @end smallexample |
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838 |
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839 Upon entering the @code{@@synchronized} block, a thread of execution |
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840 shall first check whether a lock has been placed on the corresponding |
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841 @code{guard} object by another thread. If it has, the current thread |
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842 shall wait until the other thread relinquishes its lock. Once |
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843 @code{guard} becomes available, the current thread will place its own |
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844 lock on it, execute the code contained in the @code{@@synchronized} |
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845 block, and finally relinquish the lock (thereby making @code{guard} |
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846 available to other threads). |
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847 |
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848 Unlike Java, Objective-C does not allow for entire methods to be |
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849 marked @code{@@synchronized}. Note that throwing exceptions out of |
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850 @code{@@synchronized} blocks is allowed, and will cause the guarding |
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851 object to be unlocked properly. |
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852 |
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853 Because of the interactions between synchronization and exception |
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854 handling, you can only use @code{@@synchronized} when compiling with |
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855 exceptions enabled, that is with the command line option |
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856 @option{-fobjc-exceptions}. |
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857 |
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858 |
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859 @c ========================================================================= |
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860 @node Fast enumeration |
| 111 | 861 @section Fast Enumeration |
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862 |
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863 @menu |
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864 * Using fast enumeration:: |
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865 * c99-like fast enumeration syntax:: |
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866 * Fast enumeration details:: |
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867 * Fast enumeration protocol:: |
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868 @end menu |
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869 |
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870 @c ================================ |
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871 @node Using fast enumeration |
| 111 | 872 @subsection Using Fast Enumeration |
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873 |
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874 GNU Objective-C provides support for the fast enumeration syntax: |
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875 |
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876 @smallexample |
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877 id array = @dots{}; |
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878 id object; |
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879 |
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880 for (object in array) |
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881 @{ |
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882 /* Do something with 'object' */ |
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883 @} |
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884 @end smallexample |
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885 |
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886 @code{array} needs to be an Objective-C object (usually a collection |
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887 object, for example an array, a dictionary or a set) which implements |
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888 the ``Fast Enumeration Protocol'' (see below). If you are using a |
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889 Foundation library such as GNUstep Base or Apple Cocoa Foundation, all |
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890 collection objects in the library implement this protocol and can be |
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891 used in this way. |
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892 |
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893 The code above would iterate over all objects in @code{array}. For |
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894 each of them, it assigns it to @code{object}, then executes the |
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895 @code{Do something with 'object'} statements. |
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896 |
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897 Here is a fully worked-out example using a Foundation library (which |
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898 provides the implementation of @code{NSArray}, @code{NSString} and |
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899 @code{NSLog}): |
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900 |
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901 @smallexample |
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902 NSArray *array = [NSArray arrayWithObjects: @@"1", @@"2", @@"3", nil]; |
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903 NSString *object; |
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904 |
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905 for (object in array) |
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906 NSLog (@@"Iterating over %@@", object); |
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907 @end smallexample |
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908 |
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909 |
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910 @c ================================ |
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911 @node c99-like fast enumeration syntax |
| 111 | 912 @subsection C99-Like Fast Enumeration Syntax |
|
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913 |
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914 A c99-like declaration syntax is also allowed: |
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915 |
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916 @smallexample |
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917 id array = @dots{}; |
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918 |
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919 for (id object in array) |
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920 @{ |
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921 /* Do something with 'object' */ |
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922 @} |
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923 @end smallexample |
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924 |
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925 this is completely equivalent to: |
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926 |
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927 @smallexample |
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928 id array = @dots{}; |
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929 |
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930 @{ |
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931 id object; |
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932 for (object in array) |
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933 @{ |
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934 /* Do something with 'object' */ |
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935 @} |
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936 @} |
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937 @end smallexample |
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938 |
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939 but can save some typing. |
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940 |
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941 Note that the option @option{-std=c99} is not required to allow this |
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942 syntax in Objective-C. |
