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Patrick Beardab8f4da2012-04-19 01:30:47 +0000[diff] [blame]7 <title>Objective-C Literals</title>
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22
23<h1>Objective-C Literals</h1>
24
25<h2>Introduction</h2>
26
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]27<p>Three new features were introduced into clang at the same time: <i>NSNumber Literals</i> provide a syntax for creating <code>NSNumber</code> from scalar literal expressions; <i>Collection Literals</i> provide a short-hand for creating arrays and dictionaries; <i>Object Subscripting</i> provides a way to use subscripting with Objective-C objects. Users of Apple compiler releases can use these features starting with the Apple LLVM Compiler 4.0. Users of open-source LLVM.org compiler releases can use these features starting with clang v3.1.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]28
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]29<p>These language additions simplify common Objective-C programming patterns, make programs more concise, and improve the safety of container creation.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]30
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]31<p>This document describes how the features are implemented in clang, and how to use them in your own programs.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]32
33<h2>NSNumber Literals</h2>
34
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]35<p>The framework class <code>NSNumber</code> is used to wrap scalar values inside objects: signed and unsigned integers (<code>char</code>, <code>short</code>, <code>int</code>, <code>long</code>, <code>long long</code>), floating point numbers (<code>float</code>, <code>double</code>), and boolean values (<code>BOOL</code>, C++ <code>bool</code>). Scalar values wrapped in objects are also known as <i>boxed</i> values.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]36
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]37<p>In Objective-C, any character, numeric or boolean literal prefixed with the <code>'@'</code> character will evaluate to a pointer to an <code>NSNumber</code> object initialized with that value. C's type suffixes may be used to control the size of numeric literals.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]38
39<h3>Examples</h3>
40
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]41<p>The following program illustrates the rules for <code>NSNumber</code> literals:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]42
43<pre>
44void main(int argc, const char *argv[]) {
45 // character literals.
46 NSNumber *theLetterZ = @'Z'; // equivalent to [NSNumber numberWithChar:'Z']
47
48 // integral literals.
49 NSNumber *fortyTwo = @42; // equivalent to [NSNumber numberWithInt:42]
50 NSNumber *fortyTwoUnsigned = @42U; // equivalent to [NSNumber numberWithUnsignedInt:42U]
51 NSNumber *fortyTwoLong = @42L; // equivalent to [NSNumber numberWithLong:42L]
52 NSNumber *fortyTwoLongLong = @42LL; // equivalent to [NSNumber numberWithLongLong:42LL]
53
54 // floating point literals.
55 NSNumber *piFloat = @3.141592654F; // equivalent to [NSNumber numberWithFloat:3.141592654F]
56 NSNumber *piDouble = @3.1415926535; // equivalent to [NSNumber numberWithDouble:3.1415926535]
57
58 // BOOL literals.
59 NSNumber *yesNumber = @YES; // equivalent to [NSNumber numberWithBool:YES]
60 NSNumber *noNumber = @NO; // equivalent to [NSNumber numberWithBool:NO]
61
62#ifdef __cplusplus
63 NSNumber *trueNumber = @true; // equivalent to [NSNumber numberWithBool:(BOOL)true]
64 NSNumber *falseNumber = @false; // equivalent to [NSNumber numberWithBool:(BOOL)false]
65#endif
66}
67</pre>
68
69<h3>Discussion</h3>
70
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]71<p>NSNumber literals only support literal scalar values after the <code>'@'</code>. Consequently, <code>@INT_MAX</code> works, but <code>@INT_MIN</code> does not, because they are defined like this:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]72
73<pre>
74#define INT_MAX 2147483647 /* max value for an int */
75#define INT_MIN (-2147483647-1) /* min value for an int */
76</pre>
77
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]78<p>The definition of <code>INT_MIN</code> is not a simple literal, but a parenthesized expression. Parenthesized
79expressions are supported using the <a href="#objc_boxed_expressions">boxed expression</a> syntax, which is described in the next section.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]80
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]81<p>Because <code>NSNumber</code> does not currently support wrapping <code>long double</code> values, the use of a <code>long double NSNumber</code> literal (e.g. <code>@123.23L</code>) will be rejected by the compiler.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]82
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]83<p>Previously, the <code>BOOL</code> type was simply a typedef for <code>signed char</code>, and <code>YES</code> and <code>NO</code> were macros that expand to <code>(BOOL)1</code> and <code>(BOOL)0</code> respectively. To support <code>@YES</code> and <code>@NO</code> expressions, these macros are now defined using new language keywords in <code>&LT;objc/objc.h&GT;</code>:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]84
85<pre>
86#if __has_feature(objc_bool)
87#define YES __objc_yes
88#define NO __objc_no
89#else
90#define YES ((BOOL)1)
91#define NO ((BOOL)0)
92#endif
93</pre>
94
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]95<p>The compiler implicitly converts <code>__objc_yes</code> and <code>__objc_no</code> to <code>(BOOL)1</code> and <code>(BOOL)0</code>. The keywords are used to disambiguate <code>BOOL</code> and integer literals.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]96
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]97<p>Objective-C++ also supports <code>@true</code> and <code>@false</code> expressions, which are equivalent to <code>@YES</code> and <code>@NO</code>.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]98
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]99<!-- ======================================================================= -->
100<h2 id="objc_boxed_expressions">Boxed Expressions</h2>
101<!-- ======================================================================= -->
102
103<p>Objective-C provides a new syntax for boxing C expressions:</p>
104
105<pre>
106<code>@( <em>expression</em> )</code>
107</pre>
108
109<p>Expressions of scalar (numeric, enumerated, BOOL) and C string pointer types
110are supported:</p>
111
112<pre>
113// numbers.
Patrick Beard16c111b2012-04-19 20:48:09 +0000[diff] [blame]114NSNumber *smallestInt = @(-INT_MAX - 1); // [NSNumber numberWithInt:(-INT_MAX - 1)]
115NSNumber *piOverTwo = @(M_PI / 2); // [NSNumber numberWithDouble:(M_PI / 2)]
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]116
117// enumerated types.
118typedef enum { Red, Green, Blue } Color;
Patrick Beard16c111b2012-04-19 20:48:09 +0000[diff] [blame]119NSNumber *favoriteColor = @(Green); // [NSNumber numberWithInt:((int)Green)]
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]120
121// strings.
Patrick Beard16c111b2012-04-19 20:48:09 +0000[diff] [blame]122NSString *path = @(getenv("PATH")); // [NSString stringWithUTF8String:(getenv("PATH"))]
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]123NSArray *pathComponents = [path componentsSeparatedByString:@":"];
124</pre>
125
126<h3>Boxed Enums</h3>
127
128<p>
129Cocoa frameworks frequently define constant values using <em>enums.</em> Although enum values are integral, they may not be used directly as boxed literals (this avoids conflicts with future <code>'@'</code>-prefixed Objective-C keywords). Instead, an enum value must be placed inside a boxed expression. The following example demonstrates configuring an <code>AVAudioRecorder</code> using a dictionary that contains a boxed enumeration value:
130</p>
131
132<pre>
133enum {
134 AVAudioQualityMin = 0,
135 AVAudioQualityLow = 0x20,
136 AVAudioQualityMedium = 0x40,
137 AVAudioQualityHigh = 0x60,
138 AVAudioQualityMax = 0x7F
139};
140
141- (AVAudioRecorder *)recordToFile:(NSURL *)fileURL {
142 NSDictionary *settings = @{ AVEncoderAudioQualityKey : @(AVAudioQualityMax) };
143 return [[AVAudioRecorder alloc] initWithURL:fileURL settings:settings error:NULL];
144}
145</pre>
146
147<p>
148The expression <code>@(AVAudioQualityMax)</code> converts <code>AVAudioQualityMax</code> to an integer type, and boxes the value accordingly. If the enum has a <a href="http://clang.llvm.org/docs/LanguageExtensions.html#objc_fixed_enum">fixed underlying type</a> as in:
149</p>
150
151<pre>
152typedef enum : unsigned char { Red, Green, Blue } Color;
153NSNumber *red = @(Red), *green = @(Green), *blue = @(Blue); // => [NSNumber numberWithUnsignedChar:]
154</pre>
155
156<p>
157then the fixed underlying type will be used to select the correct <code>NSNumber</code> creation method.
158</p>
159
160<h3>Boxed C Strings</h3>
161
162<p>
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]163A C string literal prefixed by the <code>'@'</code> token denotes an <code>NSString</code> literal in the same way a numeric literal prefixed by the <code>'@'</code> token denotes an <code>NSNumber</code> literal. When the type of the parenthesized expression is <code>(char *)</code> or <code>(const char *)</code>, the result of the boxed expression is a pointer to an <code>NSString</code> object containing equivalent character data, which is assumed to be '\0'-terminated and UTF-8 encoded. The following example converts C-style command line arguments into <code>NSString</code> objects.
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]164</p>
165
166<pre>
167// Partition command line arguments into positional and option arguments.
168NSMutableArray *args = [NSMutableArray new];
169NSMutableDictionary *options = [NSMutableArray new];
170while (--argc) {
171 const char *arg = *++argv;
172 if (strncmp(arg, "--", 2) == 0) {
173 options[@(arg + 2)] = @(*++argv); // --key value
174 } else {
175 [args addObject:@(arg)]; // positional argument
176 }
177}
178</pre>
179
180<p>
181As with all C pointers, character pointer expressions can involve arbitrary pointer arithmetic, therefore programmers must ensure that the character data is valid. Passing <code>NULL</code> as the character pointer will raise an exception at runtime. When possible, the compiler will reject <code>NULL</code> character pointers used in boxed expressions.
182</p>
183
184<h3>Availability</h3>
185
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]186<p>Boxed expressions will be available in clang 3.2. It is not currently available in any Apple compiler.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]187
188<h2>Container Literals</h2>
189
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]190<p>Objective-C now supports a new expression syntax for creating immutable array and dictionary container objects.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]191
192<h3>Examples</h3>
193
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]194<p>Immutable array expression:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]195
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]196<pre>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]197NSArray *array = @[ @"Hello", NSApp, [NSNumber numberWithInt:42] ];
198</pre>
199
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]200<p>This creates an <code>NSArray</code> with 3 elements. The comma-separated sub-expressions of an array literal can be any Objective-C object pointer typed expression.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]201
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]202<p>Immutable dictionary expression:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]203
204<pre>
205NSDictionary *dictionary = @{
206 @"name" : NSUserName(),
207 @"date" : [NSDate date],
208 @"processInfo" : [NSProcessInfo processInfo]
209};
210</pre>
211
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]212<p>This creates an <code>NSDictionary</code> with 3 key/value pairs. Value sub-expressions of a dictionary literal must be Objective-C object pointer typed, as in array literals. Key sub-expressions must be of an Objective-C object pointer type that implements the <code>&LT;NSCopying&GT;</code> protocol.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]213
214<h3>Discussion</h3>
215
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]216<p>Neither keys nor values can have the value <code>nil</code> in containers. If the compiler can prove that a key or value is <code>nil</code> at compile time, then a warning will be emitted. Otherwise, a runtime error will occur.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]217
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]218<p>Using array and dictionary literals is safer than the variadic creation forms commonly in use today. Array literal expressions expand to calls to <code>+[NSArray arrayWithObjects:count:]</code>, which validates that all objects are non-<code>nil</code>. The variadic form, <code>+[NSArray arrayWithObjects:]</code> uses <code>nil</code> as an argument list terminator, which can lead to malformed array objects. Dictionary literals are similarly created with <code>+[NSDictionary dictionaryWithObjects:forKeys:count:]</code> which validates all objects and keys, unlike <code>+[NSDictionary dictionaryWithObjectsAndKeys:]</code> which also uses a <code>nil</code> parameter as an argument list terminator.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]219
220<h2>Object Subscripting</h2>
221
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]222<p>Objective-C object pointer values can now be used with C's subscripting operator.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]223
224<h3>Examples</h3>
225
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]226<p>The following code demonstrates the use of object subscripting syntax with <code>NSMutableArray</code> and <code>NSMutableDictionary</code> objects:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]227
228<pre>
229NSMutableArray *array = ...;
230NSUInteger idx = ...;
231id newObject = ...;
232id oldObject = array[idx];
233array[idx] = newObject; // replace oldObject with newObject
234
235NSMutableDictionary *dictionary = ...;
236NSString *key = ...;
237oldObject = dictionary[key];
238dictionary[key] = newObject; // replace oldObject with newObject
239</pre>
240
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]241<p>The next section explains how subscripting expressions map to accessor methods.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]242
243<h3>Subscripting Methods</h3>
244
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]245<p>Objective-C supports two kinds of subscript expressions: <i>array-style</i> subscript expressions use integer typed subscripts; <i>dictionary-style</i> subscript expressions use Objective-C object pointer typed subscripts. Each type of subscript expression is mapped to a message send using a predefined selector. The advantage of this design is flexibility: class designers are free to introduce subscripting by declaring methods or by adopting protocols. Moreover, because the method names are selected by the type of the subscript, an object can be subscripted using both array and dictionary styles.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]246
247<h4>Array-Style Subscripting</h4>
248
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]249<p>When the subscript operand has an integral type, the expression is rewritten to use one of two different selectors, depending on whether the element is being read or written. When an expression reads an element using an integral index, as in the following example:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]250
251<pre>
252NSUInteger idx = ...;
253id value = object[idx];
254</pre>
255
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]256<p>it is translated into a call to <code>objectAtIndexedSubscript:</code></p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]257
258<pre>
259id value = [object objectAtIndexedSubscript:idx];
260</pre>
261
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]262<p>When an expression writes an element using an integral index:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]263
264<pre>
265object[idx] = newValue;
266</pre>
267
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]268<p>it is translated to a call to <code>setObject:atIndexedSubscript:</code></p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]269
270<pre>
271[object setObject:newValue atIndexedSubscript:idx];
272</pre>
273
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]274<p>These message sends are then type-checked and performed just like explicit message sends. The method used for objectAtIndexedSubscript: must be declared with an argument of integral type and a return value of some Objective-C object pointer type. The method used for setObject:atIndexedSubscript: must be declared with its first argument having some Objective-C pointer type and its second argument having integral type.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]275
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]276<p>The meaning of indexes is left up to the declaring class. The compiler will coerce the index to the appropriate argument type of the method it uses for type-checking. For an instance of <code>NSArray</code>, reading an element using an index outside the range <code>[0, array.count)</code> will raise an exception. For an instance of <code>NSMutableArray</code>, assigning to an element using an index within this range will replace that element, but assigning to an element using an index outside this range will raise an exception; no syntax is provided for inserting, appending, or removing elements for mutable arrays.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]277
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]278<p>A class need not declare both methods in order to take advantage of this language feature. For example, the class <code>NSArray</code> declares only <code>objectAtIndexedSubscript:</code>, so that assignments to elements will fail to type-check; moreover, its subclass <code>NSMutableArray</code> declares <code>setObject:atIndexedSubscript:</code>.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]279
280<h4>Dictionary-Style Subscripting</h4>
281
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]282<p>When the subscript operand has an Objective-C object pointer type, the expression is rewritten to use one of two different selectors, depending on whether the element is being read from or written to. When an expression reads an element using an Objective-C object pointer subscript operand, as in the following example:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]283
284<pre>
285id key = ...;
286id value = object[key];
287</pre>
288
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]289<p>it is translated into a call to the <code>objectForKeyedSubscript:</code> method:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]290
291<pre>
292id value = [object objectForKeyedSubscript:key];
293</pre>
294
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]295<p>When an expression writes an element using an Objective-C object pointer subscript:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]296
297<pre>
298object[key] = newValue;
299</pre>
300
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]301<p>it is translated to a call to <code>setObject:forKeyedSubscript:</code></p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]302
303<pre>
304[object setObject:newValue forKeyedSubscript:key];
305</pre>
306
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]307<p>The behavior of <code>setObject:forKeyedSubscript:</code> is class-specific; but in general it should replace an existing value if one is already associated with a key, otherwise it should add a new value for the key. No syntax is provided for removing elements from mutable dictionaries.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]308
309<h3>Discussion</h3>
310
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]311<p>An Objective-C subscript expression occurs when the base operand of the C subscript operator has an Objective-C object pointer type. Since this potentially collides with pointer arithmetic on the value, these expressions are only supported under the modern Objective-C runtime, which categorically forbids such arithmetic.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]312
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]313<p>Currently, only subscripts of integral or Objective-C object pointer type are supported. In C++, a class type can be used if it has a single conversion function to an integral or Objective-C pointer type, in which case that conversion is applied and analysis continues as appropriate. Otherwise, the expression is ill-formed.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]314
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]315<p>An Objective-C object subscript expression is always an l-value. If the expression appears on the left-hand side of a simple assignment operator (=), the element is written as described below. If the expression appears on the left-hand side of a compound assignment operator (e.g. +=), the program is ill-formed, because the result of reading an element is always an Objective-C object pointer and no binary operators are legal on such pointers. If the expression appears in any other position, the element is read as described below. It is an error to take the address of a subscript expression, or (in C++) to bind a reference to it.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]316
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]317<p>Programs can use object subscripting with Objective-C object pointers of type <code>id</code>. Normal dynamic message send rules apply; the compiler must see <i>some</i> declaration of the subscripting methods, and will pick the declaration seen first.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]318
319<h2>Grammar Additions</h2>
320
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]321<p>To support the new syntax described above, the Objective-C <code>@</code>-expression grammar has the following new productions:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]322
323<pre>
324objc-at-expression : '@' (string-literal | encode-literal | selector-literal | protocol-literal | object-literal)
325 ;
326
327object-literal : ('+' | '-')? numeric-constant
328 | character-constant
329 | boolean-constant
330 | array-literal
331 | dictionary-literal
332 ;
333
334boolean-constant : '__objc_yes' | '__objc_no' | 'true' | 'false' /* boolean keywords. */
335 ;
336
337array-literal : '[' assignment-expression-list ']'
338 ;
339
340assignment-expression-list : assignment-expression (',' assignment-expression-list)?
341 | /* empty */
342 ;
343
344dictionary-literal : '{' key-value-list '}'
345 ;
346
347key-value-list : key-value-pair (',' key-value-list)?
348 | /* empty */
349 ;
350
351key-value-pair : assignment-expression ':' assignment-expression
352 ;
353</pre>
354
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]355<p>Note: <code>@true</code> and <code>@false</code> are only supported in Objective-C++.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]356
357<h2>Availability Checks</h2>
358
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]359<p>Programs test for the new features by using clang's __has_feature checks. Here are examples of their use:</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]360
361<pre>
362#if __has_feature(objc_array_literals)
363 // new way.
364 NSArray *elements = @[ @"H", @"He", @"O", @"C" ];
365#else
366 // old way (equivalent).
367 id objects[] = { @"H", @"He", @"O", @"C" };
368 NSArray *elements = [NSArray arrayWithObjects:objects count:4];
369#endif
370
371#if __has_feature(objc_dictionary_literals)
372 // new way.
373 NSDictionary *masses = @{ @"H" : @1.0078, @"He" : @4.0026, @"O" : @15.9990, @"C" : @12.0096 };
374#else
375 // old way (equivalent).
376 id keys[] = { @"H", @"He", @"O", @"C" };
Patrick Bearda62c3802012-03-20 22:24:08 +0000[diff] [blame]377 id values[] = { [NSNumber numberWithDouble:1.0078], [NSNumber numberWithDouble:4.0026],
378 [NSNumber numberWithDouble:15.9990], [NSNumber numberWithDouble:12.0096] };
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]379 NSDictionary *masses = [NSDictionary dictionaryWithObjects:objects forKeys:keys count:4];
380#endif
381
382#if __has_feature(objc_subscripting)
383 NSUInteger i, count = elements.count;
384 for (i = 0; i < count; ++i) {
385 NSString *element = elements[i];
386 NSNumber *mass = masses[element];
387 NSLog(@"the mass of %@ is %@", element, mass);
388 }
389#else
390 NSUInteger i, count = [elements count];
391 for (i = 0; i < count; ++i) {
392 NSString *element = [elements objectAtIndex:i];
393 NSNumber *mass = [masses objectForKey:element];
394 NSLog(@"the mass of %@ is %@", element, mass);
395 }
396#endif
397</pre>
398
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]399<p>Code can use also <code>__has_feature(objc_bool)</code> to check for the availability of numeric literals support. This checks for the new <code>__objc_yes / __objc_no</code> keywords, which enable the use of <code>@YES / @NO</code> literals.</p>
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]400
Patrick Bearddd9fe592012-04-19 14:33:55 +0000[diff] [blame]401<p>To check whether boxed expressions are supported, use <code>__has_feature(objc_boxed_expressions)</code> feature macro.</p>
Patrick Beardeb382ec2012-04-19 00:25:12 +0000[diff] [blame]402
Patrick Beardaf39ba12012-03-20 20:50:45 +0000[diff] [blame]403</div>
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