Mercurial > hg > CbC > CbC_gcc
annotate gcc/doc/objc.texi @ 67:f6334be47118
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| author | nobuyasu <dimolto@cr.ie.u-ryukyu.ac.jp> |
|---|---|
| date | Tue, 22 Mar 2011 17:18:12 +0900 |
| parents | a06113de4d67 |
| children | 04ced10e8804 |
| rev | line source |
|---|---|
| 0 | 1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
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2 @c 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2010 |
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3 @c Free Software Foundation, Inc. |
| 0 | 4 @c This is part of the GCC manual. |
| 5 @c For copying conditions, see the file gcc.texi. | |
| 6 | |
| 7 @node Objective-C | |
| 8 @comment node-name, next, previous, up | |
| 9 | |
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10 @chapter GNU Objective-C features |
| 0 | 11 |
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12 This document is meant to describe some of the GNU Objective-C |
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13 features. It is not intended to teach you Objective-C. There are |
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14 several resources on the Internet that present the language. |
| 0 | 15 |
| 16 @menu | |
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17 * GNU Objective-C runtime API:: |
| 0 | 18 * Executing code before main:: |
| 19 * Type encoding:: | |
| 20 * Garbage Collection:: | |
| 21 * Constant string objects:: | |
| 22 * compatibility_alias:: | |
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23 * Exceptions:: |
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24 * Synchronization:: |
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25 * Fast enumeration:: |
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26 * Messaging with the GNU Objective-C runtime:: |
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27 @end menu |
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28 |
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29 @c ========================================================================= |
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30 @node GNU Objective-C runtime API |
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31 @section GNU Objective-C runtime API |
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32 |
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33 This section is specific for the GNU Objective-C runtime. If you are |
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34 using a different runtime, you can skip it. |
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35 |
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36 The GNU Objective-C runtime provides an API that allows you to |
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37 interact with the Objective-C runtime system, querying the live |
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38 runtime structures and even manipulating them. This allows you for |
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39 example to inspect and navigate classes, methods and protocols; to |
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40 define new classes or new methods, and even to modify existing classes |
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41 or protocols. |
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42 |
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43 If you are using a ``Foundation'' library such as GNUstep-Base, this |
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44 library will provide you with a rich set of functionality to do most |
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45 of the inspection tasks, and you probably will only need direct access |
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46 to the GNU Objective-C runtime API to define new classes or methods. |
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47 |
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48 @menu |
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49 * Modern GNU Objective-C runtime API:: |
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50 * Traditional GNU Objective-C runtime API:: |
| 0 | 51 @end menu |
| 52 | |
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53 @c ========================================================================= |
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54 @node Modern GNU Objective-C runtime API |
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55 @subsection Modern GNU Objective-C runtime API |
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56 |
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57 The GNU Objective-C runtime provides an API which is similar to the |
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58 one provided by the ``Objective-C 2.0'' Apple/NeXT Objective-C |
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59 runtime. The API is documented in the public header files of the GNU |
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60 Objective-C runtime: |
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61 |
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62 @itemize @bullet |
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63 |
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64 @item |
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65 @file{objc/objc.h}: this is the basic Objective-C header file, |
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66 defining the basic Objective-C types such as @code{id}, @code{Class} |
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67 and @code{BOOL}. You have to include this header to do almost |
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68 anything with Objective-C. |
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69 |
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70 @item |
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71 @file{objc/runtime.h}: this header declares most of the public runtime |
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72 API functions allowing you to inspect and manipulate the Objective-C |
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73 runtime data structures. These functions are fairly standardized |
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74 across Objective-C runtimes and are almost identical to the Apple/NeXT |
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75 Objective-C runtime ones. It does not declare functions in some |
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76 specialized areas (constructing and forwarding message invocations, |
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77 threading) which are in the other headers below. You have to include |
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78 @file{objc/objc.h} and @file{objc/runtime.h} to use any of the |
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79 functions, such as @code{class_getName()}, declared in |
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80 @file{objc/runtime.h}. |
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81 |
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82 @item |
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83 @file{objc/message.h}: this header declares public functions used to |
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84 construct, deconstruct and forward message invocations. Because |
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85 messaging is done in quite a different way on different runtimes, |
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86 functions in this header are specific to the GNU Objective-C runtime |
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87 implementation. |
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88 |
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89 @item |
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90 @file{objc/objc-exception.h}: this header declares some public |
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91 functions related to Objective-C exceptions. For example functions in |
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92 this header allow you to throw an Objective-C exception from plain |
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93 C/C++ code. |
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94 |
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95 @item |
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96 @file{objc/objc-sync.h}: this header declares some public functions |
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97 related to the Objective-C @code{@@synchronized()} syntax, allowing |
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98 you to emulate an Objective-C @code{@@synchronized()} block in plain |
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99 C/C++ code. |
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100 |
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101 @item |
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102 @file{objc/thr.h}: this header declares a public runtime API threading |
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103 layer that is only provided by the GNU Objective-C runtime. It |
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104 declares functions such as @code{objc_mutex_lock()}, which provide a |
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105 platform-independent set of threading functions. |
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106 |
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107 @end itemize |
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108 |
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109 The header files contain detailed documentation for each function in |
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110 the GNU Objective-C runtime API. |
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111 |
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112 @c ========================================================================= |
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113 @node Traditional GNU Objective-C runtime API |
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114 @subsection Traditional GNU Objective-C runtime API |
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115 |
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116 The GNU Objective-C runtime used to provide a different API, which we |
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117 call the ``traditional'' GNU Objective-C runtime API. Functions |
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118 belonging to this API are easy to recognize because they use a |
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119 different naming convention, such as @code{class_get_super_class()} |
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120 (traditional API) instead of @code{class_getSuperclass()} (modern |
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121 API). Software using this API includes the file |
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122 @file{objc/objc-api.h} where it is declared. |
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123 |
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124 The traditional API is deprecated but it is still supported in this |
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125 release of the runtime; you can access it as usual by including |
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126 @file{objc/objc-api.h}. |
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127 |