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943 |
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944 @c ================================ |
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945 @node Fast enumeration details |
| 111 | 946 @subsection Fast Enumeration Details |
|
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947 |
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948 Here is a more technical description with the gory details. Consider the code |
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949 |
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950 @smallexample |
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951 for (@var{object expression} in @var{collection expression}) |
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952 @{ |
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953 @var{statements} |
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954 @} |
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955 @end smallexample |
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956 |
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957 here is what happens when you run it: |
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958 |
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959 @itemize @bullet |
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960 @item |
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961 @code{@var{collection expression}} is evaluated exactly once and the |
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962 result is used as the collection object to iterate over. This means |
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963 it is safe to write code such as @code{for (object in [NSDictionary |
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964 keyEnumerator]) @dots{}}. |
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965 |
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966 @item |
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967 the iteration is implemented by the compiler by repeatedly getting |
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968 batches of objects from the collection object using the fast |
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969 enumeration protocol (see below), then iterating over all objects in |
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970 the batch. This is faster than a normal enumeration where objects are |
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971 retrieved one by one (hence the name ``fast enumeration''). |
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972 |
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973 @item |
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974 if there are no objects in the collection, then |
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975 @code{@var{object expression}} is set to @code{nil} and the loop |
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976 immediately terminates. |
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977 |
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978 @item |
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979 if there are objects in the collection, then for each object in the |
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980 collection (in the order they are returned) @code{@var{object expression}} |
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981 is set to the object, then @code{@var{statements}} are executed. |
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982 |
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983 @item |
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984 @code{@var{statements}} can contain @code{break} and @code{continue} |
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985 commands, which will abort the iteration or skip to the next loop |
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986 iteration as expected. |
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987 |
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988 @item |
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989 when the iteration ends because there are no more objects to iterate |
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990 over, @code{@var{object expression}} is set to @code{nil}. This allows |
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991 you to determine whether the iteration finished because a @code{break} |
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992 command was used (in which case @code{@var{object expression}} will remain |
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993 set to the last object that was iterated over) or because it iterated |
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994 over all the objects (in which case @code{@var{object expression}} will be |
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995 set to @code{nil}). |
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996 |
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997 @item |
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998 @code{@var{statements}} must not make any changes to the collection |
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999 object; if they do, it is a hard error and the fast enumeration |
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1000 terminates by invoking @code{objc_enumerationMutation}, a runtime |
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1001 function that normally aborts the program but which can be customized |
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1002 by Foundation libraries via @code{objc_set_mutation_handler} to do |
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1003 something different, such as raising an exception. |
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1004 |
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1005 @end itemize |
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1006 |
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1007 @c ================================ |
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1008 @node Fast enumeration protocol |
| 111 | 1009 @subsection Fast Enumeration Protocol |
|
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1010 |
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1011 If you want your own collection object to be usable with fast |
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1012 enumeration, you need to have it implement the method |
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1013 |
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1014 @smallexample |
| 111 | 1015 - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state |
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1016 objects: (id *)objects |
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1017 count: (unsigned long)len; |
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1018 @end smallexample |
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1019 |
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1020 where @code{NSFastEnumerationState} must be defined in your code as follows: |
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1021 |
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1022 @smallexample |
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1023 typedef struct |
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1024 @{ |
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1025 unsigned long state; |
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1026 id *itemsPtr; |
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1027 unsigned long *mutationsPtr; |
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1028 unsigned long extra[5]; |
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1029 @} NSFastEnumerationState; |
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1030 @end smallexample |
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1031 |
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1032 If no @code{NSFastEnumerationState} is defined in your code, the |
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1033 compiler will automatically replace @code{NSFastEnumerationState *} |
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1034 with @code{struct __objcFastEnumerationState *}, where that type is |
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1035 silently defined by the compiler in an identical way. This can be |
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1036 confusing and we recommend that you define |
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1037 @code{NSFastEnumerationState} (as shown above) instead. |
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1038 |
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1039 The method is called repeatedly during a fast enumeration to retrieve |
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1040 batches of objects. Each invocation of the method should retrieve the |
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1041 next batch of objects. |
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1042 |
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1043 The return value of the method is the number of objects in the current |
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1044 batch; this should not exceed @code{len}, which is the maximum size of |
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1045 a batch as requested by the caller. The batch itself is returned in |
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1046 the @code{itemsPtr} field of the @code{NSFastEnumerationState} struct. |
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1047 |
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1048 To help with returning the objects, the @code{objects} array is a C |
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1049 array preallocated by the caller (on the stack) of size @code{len}. |
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1050 In many cases you can put the objects you want to return in that |
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1051 @code{objects} array, then do @code{itemsPtr = objects}. But you |
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1052 don't have to; if your collection already has the objects to return in |
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1053 some form of C array, it could return them from there instead. |
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1054 |
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1055 The @code{state} and @code{extra} fields of the |
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1056 @code{NSFastEnumerationState} structure allows your collection object |
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1057 to keep track of the state of the enumeration. In a simple array |
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1058 implementation, @code{state} may keep track of the index of the last |
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1059 object that was returned, and @code{extra} may be unused. |
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1060 |
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1061 The @code{mutationsPtr} field of the @code{NSFastEnumerationState} is |
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1062 used to keep track of mutations. It should point to a number; before |
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1063 working on each object, the fast enumeration loop will check that this |
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1064 number has not changed. If it has, a mutation has happened and the |
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1065 fast enumeration will abort. So, @code{mutationsPtr} could be set to |
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1066 point to some sort of version number of your collection, which is |
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1067 increased by one every time there is a change (for example when an |
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1068 object is added or removed). Or, if you are content with less strict |
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1069 mutation checks, it could point to the number of objects in your |
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1070 collection or some other value that can be checked to perform an |
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1071 approximate check that the collection has not been mutated. |
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1072 |
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1073 Finally, note how we declared the @code{len} argument and the return |
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1074 value to be of type @code{unsigned long}. They could also be declared |
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1075 to be of type @code{unsigned int} and everything would still work. |
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1076 |
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1077 @c ========================================================================= |
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1078 @node Messaging with the GNU Objective-C runtime |
| 111 | 1079 @section Messaging with the GNU Objective-C Runtime |
|
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1080 |
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1081 This section is specific for the GNU Objective-C runtime. If you are |
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1082 using a different runtime, you can skip it. |
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1083 |
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1084 The implementation of messaging in the GNU Objective-C runtime is |
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1085 designed to be portable, and so is based on standard C. |
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1086 |
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1087 Sending a message in the GNU Objective-C runtime is composed of two |
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1088 separate steps. First, there is a call to the lookup function, |
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1089 @code{objc_msg_lookup ()} (or, in the case of messages to super, |
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1090 @code{objc_msg_lookup_super ()}). This runtime function takes as |
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1091 argument the receiver and the selector of the method to be called; it |
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1092 returns the @code{IMP}, that is a pointer to the function implementing |
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1093 the method. The second step of method invocation consists of casting |
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1094 this pointer function to the appropriate function pointer type, and |
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1095 calling the function pointed to it with the right arguments. |
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1096 |
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1097 For example, when the compiler encounters a method invocation such as |
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1098 @code{[object init]}, it compiles it into a call to |
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1099 @code{objc_msg_lookup (object, @@selector(init))} followed by a cast |
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1100 of the returned value to the appropriate function pointer type, and |
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1101 then it calls it. |
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1102 |
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1103 @menu |
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1104 * Dynamically registering methods:: |
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1105 * Forwarding hook:: |
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1106 @end menu |
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1107 |
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1108 @c ========================================================================= |
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1109 @node Dynamically registering methods |
| 111 | 1110 @subsection Dynamically Registering Methods |
|
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1111 |
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1112 If @code{objc_msg_lookup()} does not find a suitable method |
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1113 implementation, because the receiver does not implement the required |
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1114 method, it tries to see if the class can dynamically register the |
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1115 method. |
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1116 |
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1117 To do so, the runtime checks if the class of the receiver implements |
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1118 the method |
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1119 |
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1120 @smallexample |
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1121 + (BOOL) resolveInstanceMethod: (SEL)selector; |
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1122 @end smallexample |
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1123 |
| 111 | 1124 in the case of an instance method, or |
|
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1125 |
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1126 @smallexample |
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1127 + (BOOL) resolveClassMethod: (SEL)selector; |
