Author: pcbeard
Date: Tue Mar 20 15:50:45 2012
New Revision: 153123
URL: http://llvm.org/viewvc/llvm-project?rev=153123&view=rev
Log:
Objective-C literals documentation.
Added:
cfe/trunk/docs/ObjectiveCLiterals.html
Added: cfe/trunk/docs/ObjectiveCLiterals.html
URL:
http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ObjectiveCLiterals.html?rev=153123&view=auto
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+<div id="content">
+
+<h1>Objective-C Literals</h1>
+
+<h2>Introduction</h2>
+
+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>
+
+These language additions simplify common Objective-C programming patterns,
make programs more concise, and improve the safety of container creation.<p>
+
+This document describes how the features are implemented in clang, and how to
use them in your own programs.<p>
+
+<h2>NSNumber Literals</h2>
+
+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>
+
+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.
+
+<h3>Examples</h3>
+
+The following program illustrates the rules for <code>NSNumber</code>
literals:<p>
+
+<pre>
+void main(int argc, const char *argv[]) {
+ // character literals.
+ NSNumber *theLetterZ = @'Z'; // equivalent to [NSNumber
numberWithChar:'Z']
+
+ // integral literals.
+ NSNumber *fortyTwo = @42; // equivalent to [NSNumber
numberWithInt:42]
+ NSNumber *fortyTwoUnsigned = @42U; // equivalent to [NSNumber
numberWithUnsignedInt:42U]
+ NSNumber *fortyTwoLong = @42L; // equivalent to [NSNumber
numberWithLong:42L]
+ NSNumber *fortyTwoLongLong = @42LL; // equivalent to [NSNumber
numberWithLongLong:42LL]
+
+ // floating point literals.
+ NSNumber *piFloat = @3.141592654F; // equivalent to [NSNumber
numberWithFloat:3.141592654F]
+ NSNumber *piDouble = @3.1415926535; // equivalent to [NSNumber
numberWithDouble:3.1415926535]
+
+ // BOOL literals.
+ NSNumber *yesNumber = @YES; // equivalent to [NSNumber
numberWithBool:YES]
+ NSNumber *noNumber = @NO; // equivalent to [NSNumber
numberWithBool:NO]
+
+#ifdef __cplusplus
+ NSNumber *trueNumber = @true; // equivalent to [NSNumber
numberWithBool:(BOOL)true]
+ NSNumber *falseNumber = @false; // equivalent to [NSNumber
numberWithBool:(BOOL)false]
+#endif
+}
+</pre>
+
+<h3>Discussion</h3>
+
+NSNumber literals only support literal scalar values after the '@'.
Consequently, @INT_MAX works, but @INT_MIN does not, because they are defined
like this:<p>
+
+<pre>
+#define INT_MAX 2147483647 /* max value for an int */
+#define INT_MIN (-2147483647-1) /* min value for an int */
+</pre>
+
+The definition of INT_MIN is not a simple literal, but a parenthesized
expression. This is by design, but may be improved in subsequent compiler
releases.<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>
+
+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><objc/objc.h></code>:<p>
+
+<pre>
+#if __has_feature(objc_bool)
+#define YES __objc_yes
+#define NO __objc_no
+#else
+#define YES ((BOOL)1)
+#define NO ((BOOL)0)
+#endif
+</pre>
+
+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>
+
+Objective-C++ also supports <code>@true</code> and <code>@false</code>
expressions, which are equivalent to <code>@YES</code> and <code>@NO</code>.
+
+
+<h2>Container Literals</h2>
+
+Objective-C now supports a new expression syntax for creating immutable array
and dictionary container objects.
+
+<h3>Examples</h3>
+
+Immutable array expression:<p>
+
+ <pre>
+NSArray *array = @[ @"Hello", NSApp, [NSNumber numberWithInt:42] ];
+</pre>
+
+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>
+
+Immutable dictionary expression:<p>
+
+<pre>
+NSDictionary *dictionary = @{
+ @"name" : NSUserName(),
+ @"date" : [NSDate date],
+ @"processInfo" : [NSProcessInfo processInfo]
+};
+</pre>
+
+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-expresions must be of an Objective-C
object pointer type that implements the <code><NSCopying></code>
protocol.<p>
+
+<h3>Discussion</h3>
+
+Nil keys or nil values are not supported in containers. If the compiler can
prove that a key or value is nil at compile time, then a warning will be
emitted. Otherwise, a runtime error will occur.<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>
+
+<h2>Object Subscripting</h2>
+
+Objective-C object pointer values can now be used with C's subscripting
operator.<p>
+
+<h3>Examples</h3>
+
+The following code demonstrates the use of object subscripting syntax with
<code>NSMutableArray</code> and <code>NSMutableDictionary</code> objects:<p>
+
+<pre>
+NSMutableArray *array = ...;
+NSUInteger idx = ...;
+id newObject = ...;
+id oldObject = array[idx];
+array[idx] = newObject; // replace oldObject with newObject
+
+NSMutableDictionary *dictionary = ...;
+NSString *key = ...;
+oldObject = dictionary[key];
+dictionary[key] = newObject; // replace oldObject with newObject
+</pre>
+
+The next section explains how subscripting expressions map to accessor
methods.<p>
+
+<h3>Subscripting Methods</h3>
+
+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.
+
+<h4>Array-Style Subscripting</h4>
+
+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>
+
+<pre>
+NSUInteger idx = ...;
+id value = object[idx];
+</pre>
+
+it is translated into a call to <code>objectAtIndexedSubscript:</code><p>
+
+<pre>
+id value = [object objectAtIndexedSubscript:idx];
+</pre>
+
+When an expression writes an element using an integral index:<p>
+
+<pre>
+object[idx] = newValue;
+</pre>
+
+it is translated to a call to <code>setObject:atIndexedSubscript:</code><p>
+
+<pre>
+[object setObject:newValue atIndexedSubscript:idx];
+</pre>
+
+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>
+
+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>
+
+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>.
+
+<h4>Dictionary-Style Subscripting</h4>
+
+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>
+
+<pre>
+id key = ...;
+id value = object[key];
+</pre>
+
+it is translated into a call to the <code>objectForKeyedSubscript:</code>
method:<p>
+
+<pre>
+id value = [object objectForKeyedSubscript:key];
+</pre>
+
+When an expression writes an element using an Objective-C object pointer
subscript:<p>
+
+<pre>
+object[key] = newValue;
+</pre>
+
+it is translated to a call to <code>setObject:forKeyedSubscript:</code>
+
+<pre>
+[object setObject:newValue forKeyedSubscript:key];
+</pre>
+
+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>
+
+<h3>Discussion</h3>
+
+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>
+
+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>
+
+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>
+
+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>
+
+<h2>Grammar Additions</h2>
+
+To support the new syntax described above, the Objective-C
<code>@</code>-expression grammar has the following new productions:<p>
+
+<pre>
+objc-at-expression : '@' (string-literal | encode-literal | selector-literal |
protocol-literal | object-literal)
+ ;
+
+object-literal : ('+' | '-')? numeric-constant
+ | character-constant
+ | boolean-constant
+ | array-literal
+ | dictionary-literal
+ ;
+
+boolean-constant : '__objc_yes' | '__objc_no' | 'true' | 'false' /* boolean
keywords. */
+ ;
+
+array-literal : '[' assignment-expression-list ']'
+ ;
+
+assignment-expression-list : assignment-expression (','
assignment-expression-list)?
+ | /* empty */
+ ;
+
+dictionary-literal : '{' key-value-list '}'
+ ;
+
+key-value-list : key-value-pair (',' key-value-list)?
+ | /* empty */
+ ;
+
+key-value-pair : assignment-expression ':' assignment-expression
+ ;
+</pre>
+
+Note: <code>@true</code> and <code>@false</code> are only supported in
Objective-C++.<p>
+
+<h2>Availability Checks</h2>
+
+Programs test for the new features by using clang's __has_feature checks. Here
are examples of their use:<p>
+
+<pre>
+#if __has_feature(objc_array_literals)
+ // new way.
+ NSArray *elements = @[ @"H", @"He", @"O", @"C" ];
+#else
+ // old way (equivalent).
+ id objects[] = { @"H", @"He", @"O", @"C" };
+ NSArray *elements = [NSArray arrayWithObjects:objects count:4];
+#endif
+
+#if __has_feature(objc_dictionary_literals)
+ // new way.
+ NSDictionary *masses = @{ @"H" : @1.0078, @"He" : @4.0026, @"O" :
@15.9990, @"C" : @12.0096 };
+#else
+ // old way (equivalent).
+ id keys[] = { @"H", @"He", @"O", @"C" };
+ id values[] = { [NSNumber numberWithFloat:1.0078], [NSNumber
numberWithFloat:4.0026],
+ [NSNumber numberWithFloat:15.9990], [NSNumber
numberWithFloat:12.0096] };
+ NSDictionary *masses = [NSDictionary dictionaryWithObjects:objects
forKeys:keys count:4];
+#endif
+
+#if __has_feature(objc_subscripting)
+ NSUInteger i, count = elements.count;
+ for (i = 0; i < count; ++i) {
+ NSString *element = elements[i];
+ NSNumber *mass = masses[element];
+ NSLog(@"the mass of %@ is %@", element, mass);
+ }
+#else
+ NSUInteger i, count = [elements count];
+ for (i = 0; i < count; ++i) {
+ NSString *element = [elements objectAtIndex:i];
+ NSNumber *mass = [masses objectForKey:element];
+ NSLog(@"the mass of %@ is %@", element, mass);
+ }
+#endif
+</pre>
+
+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>
+
+</div>
+</body>
+</html>
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