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128 If you are using the traditional API you are urged to upgrade your |
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129 software to use the modern API because the traditional API requires |
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130 access to private runtime internals to do anything serious with it; |
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131 for this reason, there is no guarantee that future releases of the GNU |
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132 Objective-C runtime library will be able to provide a fully compatible |
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133 @file{objc/objc-api.h} as the private runtime internals change. It is |
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134 expected that the next release will hide a number of runtime internals |
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135 making the traditional API nominally supported but fairly useless |
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136 beyond very simple use cases. |
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137 |
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138 Finally, you can not include both @file{objc/objc-api.h} and |
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139 @file{objc/runtime.h} at the same time. The traditional and modern |
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140 APIs unfortunately have some conflicting declarations (such as the one |
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141 for @code{Method}) and can not be used at the same time. |
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142 |
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143 @c ========================================================================= |
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144 @node Executing code before main |
| 0 | 145 @section @code{+load}: Executing code before main |
| 146 | |
|
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147 This section is specific for the GNU Objective-C runtime. If you are |
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148 using a different runtime, you can skip it. |
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149 |
| 0 | 150 The GNU Objective-C runtime provides a way that allows you to execute |
| 151 code before the execution of the program enters the @code{main} | |
| 152 function. The code is executed on a per-class and a per-category basis, | |
| 153 through a special class method @code{+load}. | |
| 154 | |
| 155 This facility is very useful if you want to initialize global variables | |
| 156 which can be accessed by the program directly, without sending a message | |
| 157 to the class first. The usual way to initialize global variables, in the | |
| 158 @code{+initialize} method, might not be useful because | |
| 159 @code{+initialize} is only called when the first message is sent to a | |
| 160 class object, which in some cases could be too late. | |
| 161 | |
| 162 Suppose for example you have a @code{FileStream} class that declares | |
| 163 @code{Stdin}, @code{Stdout} and @code{Stderr} as global variables, like | |
| 164 below: | |
| 165 | |
| 166 @smallexample | |
| 167 | |
| 168 FileStream *Stdin = nil; | |
| 169 FileStream *Stdout = nil; | |
| 170 FileStream *Stderr = nil; | |
| 171 | |
| 172 @@implementation FileStream | |
| 173 | |
| 174 + (void)initialize | |
| 175 @{ | |
| 176 Stdin = [[FileStream new] initWithFd:0]; | |
| 177 Stdout = [[FileStream new] initWithFd:1]; | |
| 178 Stderr = [[FileStream new] initWithFd:2]; | |
| 179 @} | |
| 180 | |
| 181 /* @r{Other methods here} */ | |
| 182 @@end | |
| 183 | |
| 184 @end smallexample | |
| 185 | |
| 186 In this example, the initialization of @code{Stdin}, @code{Stdout} and | |
| 187 @code{Stderr} in @code{+initialize} occurs too late. The programmer can | |
| 188 send a message to one of these objects before the variables are actually | |
| 189 initialized, thus sending messages to the @code{nil} object. The | |
| 190 @code{+initialize} method which actually initializes the global | |
| 191 variables is not invoked until the first message is sent to the class | |
| 192 object. The solution would require these variables to be initialized | |
| 193 just before entering @code{main}. | |
| 194 | |
| 195 The correct solution of the above problem is to use the @code{+load} | |
| 196 method instead of @code{+initialize}: | |
| 197 | |
| 198 @smallexample | |
| 199 | |
| 200 @@implementation FileStream | |
| 201 | |
| 202 + (void)load | |
| 203 @{ | |
| 204 Stdin = [[FileStream new] initWithFd:0]; | |
| 205 Stdout = [[FileStream new] initWithFd:1]; | |
| 206 Stderr = [[FileStream new] initWithFd:2]; | |
| 207 @} | |
| 208 | |
| 209 /* @r{Other methods here} */ | |
| 210 @@end | |
| 211 | |
| 212 @end smallexample | |
| 213 | |
| 214 The @code{+load} is a method that is not overridden by categories. If a | |
| 215 class and a category of it both implement @code{+load}, both methods are | |
| 216 invoked. This allows some additional initializations to be performed in | |
| 217 a category. | |
| 218 | |
| 219 This mechanism is not intended to be a replacement for @code{+initialize}. | |
| 220 You should be aware of its limitations when you decide to use it | |
| 221 instead of @code{+initialize}. | |
| 222 | |
| 223 @menu | |
| 224 * What you can and what you cannot do in +load:: | |
| 225 @end menu | |
| 226 | |
| 227 | |
|
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228 @node What you can and what you cannot do in +load |
| 0 | 229 @subsection What you can and what you cannot do in @code{+load} |
| 230 | |
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231 @code{+load} is to be used only as a last resort. Because it is |
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232 executed very early, most of the Objective-C runtime machinery will |
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233 not be ready when @code{+load} is executed; hence @code{+load} works |
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234 best for executing C code that is independent on the Objective-C |
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235 runtime. |
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236 |
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237 The @code{+load} implementation in the GNU runtime guarantees you the |
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238 following things: |
| 0 | 239 |
| 240 @itemize @bullet | |
| 241 | |
| 242 @item | |
| 243 you can write whatever C code you like; | |
| 244 | |
| 245 @item | |
| 246 you can allocate and send messages to objects whose class is implemented | |
| 247 in the same file; | |
| 248 | |
| 249 @item | |
|
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250 the @code{+load} implementation of all super classes of a class are |
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251 executed before the @code{+load} of that class is executed; |
| 0 | 252 |
| 253 @item | |
| 254 the @code{+load} implementation of a class is executed before the | |
| 255 @code{+load} implementation of any category. | |
| 256 | |
| 257 @end itemize | |
| 258 | |
| 259 In particular, the following things, even if they can work in a | |
| 260 particular case, are not guaranteed: | |
| 261 | |
| 262 @itemize @bullet | |
| 263 | |
| 264 @item | |
| 265 allocation of or sending messages to arbitrary objects; | |
| 266 | |
| 267 @item | |
| 268 allocation of or sending messages to objects whose classes have a | |
| 269 category implemented in the same file; | |
| 270 | |
|
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271 @item |
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272 sending messages to Objective-C constant strings (@code{@@"this is a |
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273 constant string"}); |
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274 |
| 0 | 275 @end itemize |
| 276 | |
| 277 You should make no assumptions about receiving @code{+load} in sibling | |
| 278 classes when you write @code{+load} of a class. The order in which | |
| 279 sibling classes receive @code{+load} is not guaranteed. | |
| 280 | |
| 281 The order in which @code{+load} and @code{+initialize} are called could | |
| 282 be problematic if this matters. If you don't allocate objects inside | |
| 283 @code{+load}, it is guaranteed that @code{+load} is called before | |
| 284 @code{+initialize}. If you create an object inside @code{+load} the | |
| 285 @code{+initialize} method of object's class is invoked even if | |
| 286 @code{+load} was not invoked. Note if you explicitly call @code{+load} | |
| 287 on a class, @code{+initialize} will be called first. To avoid possible | |
| 288 problems try to implement only one of these methods. | |
| 289 | |
| 290 The @code{+load} method is also invoked when a bundle is dynamically | |
| 291 loaded into your running program. This happens automatically without any | |
| 292 intervening operation from you. When you write bundles and you need to | |
| 293 write @code{+load} you can safely create and send messages to objects whose | |
| 294 classes already exist in the running program. The same restrictions as | |
| 295 above apply to classes defined in bundle. | |
| 296 | |
| 297 | |
| 298 | |
|
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299 @node Type encoding |
| 0 | 300 @section Type encoding |
| 301 | |
|
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302 This is an advanced section. Type encodings are used extensively by |
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303 the compiler and by the runtime, but you generally do not need to know |
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304 about them to use Objective-C. |
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305 |
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306 The Objective-C compiler generates type encodings for all the types. |
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307 These type encodings are used at runtime to find out information about |
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308 selectors and methods and about objects and classes. |
| 0 | 309 |
| 310 The types are encoded in the following way: | |
| 311 | |
| 312 @c @sp 1 | |
| 313 | |
| 314 @multitable @columnfractions .25 .75 | |
| 315 @item @code{_Bool} | |
| 316 @tab @code{B} | |
| 317 @item @code{char} | |
| 318 @tab @code{c} | |
| 319 @item @code{unsigned char} | |
| 320 @tab @code{C} | |
| 321 @item @code{short} | |
| 322 @tab @code{s} | |
| 323 @item @code{unsigned short} | |
| 324 @tab @code{S} | |
| 325 @item @code{int} | |
| 326 @tab @code{i} | |
| 327 @item @code{unsigned int} | |
| 328 @tab @code{I} | |
| 329 @item @code{long} | |
| 330 @tab @code{l} | |
| 331 @item @code{unsigned long} | |
| 332 @tab @code{L} | |
| 333 @item @code{long long} | |
| 334 @tab @code{q} | |
| 335 @item @code{unsigned long long} | |
| 336 @tab @code{Q} | |
| 337 @item @code{float} | |
| 338 @tab @code{f} | |
| 339 @item @code{double} | |
| 340 @tab @code{d} | |
|
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341 @item @code{long double} |
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342 @tab @code{D} |
| 0 | 343 @item @code{void} |
| 344 @tab @code{v} | |
| 345 @item @code{id} | |
| 346 @tab @code{@@} | |
| 347 @item @code{Class} | |
| 348 @tab @code{#} | |
| 349 @item @code{SEL} | |
| 350 @tab @code{:} | |
| 351 @item @code{char*} | |
| 352 @tab @code{*} | |
|
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353 @item @code{enum} |
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354 @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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355 values. Often the compiler users @code{unsigned int}, which is then encoded as @code{I}. |
| 0 | 356 @item unknown type |
| 357 @tab @code{?} | |
| 358 @item Complex types | |
| 359 @tab @code{j} followed by the inner type. For example @code{_Complex double} is encoded as "jd". | |
| 360 @item bit-fields | |
| 361 @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) | |
| 362 @end multitable | |
| 363 | |
| 364 @c @sp 1 | |
| 365 | |
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366 The encoding of bit-fields has changed to allow bit-fields to be |
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367 properly handled by the runtime functions that compute sizes and |
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368 alignments of types that contain bit-fields. The previous encoding |
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369 contained only the size of the bit-field. Using only this information |
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370 it is not possible to reliably compute the size occupied by the |
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371 bit-field. This is very important in the presence of the Boehm's |
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372 garbage collector because the objects are allocated using the typed |
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373 memory facility available in this collector. The typed memory |
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374 allocation requires information about where the pointers are located |
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375 inside the object. |
| 0 | 376 |
| 377 The position in the bit-field is the position, counting in bits, of the | |
| 378 bit closest to the beginning of the structure. | |
| 379 | |
| 380 The non-atomic types are encoded as follows: | |
| 381 | |
| 382 @c @sp 1 | |
| 383 | |
| 384 @multitable @columnfractions .2 .8 | |
| 385 @item pointers | |
| 386 @tab @samp{^} followed by the pointed type. | |
| 387 @item arrays | |
| 388 @tab @samp{[} followed by the number of elements in the array followed by the type of the elements followed by @samp{]} | |
| 389 @item structures | |
| 390 @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{@}} | |
| 391 @item unions | |
| 392 @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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393 @item vectors |
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394 @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 | 395 @end multitable |
| 396 | |
| 397 Here are some types and their encodings, as they are generated by the | |
| 398 compiler on an i386 machine: | |
| 399 | |
| 400 @sp 1 | |
| 401 | |
| 402 @multitable @columnfractions .25 .75 | |
| 403 @item Objective-C type | |
| 404 @tab Compiler encoding | |
| 405 @item | |
| 406 @smallexample | |
| 407 int a[10]; | |
| 408 @end smallexample | |
| 409 @tab @code{[10i]} | |
| 410 @item | |
| 411 @smallexample | |
| 412 struct @{ | |
| 413 int i; | |
| 414 float f[3]; | |
| 415 int a:3; | |
| 416 int b:2; | |
| 417 char c; | |
| 418 @} | |
| 419 @end smallexample | |
| 420 @tab @code{@{?=i[3f]b128i3b131i2c@}} | |
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421 @item |
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422 @smallexample |
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423 int a __attribute__ ((vector_size (16))); |
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424 @end smallexample |
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425 @tab @code{![16,16i]} (alignment would depend on the machine) |
| 0 | 426 @end multitable |
| 427 | |
| 428 @sp 1 | |
| 429 | |
| 430 In addition to the types the compiler also encodes the type | |
| 431 specifiers. The table below describes the encoding of the current | |
| 432 Objective-C type specifiers: | |
| 433 | |
| 434 @sp 1 | |
| 435 | |
| 436 @multitable @columnfractions .25 .75 | |
| 437 @item Specifier | |
| 438 @tab Encoding | |
| 439 @item @code{const} | |
| 440 @tab @code{r} | |
| 441 @item @code{in} | |
| 442 @tab @code{n} | |
| 443 @item @code{inout} | |
| 444 @tab @code{N} | |
| 445 @item @code{out} | |
| 446 @tab @code{o} | |
| 447 @item @code{bycopy} | |
| 448 @tab @code{O} | |
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449 @item @code{byref} |
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450 @tab @code{R} |
| 0 | 451 @item @code{oneway} |
| 452 @tab @code{V} | |
| 453 @end multitable | |
| 454 | |
| 455 @sp 1 | |
| 456 | |
| 457 The type specifiers are encoded just before the type. Unlike types | |
| 458 however, the type specifiers are only encoded when they appear in method | |
| 459 argument types. | |
| 460 | |
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461 Note how @code{const} interacts with pointers: |
| 0 | 462 |
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463 @sp 1 |
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464 |
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465 @multitable @columnfractions .25 .75 |
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466 @item Objective-C type |
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467 @tab Compiler encoding |
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468 @item |
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469 @smallexample |
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470 const int |
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471 @end smallexample |
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472 @tab @code{ri} |
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473 @item |
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474 @smallexample |
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475 const int* |
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476 @end smallexample |
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477 @tab @code{^ri} |
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478 @item |
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479 @smallexample |
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480 int *const |
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481 @end smallexample |
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482 @tab @code{r^i} |
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483 @end multitable |
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484 |
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485 @sp 1 |
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486 |
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487 @code{const int*} is a pointer to a @code{const int}, and so is |
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488 encoded as @code{^ri}. @code{int* const}, instead, is a @code{const} |
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489 pointer to an @code{int}, and so is encoded as @code{r^i}. |
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490 |
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491 Finally, there is a complication when encoding @code{const char *} |
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492 versus @code{char * const}. Because @code{char *} is encoded as |
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493 @code{*} and not as @code{^c}, there is no way to express the fact |
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494 that @code{r} applies to the pointer or to the pointee. |
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495 |
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496 Hence, it is assumed as a convention that @code{r*} means @code{const |
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497 char *} (since it is what is most often meant), and there is no way to |
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498 encode @code{char *const}. @code{char *const} would simply be encoded |
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499 as @code{*}, and the @code{const} is lost. |
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500 |
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501 @menu |
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502 * Legacy type encoding:: |
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503 * @@encode:: |
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504 * Method signatures:: |
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505 @end menu |
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506 |
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507 @node Legacy type encoding |
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508 @subsection Legacy type encoding |
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509 |
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510 Unfortunately, historically GCC used to have a number of bugs in its |
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511 encoding code. The NeXT runtime expects GCC to emit type encodings in |
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512 this historical format (compatible with GCC-3.3), so when using the |
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513 NeXT runtime, GCC will introduce on purpose a number of incorrect |
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514 encodings: |
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515 |
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516 @itemize @bullet |
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517 |
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518 @item |
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519 the read-only qualifier of the pointee gets emitted before the '^'. |
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520 The read-only qualifier of the pointer itself gets ignored, unless it |
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521 is a typedef. Also, the 'r' is only emitted for the outermost type. |
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522 |
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523 @item |
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524 32-bit longs are encoded as 'l' or 'L', but not always. For typedefs, |
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525 the compiler uses 'i' or 'I' instead if encoding a struct field or a |
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526 pointer. |
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527 |
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528 @item |
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529 @code{enum}s are always encoded as 'i' (int) even if they are actually |
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530 unsigned or long. |
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531 |
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532 @end itemize |
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533 |
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534 In addition to that, the NeXT runtime uses a different encoding for |
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535 bitfields. It encodes them as @code{b} followed by the size, without |
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536 a bit offset or the underlying field type. |
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537 |
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538 @node @@encode |
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539 @subsection @@encode |
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540 |
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541 GNU Objective-C supports the @code{@@encode} syntax that allows you to |
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542 create a type encoding from a C/Objective-C type. For example, |
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543 @code{@@encode(int)} is compiled by the compiler into @code{"i"}. |
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544 |
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545 @code{@@encode} does not support type qualifiers other than |
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546 @code{const}. For example, @code{@@encode(const char*)} is valid and |
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547 is compiled into @code{"r*"}, while @code{@@encode(bycopy char *)} is |
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548 invalid and will cause a compilation error. |
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549 |
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550 @node Method signatures |
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551 @subsection Method signatures |
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552 |
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553 This section documents the encoding of method types, which is rarely |
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554 needed to use Objective-C. You should skip it at a first reading; the |
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555 runtime provides functions that will work on methods and can walk |
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556 through the list of parameters and interpret them for you. These |
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557 functions are part of the public ``API'' and are the preferred way to |
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558 interact with method signatures from user code. |
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559 |
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560 But if you need to debug a problem with method signatures and need to |
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561 know how they are implemented (i.e., the ``ABI''), read on. |
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562 |
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563 Methods have their ``signature'' encoded and made available to the |
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564 runtime. The ``signature'' encodes all the information required to |
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565 dynamically build invocations of the method at runtime: return type |
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566 and arguments. |
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567 |
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568 The ``signature'' is a null-terminated string, composed of the following: |
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569 |
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570 @itemize @bullet |
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571 |
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572 @item |
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573 The return type, including type qualifiers. For example, a method |
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574 returning @code{int} would have @code{i} here. |
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575 |
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576 @item |
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577 The total size (in bytes) required to pass all the parameters. This |
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578 includes the two hidden parameters (the object @code{self} and the |
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579 method selector @code{_cmd}). |
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580 |
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581 @item |
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582 Each argument, with the type encoding, followed by the offset (in |
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583 bytes) of the argument in the list of parameters. |
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584 |
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585 @end itemize |
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586 |
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587 For example, a method with no arguments and returning @code{int} would |
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588 have the signature @code{i8@@0:4} if the size of a pointer is 4. The |
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589 signature is interpreted as follows: the @code{i} is the return type |
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590 (an @code{int}), the @code{8} is the total size of the parameters in |
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591 bytes (two pointers each of size 4), the @code{@@0} is the first |
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592 parameter (an object at byte offset @code{0}) and @code{:4} is the |
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593 second parameter (a @code{SEL} at byte offset @code{4}). |
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594 |
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595 You can easily find more examples by running the ``strings'' program |
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596 on an Objective-C object file compiled by GCC. You'll see a lot of |
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597 strings that look very much like @code{i8@@0:4}. They are signatures |
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598 of Objective-C methods. |
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599 |
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600 |
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601 @node Garbage Collection |
| 0 | 602 @section Garbage Collection |
| 603 | |
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604 This section is specific for the GNU Objective-C runtime. If you are |
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605 using a different runtime, you can skip it. |
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606 |
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607 Support for garbage collection with the GNU runtime has been added by |
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608 using a powerful conservative garbage collector, known as the |
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609 Boehm-Demers-Weiser conservative garbage collector. |
| 0 | 610 |
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611 To enable the support for it you have to configure the compiler using |
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612 an additional argument, @w{@option{--enable-objc-gc}}. This will |
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613 build the boehm-gc library, and build an additional runtime library |
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614 which has several enhancements to support the garbage collector. The |
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615 new library has a new name, @file{libobjc_gc.a} to not conflict with |
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616 the non-garbage-collected library. |
| 0 | 617 |
| 618 When the garbage collector is used, the objects are allocated using the | |
| 619 so-called typed memory allocation mechanism available in the | |
| 620 Boehm-Demers-Weiser collector. This mode requires precise information on | |
| 621 where pointers are located inside objects. This information is computed | |
| 622 once per class, immediately after the class has been initialized. | |
| 623 | |
| 624 There is a new runtime function @code{class_ivar_set_gcinvisible()} | |
| 625 which can be used to declare a so-called @dfn{weak pointer} | |
| 626 reference. Such a pointer is basically hidden for the garbage collector; | |
| 627 this can be useful in certain situations, especially when you want to | |
| 628 keep track of the allocated objects, yet allow them to be | |
| 629 collected. This kind of pointers can only be members of objects, you | |
| 630 cannot declare a global pointer as a weak reference. Every type which is | |
| 631 a pointer type can be declared a weak pointer, including @code{id}, | |
| 632 @code{Class} and @code{SEL}. | |
| 633 | |
| 634 Here is an example of how to use this feature. Suppose you want to | |
| 635 implement a class whose instances hold a weak pointer reference; the | |
| 636 following class does this: | |
| 637 | |
| 638 @smallexample | |
| 639 | |
| 640 @@interface WeakPointer : Object | |
| 641 @{ | |
| 642 const void* weakPointer; | |
| 643 @} | |
| 644 | |
| 645 - initWithPointer:(const void*)p; | |
| 646 - (const void*)weakPointer; | |
| 647 @@end | |
| 648 | |
| 649 | |
| 650 @@implementation WeakPointer | |
| 651 | |
| 652 + (void)initialize | |
| 653 @{ | |
| 654 class_ivar_set_gcinvisible (self, "weakPointer", YES); | |
| 655 @} | |
| 656 | |
| 657 - initWithPointer:(const void*)p | |
| 658 @{ | |
| 659 weakPointer = p; | |
| 660 return self; | |
| 661 @} | |
| 662 | |
| 663 - (const void*)weakPointer | |
| 664 @{ | |
| 665 return weakPointer; | |
| 666 @} | |
| 667 | |
| 668 @@end | |
| 669 | |
| 670 @end smallexample | |
| 671 | |
| 672 Weak pointers are supported through a new type character specifier | |
| 673 represented by the @samp{!} character. The | |
| 674 @code{class_ivar_set_gcinvisible()} function adds or removes this | |
| 675 specifier to the string type description of the instance variable named | |
| 676 as argument. | |
| 677 | |
| 678 @c ========================================================================= | |
| 679 @node Constant string objects | |
| 680 @section Constant string objects | |
| 681 | |
| 682 GNU Objective-C provides constant string objects that are generated | |
| 683 directly by the compiler. You declare a constant string object by | |
| 684 prefixing a C constant string with the character @samp{@@}: | |
| 685 | |
| 686 @smallexample | |
| 687 id myString = @@"this is a constant string object"; | |
| 688 @end smallexample | |
| 689 | |
| 690 The constant string objects are by default instances of the | |
| 691 @code{NXConstantString} class which is provided by the GNU Objective-C | |
| 692 runtime. To get the definition of this class you must include the | |
| 693 @file{objc/NXConstStr.h} header file. | |
| 694 | |
| 695 User defined libraries may want to implement their own constant string | |
| 696 class. To be able to support them, the GNU Objective-C compiler provides | |
| 697 a new command line options @option{-fconstant-string-class=@var{class-name}}. | |
| 698 The provided class should adhere to a strict structure, the same | |
| 699 as @code{NXConstantString}'s structure: | |
| 700 | |
| 701 @smallexample | |
| 702 | |
| 703 @@interface MyConstantStringClass | |
| 704 @{ | |
| 705 Class isa; | |
| 706 char *c_string; | |
| 707 unsigned int len; | |
| 708 @} | |
| 709 @@end | |
| 710 | |
| 711 @end smallexample | |
| 712 | |
| 713 @code{NXConstantString} inherits from @code{Object}; user class | |
| 714 libraries may choose to inherit the customized constant string class | |
| 715 from a different class than @code{Object}. There is no requirement in | |
| 716 the methods the constant string class has to implement, but the final | |
| 717 ivar layout of the class must be the compatible with the given | |
| 718 structure. | |
| 719 | |
| 720 When the compiler creates the statically allocated constant string | |
| 721 object, the @code{c_string} field will be filled by the compiler with | |
| 722 the string; the @code{length} field will be filled by the compiler with | |
| 723 the string length; the @code{isa} pointer will be filled with | |
| 724 @code{NULL} by the compiler, and it will later be fixed up automatically | |
| 725 at runtime by the GNU Objective-C runtime library to point to the class | |
| 726 which was set by the @option{-fconstant-string-class} option when the | |
| 727 object file is loaded (if you wonder how it works behind the scenes, the | |
| 728 name of the class to use, and the list of static objects to fixup, are | |
| 729 stored by the compiler in the object file in a place where the GNU | |
| 730 runtime library will find them at runtime). | |
| 731 | |
| 732 As a result, when a file is compiled with the | |
| 733 @option{-fconstant-string-class} option, all the constant string objects | |
| 734 will be instances of the class specified as argument to this option. It | |
| 735 is possible to have multiple compilation units referring to different | |
| 736 constant string classes, neither the compiler nor the linker impose any | |
| 737 restrictions in doing this. | |
| 738 | |
| 739 @c ========================================================================= | |
| 740 @node compatibility_alias | |
| 741 @section compatibility_alias | |
| 742 | |
| 743 The keyword @code{@@compatibility_alias} allows you to define a class name | |
| 744 as equivalent to another class name. For example: | |
| 745 | |
| 746 @smallexample | |
| 747 @@compatibility_alias WOApplication GSWApplication; | |
| 748 @end smallexample | |
| 749 | |
| 750 tells the compiler that each time it encounters @code{WOApplication} as | |
| 751 a class name, it should replace it with @code{GSWApplication} (that is, | |
| 752 @code{WOApplication} is just an alias for @code{GSWApplication}). | |
| 753 | |
| 754 There are some constraints on how this can be used--- | |
| 755 | |
| 756 @itemize @bullet | |
| 757 | |
| 758 @item @code{WOApplication} (the alias) must not be an existing class; | |
| 759 | |
| 760 @item @code{GSWApplication} (the real class) must be an existing class. | |
| 761 | |
| 762 @end itemize | |
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763 |
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764 @c ========================================================================= |
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765 @node Exceptions |
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766 @section Exceptions |
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767 |
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768 GNU Objective-C provides exception support built into the language, as |
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769 in the following example: |
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770 |
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771 @smallexample |
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772 @@try @{ |
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773 @dots{} |
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774 @@throw expr; |
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775 @dots{} |
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776 @} |
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777 @@catch (AnObjCClass *exc) @{ |
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778 @dots{} |
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779 @@throw expr; |
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780 @dots{} |
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781 @@throw; |
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782 @dots{} |
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783 @} |
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784 @@catch (AnotherClass *exc) @{ |
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785 @dots{} |
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786 @} |
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787 @@catch (id allOthers) @{ |
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788 @dots{} |
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789 @} |
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790 @@finally @{ |
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791 @dots{} |
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792 @@throw expr; |
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793 @dots{} |
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794 @} |
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795 @end smallexample |
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796 |
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797 The @code{@@throw} statement may appear anywhere in an Objective-C or |
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798 Objective-C++ program; when used inside of a @code{@@catch} block, the |
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799 @code{@@throw} may appear without an argument (as shown above), in |
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800 which case the object caught by the @code{@@catch} will be rethrown. |
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801 |
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802 Note that only (pointers to) Objective-C objects may be thrown and |
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803 caught using this scheme. When an object is thrown, it will be caught |
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804 by the nearest @code{@@catch} clause capable of handling objects of |
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805 that type, analogously to how @code{catch} blocks work in C++ and |
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806 Java. A @code{@@catch(id @dots{})} clause (as shown above) may also |
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807 be provided to catch any and all Objective-C exceptions not caught by |
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808 previous @code{@@catch} clauses (if any). |
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809 |
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810 The @code{@@finally} clause, if present, will be executed upon exit |
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811 from the immediately preceding @code{@@try @dots{} @@catch} section. |
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812 This will happen regardless of whether any exceptions are thrown, |
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813 caught or rethrown inside the @code{@@try @dots{} @@catch} section, |
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814 analogously to the behavior of the @code{finally} clause in Java. |
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815 |
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816 There are several caveats to using the new exception mechanism: |
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817 |
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818 @itemize @bullet |
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819 @item |
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820 The @option{-fobjc-exceptions} command line option must be used when |
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821 compiling Objective-C files that use exceptions. |
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822 |
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823 @item |
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824 With the GNU runtime, exceptions are always implemented as ``native'' |
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825 exceptions and it is recommended that the @option{-fexceptions} and |
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826 @option{-shared-libgcc} options are used when linking. |
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827 |
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828 @item |
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829 With the NeXT runtime, although currently designed to be binary |
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830 compatible with @code{NS_HANDLER}-style idioms provided by the |
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831 @code{NSException} class, the new exceptions can only be used on Mac |
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832 OS X 10.3 (Panther) and later systems, due to additional functionality |
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833 needed in the NeXT Objective-C runtime. |
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834 |
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835 @item |
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836 As mentioned above, the new exceptions do not support handling |
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837 types other than Objective-C objects. Furthermore, when used from |
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838 Objective-C++, the Objective-C exception model does not interoperate with C++ |
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839 exceptions at this time. This means you cannot @code{@@throw} an exception |
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840 from Objective-C and @code{catch} it in C++, or vice versa |
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841 (i.e., @code{throw @dots{} @@catch}). |
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842 @end itemize |
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843 |
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844 @c ========================================================================= |
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845 @node Synchronization |
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846 @section Synchronization |
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847 |
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848 GNU Objective-C provides support for synchronized blocks: |
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849 |
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850 @smallexample |
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851 @@synchronized (ObjCClass *guard) @{ |
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852 @dots{} |
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853 @} |
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854 @end smallexample |
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855 |
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856 Upon entering the @code{@@synchronized} block, a thread of execution |
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857 shall first check whether a lock has been placed on the corresponding |
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858 @code{guard} object by another thread. If it has, the current thread |
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859 shall wait until the other thread relinquishes its lock. Once |
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860 @code{guard} becomes available, the current thread will place its own |
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861 lock on it, execute the code contained in the @code{@@synchronized} |
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862 block, and finally relinquish the lock (thereby making @code{guard} |
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863 available to other threads). |
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864 |
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865 Unlike Java, Objective-C does not allow for entire methods to be |
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866 marked @code{@@synchronized}. Note that throwing exceptions out of |
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867 @code{@@synchronized} blocks is allowed, and will cause the guarding |
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868 object to be unlocked properly. |
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869 |
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870 Because of the interactions between synchronization and exception |
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871 handling, you can only use @code{@@synchronized} when compiling with |
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872 exceptions enabled, that is with the command line option |
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873 @option{-fobjc-exceptions}. |
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874 |
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875 |
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876 @c ========================================================================= |
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877 @node Fast enumeration |
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878 @section Fast enumeration |
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879 |
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880 @menu |
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881 * Using fast enumeration:: |
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882 * c99-like fast enumeration syntax:: |
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883 * Fast enumeration details:: |
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884 * Fast enumeration protocol:: |
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885 @end menu |
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886 |
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887 @c ================================ |
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888 @node Using fast enumeration |
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889 @subsection Using fast enumeration |
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890 |
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891 GNU Objective-C provides support for the fast enumeration syntax: |
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892 |
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893 @smallexample |
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894 id array = @dots{}; |
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895 id object; |
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896 |
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897 for (object in array) |
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898 @{ |
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899 /* Do something with 'object' */ |
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900 @} |
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901 @end smallexample |
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902 |
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903 @code{array} needs to be an Objective-C object (usually a collection |
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904 object, for example an array, a dictionary or a set) which implements |
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905 the ``Fast Enumeration Protocol'' (see below). If you are using a |
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906 Foundation library such as GNUstep Base or Apple Cocoa Foundation, all |
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907 collection objects in the library implement this protocol and can be |
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908 used in this way. |
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909 |
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910 The code above would iterate over all objects in @code{array}. For |
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911 each of them, it assigns it to @code{object}, then executes the |
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912 @code{Do something with 'object'} statements. |
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913 |
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914 Here is a fully worked-out example using a Foundation library (which |
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915 provides the implementation of @code{NSArray}, @code{NSString} and |
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916 @code{NSLog}): |
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917 |
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918 @smallexample |
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919 NSArray *array = [NSArray arrayWithObjects: @@"1", @@"2", @@"3", nil]; |
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920 NSString *object; |
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921 |
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922 for (object in array) |
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923 NSLog (@@"Iterating over %@@", object); |
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924 @end smallexample |
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925 |
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926 |
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927 @c ================================ |
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928 @node c99-like fast enumeration syntax |
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929 @subsection c99-like fast enumeration syntax |
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930 |
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931 A c99-like declaration syntax is also allowed: |
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932 |
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933 @smallexample |
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934 id array = @dots{}; |
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935 |
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936 for (id object in array) |
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937 @{ |
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938 /* Do something with 'object' */ |
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939 @} |
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940 @end smallexample |
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941 |
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942 this is completely equivalent to: |
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943 |
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944 @smallexample |
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945 id array = @dots{}; |
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946 |
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947 @{ |
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948 id object; |
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949 for (object in array) |
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950 @{ |
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951 /* Do something with 'object' */ |
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952 @} |
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953 @} |
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954 @end smallexample |
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955 |
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956 but can save some typing. |
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957 |
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958 Note that the option @option{-std=c99} is not required to allow this |
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959 syntax in Objective-C. |
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960 |
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961 @c ================================ |
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962 @node Fast enumeration details |
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963 @subsection Fast enumeration details |
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964 |
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965 Here is a more technical description with the gory details. Consider the code |
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966 |
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967 @smallexample |
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968 for (@var{object expression} in @var{collection expression}) |
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969 @{ |
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970 @var{statements} |
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971 @} |
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972 @end smallexample |
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973 |
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974 here is what happens when you run it: |
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975 |
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976 @itemize @bullet |
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977 @item |
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978 @code{@var{collection expression}} is evaluated exactly once and the |
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979 result is used as the collection object to iterate over. This means |
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980 it is safe to write code such as @code{for (object in [NSDictionary |
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981 keyEnumerator]) @dots{}}. |
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982 |
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983 @item |
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984 the iteration is implemented by the compiler by repeatedly getting |
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985 batches of objects from the collection object using the fast |
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986 enumeration protocol (see below), then iterating over all objects in |
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987 the batch. This is faster than a normal enumeration where objects are |
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988 retrieved one by one (hence the name ``fast enumeration''). |
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989 |
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990 @item |
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991 if there are no objects in the collection, then |
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992 @code{@var{object expression}} is set to @code{nil} and the loop |
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993 immediately terminates. |
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994 |
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995 @item |
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996 if there are objects in the collection, then for each object in the |
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997 collection (in the order they are returned) @code{@var{object expression}} |
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998 is set to the object, then @code{@var{statements}} are executed. |
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999 |
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1000 @item |
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1001 @code{@var{statements}} can contain @code{break} and @code{continue} |
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1002 commands, which will abort the iteration or skip to the next loop |
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1003 iteration as expected. |
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1004 |
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1005 @item |
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1006 when the iteration ends because there are no more objects to iterate |
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1007 over, @code{@var{object expression}} is set to @code{nil}. This allows |
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1008 you to determine whether the iteration finished because a @code{break} |
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1009 command was used (in which case @code{@var{object expression}} will remain |
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1010 set to the last object that was iterated over) or because it iterated |
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1011 over all the objects (in which case @code{@var{object expression}} will be |
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1012 set to @code{nil}). |
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1013 |
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1014 @item |
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1015 @code{@var{statements}} must not make any changes to the collection |
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1016 object; if they do, it is a hard error and the fast enumeration |
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1017 terminates by invoking @code{objc_enumerationMutation}, a runtime |
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1018 function that normally aborts the program but which can be customized |
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1019 by Foundation libraries via @code{objc_set_mutation_handler} to do |
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1020 something different, such as raising an exception. |
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1021 |
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1022 @end itemize |
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1023 |
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1024 @c ================================ |
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1025 @node Fast enumeration protocol |
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1026 @subsection Fast enumeration protocol |
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1027 |
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1028 If you want your own collection object to be usable with fast |
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1029 enumeration, you need to have it implement the method |
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1030 |
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1031 @smallexample |
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1032 - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state |
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1033 objects: (id *)objects |
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1034 count: (unsigned long)len; |
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1035 @end smallexample |
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1036 |
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1037 where @code{NSFastEnumerationState} must be defined in your code as follows: |
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1038 |
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1039 @smallexample |
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1040 typedef struct |
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1041 @{ |
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1042 unsigned long state; |
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1043 id *itemsPtr; |
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1044 unsigned long *mutationsPtr; |
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1045 unsigned long extra[5]; |
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1046 @} NSFastEnumerationState; |
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1047 @end smallexample |
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1048 |
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1049 If no @code{NSFastEnumerationState} is defined in your code, the |
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1050 compiler will automatically replace @code{NSFastEnumerationState *} |
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1051 with @code{struct __objcFastEnumerationState *}, where that type is |
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1052 silently defined by the compiler in an identical way. This can be |
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1053 confusing and we recommend that you define |
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1054 @code{NSFastEnumerationState} (as shown above) instead. |
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1055 |
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1056 The method is called repeatedly during a fast enumeration to retrieve |
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1057 batches of objects. Each invocation of the method should retrieve the |
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1058 next batch of objects. |
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1059 |
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1060 The return value of the method is the number of objects in the current |
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1061 batch; this should not exceed @code{len}, which is the maximum size of |
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1062 a batch as requested by the caller. The batch itself is returned in |
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1063 the @code{itemsPtr} field of the @code{NSFastEnumerationState} struct. |
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1064 |
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1065 To help with returning the objects, the @code{objects} array is a C |
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1066 array preallocated by the caller (on the stack) of size @code{len}. |
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1067 In many cases you can put the objects you want to return in that |
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1068 @code{objects} array, then do @code{itemsPtr = objects}. But you |
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1069 don't have to; if your collection already has the objects to return in |
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1070 some form of C array, it could return them from there instead. |
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1071 |
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1072 The @code{state} and @code{extra} fields of the |
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1073 @code{NSFastEnumerationState} structure allows your collection object |
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1074 to keep track of the state of the enumeration. In a simple array |
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1075 implementation, @code{state} may keep track of the index of the last |
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1076 object that was returned, and @code{extra} may be unused. |
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1077 |
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1078 The @code{mutationsPtr} field of the @code{NSFastEnumerationState} is |
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1079 used to keep track of mutations. It should point to a number; before |
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1080 working on each object, the fast enumeration loop will check that this |
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1081 number has not changed. If it has, a mutation has happened and the |
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1082 fast enumeration will abort. So, @code{mutationsPtr} could be set to |
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1083 point to some sort of version number of your collection, which is |
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1084 increased by one every time there is a change (for example when an |
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1085 object is added or removed). Or, if you are content with less strict |
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1086 mutation checks, it could point to the number of objects in your |
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1087 collection or some other value that can be checked to perform an |
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1088 approximate check that the collection has not been mutated. |
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1089 |
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1090 Finally, note how we declared the @code{len} argument and the return |
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1091 value to be of type @code{unsigned long}. They could also be declared |
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1092 to be of type @code{unsigned int} and everything would still work. |
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1093 |
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1094 @c ========================================================================= |
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1095 @node Messaging with the GNU Objective-C runtime |
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1096 @section Messaging with the GNU Objective-C runtime |
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1097 |
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1098 This section is specific for the GNU Objective-C runtime. If you are |
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1099 using a different runtime, you can skip it. |
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1100 |
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1101 The implementation of messaging in the GNU Objective-C runtime is |
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1102 designed to be portable, and so is based on standard C. |
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1103 |
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1104 Sending a message in the GNU Objective-C runtime is composed of two |
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1105 separate steps. First, there is a call to the lookup function, |
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1106 @code{objc_msg_lookup ()} (or, in the case of messages to super, |
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1107 @code{objc_msg_lookup_super ()}). This runtime function takes as |
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1108 argument the receiver and the selector of the method to be called; it |
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1109 returns the @code{IMP}, that is a pointer to the function implementing |
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1110 the method. The second step of method invocation consists of casting |
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1111 this pointer function to the appropriate function pointer type, and |
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1112 calling the function pointed to it with the right arguments. |
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1113 |
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1114 For example, when the compiler encounters a method invocation such as |
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1115 @code{[object init]}, it compiles it into a call to |
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1116 @code{objc_msg_lookup (object, @@selector(init))} followed by a cast |
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1117 of the returned value to the appropriate function pointer type, and |
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1118 then it calls it. |
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1119 |
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1120 @menu |
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1121 * Dynamically registering methods:: |
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1122 * Forwarding hook:: |
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1123 @end menu |
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1124 |
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1125 @c ========================================================================= |
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1126 @node Dynamically registering methods |
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1127 @subsection Dynamically registering methods |
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1128 |
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1129 If @code{objc_msg_lookup()} does not find a suitable method |
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1130 implementation, because the receiver does not implement the required |
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1131 method, it tries to see if the class can dynamically register the |
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1132 method. |
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1133 |
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1134 To do so, the runtime checks if the class of the receiver implements |
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1135 the method |
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1136 |
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1137 @smallexample |
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1138 + (BOOL) resolveInstanceMethod: (SEL)selector; |
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1139 @end smallexample |
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1140 |
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1141 in the case of an instance method, or |
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1142 |
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1143 @smallexample |
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1144 + (BOOL) resolveClassMethod: (SEL)selector; |
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1145 @end smallexample |
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1146 |
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1147 in the case of a class method. If the class implements it, the |
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1148 runtime invokes it, passing as argument the selector of the original |
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1149 method, and if it returns @code{YES}, the runtime tries the lookup |
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1150 again, which could now succeed if a matching method was added |
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1151 dynamically by @code{+resolveInstanceMethod:} or |
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1152 @code{+resolveClassMethod:}. |
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1153 |
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1154 This allows classes to dynamically register methods (by adding them to |
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1155 the class using @code{class_addMethod}) when they are first called. |
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1156 To do so, a class should implement @code{+resolveInstanceMethod:} (or, |
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1157 depending on the case, @code{+resolveClassMethod:}) and have it |
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1158 recognize the selectors of methods that can be registered dynamically |
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1159 at runtime, register them, and return @code{YES}. It should return |
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1160 @code{NO} for methods that it does not dynamically registered at |
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1161 runtime. |
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1162 |
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1163 If @code{+resolveInstanceMethod:} (or @code{+resolveClassMethod:}) is |
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1164 not implemented or returns @code{NO}, the runtime then tries the |
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1165 forwarding hook. |
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1166 |
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1167 Support for @code{+resolveInstanceMethod:} and |
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1168 @code{resolveClassMethod:} was added to the GNU Objective-C runtime in |
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1169 GCC version 4.6. |
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1170 |
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1171 @c ========================================================================= |
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1172 @node Forwarding hook |
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1173 @subsection Forwarding hook |
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1174 |
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1175 The GNU Objective-C runtime provides a hook, called |
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1176 @code{__objc_msg_forward2}, which is called by |
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1177 @code{objc_msg_lookup()} when it can't find a method implementation in |
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1178 the runtime tables and after calling @code{+resolveInstanceMethod:} |
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1179 and @code{+resolveClassMethod:} has been attempted and did not succeed |
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1180 in dynamically registering the method. |
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1181 |
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1182 To configure the hook, you set the global variable |
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1183 @code{__objc_msg_foward2} to a function with the same argument and |
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1184 return types of @code{objc_msg_lookup()}. When |
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1185 @code{objc_msg_lookup()} can not find a method implementation, it |
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1186 invokes the hook function you provided to get a method implementation |
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1187 to return. So, in practice @code{__objc_msg_forward2} allows you to |
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1188 extend @code{objc_msg_lookup()} by adding some custom code that is |
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1189 called to do a further lookup when no standard method implementation |
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1190 can be found using the normal lookup. |
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1191 |
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1192 This hook is generally reserved for ``Foundation'' libraries such as |
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1193 GNUstep Base, which use it to implement their high-level method |
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1194 forwarding API, typically based around the @code{forwardInvocation:} |
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1195 method. So, unless you are implementing your own ``Foundation'' |
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1196 library, you should not set this hook. |
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1197 |
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1198 In a typical forwarding implementation, the @code{__objc_msg_forward2} |
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1199 hook function determines the argument and return type of the method |
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1200 that is being looked up, and then creates a function that takes these |
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1201 arguments and has that return type, and returns it to the caller. |
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1202 Creating this function is non-trivial and is typically performed using |
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1203 a dedicated library such as @code{libffi}. |
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1204 |
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1205 The forwarding method implementation thus created is returned by |
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1206 @code{objc_msg_lookup()} and is executed as if it was a normal method |
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1207 implementation. When the forwarding method implementation is called, |
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1208 it is usually expected to pack all arguments into some sort of object |
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1209 (typically, an @code{NSInvocation} in a ``Foundation'' library), and |
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1210 hand it over to the programmer (@code{forwardInvocation:}) who is then |
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1211 allowed to manipulate the method invocation using a high-level API |
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1212 provided by the ``Foundation'' library. For example, the programmer |
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1213 may want to examine the method invocation arguments and name and |
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1214 potentially change them before forwarding the method invocation to one |
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1215 or more local objects (@code{performInvocation:}) or even to remote |
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1216 objects (by using Distributed Objects or some other mechanism). When |
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1217 all this completes, the return value is passed back and must be |
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1218 returned correctly to the original caller. |
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1219 |
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1220 Note that the GNU Objective-C runtime currently provides no support |
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1221 for method forwarding or method invocations other than the |
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1222 @code{__objc_msg_forward2} hook. |
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1223 |
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1224 If the forwarding hook does not exist or returns @code{NULL}, the |
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1225 runtime currently attempts forwarding using an older, deprecated API, |
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1226 and if that fails, it aborts the program. In future versions of the |
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1227 GNU Objective-C runtime, the runtime will immediately abort. |