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1128 @end smallexample |
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1129 |
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1130 in the case of a class method. If the class implements it, the |
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1131 runtime invokes it, passing as argument the selector of the original |
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1132 method, and if it returns @code{YES}, the runtime tries the lookup |
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1133 again, which could now succeed if a matching method was added |
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1134 dynamically by @code{+resolveInstanceMethod:} or |
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1135 @code{+resolveClassMethod:}. |
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1136 |
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1137 This allows classes to dynamically register methods (by adding them to |
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1138 the class using @code{class_addMethod}) when they are first called. |
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1139 To do so, a class should implement @code{+resolveInstanceMethod:} (or, |
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1140 depending on the case, @code{+resolveClassMethod:}) and have it |
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1141 recognize the selectors of methods that can be registered dynamically |
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1142 at runtime, register them, and return @code{YES}. It should return |
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1143 @code{NO} for methods that it does not dynamically registered at |
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1144 runtime. |
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1145 |
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1146 If @code{+resolveInstanceMethod:} (or @code{+resolveClassMethod:}) is |
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1147 not implemented or returns @code{NO}, the runtime then tries the |
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1148 forwarding hook. |
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1149 |
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1150 Support for @code{+resolveInstanceMethod:} and |
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1151 @code{resolveClassMethod:} was added to the GNU Objective-C runtime in |
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1152 GCC version 4.6. |
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1153 |
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1154 @c ========================================================================= |
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1155 @node Forwarding hook |
| 111 | 1156 @subsection Forwarding Hook |
|
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1157 |
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1158 The GNU Objective-C runtime provides a hook, called |
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1159 @code{__objc_msg_forward2}, which is called by |
| 111 | 1160 @code{objc_msg_lookup()} when it cannot find a method implementation in |
|
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1161 the runtime tables and after calling @code{+resolveInstanceMethod:} |
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1162 and @code{+resolveClassMethod:} has been attempted and did not succeed |
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1163 in dynamically registering the method. |
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1164 |
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1165 To configure the hook, you set the global variable |
| 111 | 1166 @code{__objc_msg_forward2} to a function with the same argument and |
|
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1167 return types of @code{objc_msg_lookup()}. When |
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1168 @code{objc_msg_lookup()} can not find a method implementation, it |
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1169 invokes the hook function you provided to get a method implementation |
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1170 to return. So, in practice @code{__objc_msg_forward2} allows you to |
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1171 extend @code{objc_msg_lookup()} by adding some custom code that is |
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1172 called to do a further lookup when no standard method implementation |
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1173 can be found using the normal lookup. |
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1174 |
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1175 This hook is generally reserved for ``Foundation'' libraries such as |
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1176 GNUstep Base, which use it to implement their high-level method |
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1177 forwarding API, typically based around the @code{forwardInvocation:} |
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1178 method. So, unless you are implementing your own ``Foundation'' |
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1179 library, you should not set this hook. |
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1180 |
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1181 In a typical forwarding implementation, the @code{__objc_msg_forward2} |
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1182 hook function determines the argument and return type of the method |
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1183 that is being looked up, and then creates a function that takes these |
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1184 arguments and has that return type, and returns it to the caller. |
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1185 Creating this function is non-trivial and is typically performed using |
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1186 a dedicated library such as @code{libffi}. |
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1187 |
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1188 The forwarding method implementation thus created is returned by |
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1189 @code{objc_msg_lookup()} and is executed as if it was a normal method |
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1190 implementation. When the forwarding method implementation is called, |
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1191 it is usually expected to pack all arguments into some sort of object |
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1192 (typically, an @code{NSInvocation} in a ``Foundation'' library), and |
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1193 hand it over to the programmer (@code{forwardInvocation:}) who is then |
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1194 allowed to manipulate the method invocation using a high-level API |
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1195 provided by the ``Foundation'' library. For example, the programmer |
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1196 may want to examine the method invocation arguments and name and |
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1197 potentially change them before forwarding the method invocation to one |
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1198 or more local objects (@code{performInvocation:}) or even to remote |
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1199 objects (by using Distributed Objects or some other mechanism). When |
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1200 all this completes, the return value is passed back and must be |
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1201 returned correctly to the original caller. |
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1202 |
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1203 Note that the GNU Objective-C runtime currently provides no support |
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1204 for method forwarding or method invocations other than the |
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1205 @code{__objc_msg_forward2} hook. |
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1206 |
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1207 If the forwarding hook does not exist or returns @code{NULL}, the |
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1208 runtime currently attempts forwarding using an older, deprecated API, |
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1209 and if that fails, it aborts the program. In future versions of the |
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1210 GNU Objective-C runtime, the runtime will immediately abort. |