This is doc/gcc.info, produced by makeinfo version 4.5 from doc/gcc.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gcc: (gcc). The GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the use of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gcc.info, Node: Pragmas, Next: Unnamed Fields, Prev: Target Builtins, Up: C Extensions Pragmas Accepted by GCC ======================= GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; *Note Function Attributes::, for further explanation. * Menu: * ARM Pragmas:: * Darwin Pragmas:: * Solaris Pragmas:: * Tru64 Pragmas::  File: gcc.info, Node: ARM Pragmas, Next: Darwin Pragmas, Up: Pragmas ARM Pragmas ----------- The ARM target defines pragmas for controlling the default addition of `long_call' and `short_call' attributes to functions. *Note Function Attributes::, for information about the effects of these attributes. `long_calls' Set all subsequent functions to have the `long_call' attribute. `no_long_calls' Set all subsequent functions to have the `short_call' attribute. `long_calls_off' Do not affect the `long_call' or `short_call' attributes of subsequent functions.  File: gcc.info, Node: Darwin Pragmas, Next: Solaris Pragmas, Prev: ARM Pragmas, Up: Pragmas Darwin Pragmas -------------- The following pragmas are available for all architectures running the Darwin operating system. These are useful for compatibility with other MacOS compilers. `mark TOKENS...' This pragma is accepted, but has no effect. `options align=ALIGNMENT' This pragma sets the alignment of fields in structures. The values of ALIGNMENT may be `mac68k', to emulate m68k alignment, or `power', to emulate PowerPC alignment. Uses of this pragma nest properly; to restore the previous setting, use `reset' for the ALIGNMENT. `segment TOKENS...' This pragma is accepted, but has no effect. `unused (VAR [, VAR]...)' This pragma declares variables to be possibly unused. GCC will not produce warnings for the listed variables. The effect is similar to that of the `unused' attribute, except that this pragma may appear anywhere within the variables' scopes.  File: gcc.info, Node: Solaris Pragmas, Next: Tru64 Pragmas, Prev: Darwin Pragmas, Up: Pragmas Solaris Pragmas --------------- For compatibility with the SunPRO compiler, the following pragma is supported. `redefine_extname OLDNAME NEWNAME' This pragma gives the C function OLDNAME the assembler label NEWNAME. The pragma must appear before the function declaration. This pragma is equivalent to the asm labels extension (*note Asm Labels::). The preprocessor defines `__PRAGMA_REDEFINE_EXTNAME' if the pragma is available.  File: gcc.info, Node: Tru64 Pragmas, Prev: Solaris Pragmas, Up: Pragmas Tru64 Pragmas ------------- For compatibility with the Compaq C compiler, the following pragma is supported. `extern_prefix STRING' This pragma renames all subsequent function and variable declarations such that STRING is prepended to the name. This effect may be terminated by using another `extern_prefix' pragma with the empty string. This pragma is similar in intent to to the asm labels extension (*note Asm Labels::) in that the system programmer wants to change the assembly-level ABI without changing the source-level API. The preprocessor defines `__PRAGMA_EXTERN_PREFIX' if the pragma is available.  File: gcc.info, Node: Unnamed Fields, Prev: Pragmas, Up: C Extensions Unnamed struct/union fields within structs/unions. ================================================== For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example: struct { int a; union { int b; float c; }; int d; } foo; In this example, the user would be able to access members of the unnamed union with code like `foo.b'. Note that only unnamed structs and unions are allowed, you may not have, for example, an unnamed `int'. You must never create such structures that cause ambiguous field definitions. For example, this structure: struct { int a; struct { int a; }; } foo; It is ambiguous which `a' is being referred to with `foo.a'. Such constructs are not supported and must be avoided. In the future, such constructs may be detected and treated as compilation errors.  File: gcc.info, Node: C++ Extensions, Next: Objective-C, Prev: C Extensions, Up: Top Extensions to the C++ Language ****************************** The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro `__GNUC__'. You can also use `__GNUG__' to test specifically for GNU C++ (*note Standard Predefined Macros: (cpp.info)Standard Predefined.). * Menu: * Min and Max:: C++ Minimum and maximum operators. * Volatiles:: What constitutes an access to a volatile object. * Restricted Pointers:: C99 restricted pointers and references. * Vague Linkage:: Where G++ puts inlines, vtables and such. * C++ Interface:: You can use a single C++ header file for both declarations and definitions. * Template Instantiation:: Methods for ensuring that exactly one copy of each needed template instantiation is emitted. * Bound member functions:: You can extract a function pointer to the method denoted by a `->*' or `.*' expression. * C++ Attributes:: Variable, function, and type attributes for C++ only. * Java Exceptions:: Tweaking exception handling to work with Java. * Deprecated Features:: Things might disappear from g++. * Backwards Compatibility:: Compatibilities with earlier definitions of C++.  File: gcc.info, Node: Min and Max, Next: Volatiles, Up: C++ Extensions Minimum and Maximum Operators in C++ ==================================== It is very convenient to have operators which return the "minimum" or the "maximum" of two arguments. In GNU C++ (but not in GNU C), `A ? B' is the "maximum", returning the larger of the numeric values A and B. These operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example. #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) You might then use `int min = MIN (i, j);' to set MIN to the minimum value of variables I and J. However, side effects in `X' or `Y' may cause unintended behavior. For example, `MIN (i++, j++)' will fail, incrementing the smaller counter twice. The GNU C `typeof' extension allows you to write safe macros that avoid this kind of problem (*note Typeof::). However, writing `MIN' and `MAX' as macros also forces you to use function-call notation for a fundamental arithmetic operation. Using GNU C++ extensions, you can write `int min = i ?' are built into the compiler, they properly handle expressions with side-effects; `int min = i++ (*ptr1)'. When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue.  File: gcc.info, Node: Restricted Pointers, Next: Vague Linkage, Prev: Volatiles, Up: C++ Extensions Restricting Pointer Aliasing ============================ As with gcc, g++ understands the C99 feature of restricted pointers, specified with the `__restrict__', or `__restrict' type qualifier. Because you cannot compile C++ by specifying the `-std=c99' language flag, `restrict' is not a keyword in C++. In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context. void fn (int *__restrict__ rptr, int &__restrict__ rref) { ... } In the body of `fn', RPTR points to an unaliased integer and RREF refers to a (different) unaliased integer. You may also specify whether a member function's THIS pointer is unaliased by using `__restrict__' as a member function qualifier. void T::fn () __restrict__ { ... } Within the body of `T::fn', THIS will have the effective definition `T *__restrict__ const this'. Notice that the interpretation of a `__restrict__' member function qualifier is different to that of `const' or `volatile' qualifier, in that it is applied to the pointer rather than the object. This is consistent with other compilers which implement restricted pointers. As with all outermost parameter qualifiers, `__restrict__' is ignored in function definition matching. This means you only need to specify `__restrict__' in a function definition, rather than in a function prototype as well.  File: gcc.info, Node: Vague Linkage, Next: C++ Interface, Prev: Restricted Pointers, Up: C++ Extensions Vague Linkage ============= There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have "vague linkage". Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever. Inline Functions Inline functions are typically defined in a header file which can be included in many different compilations. Hopefully they can usually be inlined, but sometimes an out-of-line copy is necessary, if the address of the function is taken or if inlining fails. In general, we emit an out-of-line copy in all translation units where one is needed. As an exception, we only emit inline virtual functions with the vtable, since it will always require a copy. Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function. VTables C++ virtual functions are implemented in most compilers using a lookup table, known as a vtable. The vtable contains pointers to the virtual functions provided by a class, and each object of the class contains a pointer to its vtable (or vtables, in some multiple-inheritance situations). If the class declares any non-inline, non-pure virtual functions, the first one is chosen as the "key method" for the class, and the vtable is only emitted in the translation unit where the key method is defined. _Note:_ If the chosen key method is later defined as inline, the vtable will still be emitted in every translation unit which defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there. type_info objects C++ requires information about types to be written out in order to implement `dynamic_cast', `typeid' and exception handling. For polymorphic classes (classes with virtual functions), the type_info object is written out along with the vtable so that `dynamic_cast' can determine the dynamic type of a class object at runtime. For all other types, we write out the type_info object when it is used: when applying `typeid' to an expression, throwing an object, or referring to a type in a catch clause or exception specification. Template Instantiations Most everything in this section also applies to template instantiations, but there are other options as well. *Note Where's the Template?: Template Instantiation. When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support. On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable. For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things. *Note Declarations and Definitions in One Header: C++ Interface, for another way to control placement of these constructs.  File: gcc.info, Node: C++ Interface, Next: Template Instantiation, Prev: Vague Linkage, Up: C++ Extensions Declarations and Definitions in One Header ========================================== C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an "interface" specification, describing its structure with type declarations and function prototypes. Second, you need the "implementation" itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel. With GNU C++, you can use a single header file for both purposes. _Warning:_ The mechanism to specify this is in transition. For the nonce, you must use one of two `#pragma' commands; in a future release of GNU C++, an alternative mechanism will make these `#pragma' commands unnecessary. The header file contains the full definitions, but is marked with `#pragma interface' in the source code. This allows the compiler to use the header file only as an interface specification when ordinary source files incorporate it with `#include'. In the single source file where the full implementation belongs, you can use either a naming convention or `#pragma implementation' to indicate this alternate use of the header file. `#pragma interface' `#pragma interface "SUBDIR/OBJECTS.h"' Use this directive in _header files_ that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing `#pragma interface' is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses `#pragma implementation'). Instead, the object files will contain references to be resolved at link time. The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to `#pragma implementation'. `#pragma implementation' `#pragma implementation "OBJECTS.h"' Use this pragma in a _main input file_, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use `#pragma interface'. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files. If you use `#pragma implementation' with no argument, it applies to an include file with the same basename(1) as your source file. For example, in `allclass.cc', giving just `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'. In versions of GNU C++ prior to 2.6.0 `allclass.h' was treated as an implementation file whenever you would include it from `allclass.cc' even if you never specified `#pragma implementation'. This was deemed to be more trouble than it was worth, however, and disabled. If you use an explicit `#pragma implementation', it must appear in your source file _before_ you include the affected header files. Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use `#include' to include the header file; `#pragma implementation' only specifies how to use the file--it doesn't actually include it.) There is no way to split up the contents of a single header file into multiple implementation files. `#pragma implementation' and `#pragma interface' also have an effect on function inlining. If you define a class in a header file marked with `#pragma interface', the effect on a function defined in that class is similar to an explicit `extern' declaration--the compiler emits no code at all to define an independent version of the function. Its definition is used only for inlining with its callers. Conversely, when you include the same header file in a main source file that declares it as `#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with `-fno-implement-inlines'. If any calls were not inlined, you will get linker errors. ---------- Footnotes ---------- (1) A file's "basename" was the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.  File: gcc.info, Node: Template Instantiation, Next: Bound member functions, Prev: C++ Interface, Up: C++ Extensions Where's the Template? ===================== C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model. Borland model Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them, and the linker collapses them together. The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about. This disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file, since they must be seen to be instantiated. Cfront model The AT&T C++ translator, Cfront, solved the template instantiation problem by creating the notion of a template repository, an automatically maintained place where template instances are stored. A more modern version of the repository works as follows: As individual object files are built, the compiler places any template definitions and instantiations encountered in the repository. At link time, the link wrapper adds in the objects in the repository and compiles any needed instances that were not previously emitted. The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker. The disadvantages are vastly increased complexity, and thus potential for error; for some code this can be just as transparent, but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories. Code written for this model tends to separate definitions of non-inline member templates into a separate file, which should be compiled separately. When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On other systems, g++ implements neither automatic model. A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations. In the mean time, you have the following options for dealing with template instantiations: 1. Compile your template-using code with `-frepo'. The compiler will generate files with the extension `.rpo' listing all of the template instantiations used in the corresponding object files which could be instantiated there; the link wrapper, `collect2', will then update the `.rpo' files to tell the compiler where to place those instantiations and rebuild any affected object files. The link-time overhead is negligible after the first pass, as the compiler will continue to place the instantiations in the same files. This is your best option for application code written for the Borland model, as it will just work. Code written for the Cfront model will need to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding `#include ' to the end of each template header. For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option. 2. Compile your code with `-fno-implicit-templates' to disable the implicit generation of template instances, and explicitly instantiate all the ones you use. This approach requires more knowledge of exactly which instances you need than do the others, but it's less mysterious and allows greater control. You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like #include "Foo.h" #include "Foo.cc" template class Foo; template ostream& operator << (ostream&, const Foo&); for each of the instances you need, and create a template instantiation library from those. If you are using Cfront-model code, you can probably get away with not using `-fno-implicit-templates' when compiling files that don't `#include' the member template definitions. If you use one big file to do the instantiations, you may want to compile it without `-fno-implicit-templates' so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well. g++ has extended the template instantiation syntax outlined in the Working Paper to allow forward declaration of explicit instantiations (with `extern'), instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with `inline'), and instantiation of only the static data members of a template class, without the support data or member functions (with (`static'): extern template int max (int, int); inline template class Foo; static template class Foo; 3. Do nothing. Pretend g++ does implement automatic instantiation management. Code written for the Borland model will work fine, but each translation unit will contain instances of each of the templates it uses. In a large program, this can lead to an unacceptable amount of code duplication. 4. Add `#pragma interface' to all files containing template definitions. For each of these files, add `#pragma implementation "FILENAME"' to the top of some `.C' file which `#include's it. Then compile everything with `-fexternal-templates'. The templates will then only be expanded in the translation unit which implements them (i.e. has a `#pragma implementation' line for the file where they live); all other files will use external references. If you're lucky, everything should work properly. If you get undefined symbol errors, you need to make sure that each template instance which is used in the program is used in the file which implements that template. If you don't have any use for a particular instance in that file, you can just instantiate it explicitly, using the syntax from the latest C++ working paper: template class A; template ostream& operator << (ostream&, const A&); This strategy will work with code written for either model. If you are using code written for the Cfront model, the file containing a class template and the file containing its member templates should be implemented in the same translation unit. 5. A slight variation on this approach is to use the flag `-falt-external-templates' instead. This flag causes template instances to be emitted in the translation unit that implements the header where they are first instantiated, rather than the one which implements the file where the templates are defined. This header must be the same in all translation units, or things are likely to break. *Note Declarations and Definitions in One Header: C++ Interface, for more discussion of these pragmas.  File: gcc.info, Node: Bound member functions, Next: C++ Attributes, Prev: Template Instantiation, Up: C++ Extensions Extracting the function pointer from a bound pointer to member function ======================================================================= In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the `this' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time. Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls. The syntax for this extension is extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp); For PMF constants (i.e. expressions of the form `&Klasse::Member'), no object is needed to obtain the address of the function. They can be converted to function pointers directly: fptr p1 = (fptr)(&A::foo); You must specify `-Wno-pmf-conversions' to use this extension.  File: gcc.info, Node: C++ Attributes, Next: Java Exceptions, Prev: Bound member functions, Up: C++ Extensions C++-Specific Variable, Function, and Type Attributes ==================================================== Some attributes only make sense for C++ programs. `init_priority (PRIORITY)' In Standard C++, objects defined at namespace scope are guaranteed to be initialized in an order in strict accordance with that of their definitions _in a given translation unit_. No guarantee is made for initializations across translation units. However, GNU C++ allows users to control the order of initialization of objects defined at namespace scope with the `init_priority' attribute by specifying a relative PRIORITY, a constant integral expression currently bounded between 101 and 65535 inclusive. Lower numbers indicate a higher priority. In the following example, `A' would normally be created before `B', but the `init_priority' attribute has reversed that order: Some_Class A __attribute__ ((init_priority (2000))); Some_Class B __attribute__ ((init_priority (543))); Note that the particular values of PRIORITY do not matter; only their relative ordering. `java_interface' This type attribute informs C++ that the class is a Java interface. It may only be applied to classes declared within an `extern "Java"' block. Calls to methods declared in this interface will be dispatched using GCJ's interface table mechanism, instead of regular virtual table dispatch.  File: gcc.info, Node: Java Exceptions, Next: Deprecated Features, Prev: C++ Attributes, Up: C++ Extensions Java Exceptions =============== The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is: struct S { ~S(); }; extern void bar(); // is written in Java, and may throw exceptions void foo() { S s; bar(); } The usual effect of an incorrect guess is a link failure, complaining of a missing routine called `__gxx_personality_v0'. You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing `#pragma GCC java_exceptions' at the head of the file. This `#pragma' must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them. You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.  File: gcc.info, Node: Deprecated Features, Next: Backwards Compatibility, Prev: Java Exceptions, Up: C++ Extensions Deprecated Features =================== In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already. While the list below is not exhaustive, it documents some of the options that are now deprecated: `-fexternal-templates' `-falt-external-templates' These are two of the many ways for g++ to implement template instantiation. *Note Template Instantiation::. The C++ standard clearly defines how template definitions have to be organized across implementation units. g++ has an implicit instantiation mechanism that should work just fine for standard-conforming code. `-fstrict-prototype' `-fno-strict-prototype' Previously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands. This feature has been removed, except where it is required for backwards compatibility *Note Backwards Compatibility::. The named return value extension has been deprecated, and is now removed from g++. The use of initializer lists with new expressions has been deprecated, and is now removed from g++. Floating and complex non-type template parameters have been deprecated, and are now removed from g++. The implicit typename extension has been deprecated and will be removed from g++ at some point. In some cases g++ determines that a dependant type such as `TPL::X' is a type without needing a `typename' keyword, contrary to the standard.  File: gcc.info, Node: Backwards Compatibility, Prev: Deprecated Features, Up: C++ Extensions Backwards Compatibility ======================= Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. _All such backwards compatibility features are liable to disappear in future versions of G++._ They should be considered deprecated *Note Deprecated Features::. `For scope' If a variable is declared at for scope, it used to remain in scope until the end of the scope which contained the for statement (rather than just within the for scope). G++ retains this, but issues a warning, if such a variable is accessed outside the for scope. `Implicit C language' Old C system header files did not contain an `extern "C" {...}' scope to set the language. On such systems, all header files are implicitly scoped inside a C language scope. Also, an empty prototype `()' will be treated as an unspecified number of arguments, rather than no arguments, as C++ demands.  File: gcc.info, Node: Objective-C, Next: Compatibility, Prev: C++ Extensions, Up: Top 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 . * Menu: * Executing code before main:: * Type encoding:: * Garbage Collection:: * