@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, @c 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc. @c This is part of the GCC manual. @c For copying conditions, see the file gcc.texi. @node Objective-C @comment node-name, next, previous, up @chapter GNU Objective-C runtime features This document is meant to describe some of the GNU Objective-C runtime features. It is not intended to teach you Objective-C, there are several resources on the Internet that present the language. Questions and comments about this document to Ovidiu Predescu @email{ovidiu@@cup.hp.com}. @menu * Executing code before main:: * Type encoding:: * Garbage Collection:: * Constant string objects:: * compatibility_alias:: @end menu @node Executing code before main, Type encoding, Objective-C, Objective-C @section @code{+load}: Executing code before main The GNU Objective-C runtime provides a way that allows you to execute code before the execution of the program enters the @code{main} function. The code is executed on a per-class and a per-category basis, through a special class method @code{+load}. This facility is very useful if you want to initialize global variables which can be accessed by the program directly, without sending a message to the class first. The usual way to initialize global variables, in the @code{+initialize} method, might not be useful because @code{+initialize} is only called when the first message is sent to a class object, which in some cases could be too late. Suppose for example you have a @code{FileStream} class that declares @code{Stdin}, @code{Stdout} and @code{Stderr} as global variables, like below: @smallexample FileStream *Stdin = nil; FileStream *Stdout = nil; FileStream *Stderr = nil; @@implementation FileStream + (void)initialize @{ Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; @} /* @r{Other methods here} */ @@end @end smallexample In this example, the initialization of @code{Stdin}, @code{Stdout} and @code{Stderr} in @code{+initialize} occurs too late. The programmer can send a message to one of these objects before the variables are actually initialized, thus sending messages to the @code{nil} object. The @code{+initialize} method which actually initializes the global variables is not invoked until the first message is sent to the class object. The solution would require these variables to be initialized just before entering @code{main}. The correct solution of the above problem is to use the @code{+load} method instead of @code{+initialize}: @smallexample @@implementation FileStream + (void)load @{ Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; @} /* @r{Other methods here} */ @@end @end smallexample The @code{+load} is a method that is not overridden by categories. If a class and a category of it both implement @code{+load}, both methods are invoked. This allows some additional initializations to be performed in a category. This mechanism is not intended to be a replacement for @code{+initialize}. You should be aware of its limitations when you decide to use it instead of @code{+initialize}. @menu * What you can and what you cannot do in +load:: @end menu @node What you can and what you cannot do in +load, , Executing code before main, Executing code before main @subsection What you can and what you cannot do in @code{+load} The @code{+load} implementation in the GNU runtime guarantees you the following things: @itemize @bullet @item you can write whatever C code you like; @item you can send messages to Objective-C constant strings (@code{@@"this is a constant string"}); @item you can allocate and send messages to objects whose class is implemented in the same file; @item the @code{+load} implementation of all super classes of a class are executed before the @code{+load} of that class is executed; @item the @code{+load} implementation of a class is executed before the @code{+load} implementation of any category. @end itemize In particular, the following things, even if they can work in a particular case, are not guaranteed: @itemize @bullet @item allocation of or sending messages to arbitrary objects; @item allocation of or sending messages to objects whose classes have a category implemented in the same file; @end itemize You should make no assumptions about receiving @code{+load} in sibling classes when you write @code{+load} of a class. The order in which sibling classes receive @code{+load} is not guaranteed. The order in which @code{+load} and @code{+initialize} are called could be problematic if this matters. If you don't allocate objects inside @code{+load}, it is guaranteed that @code{+load} is called before @code{+initialize}. If you create an object inside @code{+load} the @code{+initialize} method of object's class is invoked even if @code{+load} was not invoked. Note if you explicitly call @code{+load} on a class, @code{+initialize} will be called first. To avoid possible problems try to implement only one of these methods. The @code{+load} method is also invoked when a bundle is dynamically loaded into your running program. This happens automatically without any intervening operation from you. When you write bundles and you need to write @code{+load} you can safely create and send messages to objects whose classes already exist in the running program. The same restrictions as above apply to classes defined in bundle. @node Type encoding, Garbage Collection, Executing code before main, Objective-C @section Type encoding The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes. The types are encoded in the following way: @c @sp 1 @multitable @columnfractions .25 .75 @item @code{_Bool} @tab @code{B} @item @code{char} @tab @code{c} @item @code{unsigned char} @tab @code{C} @item @code{short} @tab @code{s} @item @code{unsigned short} @tab @code{S} @item @code{int} @tab @code{i} @item @code{unsigned int} @tab @code{I} @item @code{long} @tab @code{l} @item @code{unsigned long} @tab @code{L} @item @code{long long} @tab @code{q} @item @code{unsigned long long} @tab @code{Q} @item @code{float} @tab @code{f} @item @code{double} @tab @code{d} @item @code{void} @tab @code{v} @item @code{id} @tab @code{@@} @item @code{Class} @tab @code{#} @item @code{SEL} @tab @code{:} @item @code{char*} @tab @code{*} @item unknown type @tab @code{?} @item Complex types @tab @code{j} followed by the inner type. For example @code{_Complex double} is encoded as "jd". @item bit-fields @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) @end multitable @c @sp 1 The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object. The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure. The non-atomic types are encoded as follows: @c @sp 1 @multitable @columnfractions .2 .8 @item pointers @tab @samp{^} followed by the pointed type. @item arrays @tab @samp{[} followed by the number of elements in the array followed by the type of the elements followed by @samp{]} @item structures @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{@}} @item unions @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{)} @end multitable Here are some types and their encodings, as they are generated by the compiler on an i386 machine: @sp 1 @multitable @columnfractions .25 .75 @item Objective-C type @tab Compiler encoding @item @smallexample int a[10]; @end smallexample @tab @code{[10i]} @item @smallexample struct @{ int i; float f[3]; int a:3; int b:2; char c; @} @end smallexample @tab @code{@{?=i[3f]b128i3b131i2c@}} @end multitable @sp 1 In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers: @sp 1 @multitable @columnfractions .25 .75 @item Specifier @tab Encoding @item @code{const} @tab @code{r} @item @code{in} @tab @code{n} @item @code{inout} @tab @code{N} @item @code{out} @tab @code{o} @item @code{bycopy} @tab @code{O} @item @code{oneway} @tab @code{V} @end multitable @sp 1 The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types. @node Garbage Collection, Constant string objects, Type encoding, Objective-C @section Garbage Collection Support for a new memory management policy has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector. It is available from @w{@uref{http://www.hpl.hp.com/personal/Hans_Boehm/gc/}}. To enable the support for it you have to configure the compiler using an additional argument, @w{@option{--enable-objc-gc}}. You need to have garbage collector installed before building the compiler. This will build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, @file{libobjc_gc.a} to not conflict with the non-garbage-collected library. When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized. There is a new runtime function @code{class_ivar_set_gcinvisible()} which can be used to declare a so-called @dfn{weak pointer} reference. Such a pointer is basically hidden for the garbage collector; this can be useful in certain situations, especially when you want to keep track of the allocated objects, yet allow them to be collected. This kind of pointers can only be members of objects, you cannot declare a global pointer as a weak reference. Every type which is a pointer type can be declared a weak pointer, including @code{id}, @code{Class} and @code{SEL}. Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this: @smallexample @@interface WeakPointer : Object @{ const void* weakPointer; @} - initWithPointer:(const void*)p; - (const void*)weakPointer; @@end @@implementation WeakPointer + (void)initialize @{ class_ivar_set_gcinvisible (self, "weakPointer", YES); @} - initWithPointer:(const void*)p @{ weakPointer = p; return self; @} - (const void*)weakPointer @{ return weakPointer; @} @@end @end smallexample Weak pointers are supported through a new type character specifier represented by the @samp{!} character. The @code{class_ivar_set_gcinvisible()} function adds or removes this specifier to the string type description of the instance variable named as argument. @c ========================================================================= @node Constant string objects @section Constant string objects GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character @samp{@@}: @smallexample id myString = @@"this is a constant string object"; @end smallexample The constant string objects are by default instances of the @code{NXConstantString} class which is provided by the GNU Objective-C runtime. To get the definition of this class you must include the @file{objc/NXConstStr.h} header file. User defined libraries may want to implement their own constant string class. To be able to support them, the GNU Objective-C compiler provides a new command line options @option{-fconstant-string-class=@var{class-name}}. The provided class should adhere to a strict structure, the same as @code{NXConstantString}'s structure: @smallexample @@interface MyConstantStringClass @{ Class isa; char *c_string; unsigned int len; @} @@end @end smallexample @code{NXConstantString} inherits from @code{Object}; user class libraries may choose to inherit the customized constant string class from a different class than @code{Object}. There is no requirement in the methods the constant string class has to implement, but the final ivar layout of the class must be the compatible with the given structure. When the compiler creates the statically allocated constant string object, the @code{c_string} field will be filled by the compiler with the string; the @code{length} field will be filled by the compiler with the string length; the @code{isa} pointer will be filled with @code{NULL} by the compiler, and it will later be fixed up automatically at runtime by the GNU Objective-C runtime library to point to the class which was set by the @option{-fconstant-string-class} option when the object file is loaded (if you wonder how it works behind the scenes, the name of the class to use, and the list of static objects to fixup, are stored by the compiler in the object file in a place where the GNU runtime library will find them at runtime). As a result, when a file is compiled with the @option{-fconstant-string-class} option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this. @c ========================================================================= @node compatibility_alias @section compatibility_alias This is a feature of the Objective-C compiler rather than of the runtime, anyway since it is documented nowhere and its existence was forgotten, we are documenting it here. The keyword @code{@@compatibility_alias} allows you to define a class name as equivalent to another class name. For example: @smallexample @@compatibility_alias WOApplication GSWApplication; @end smallexample tells the compiler that each time it encounters @code{WOApplication} as a class name, it should replace it with @code{GSWApplication} (that is, @code{WOApplication} is just an alias for @code{GSWApplication}). There are some constraints on how this can be used--- @itemize @bullet @item @code{WOApplication} (the alias) must not be an existing class; @item @code{GSWApplication} (the real class) must be an existing class. @end itemize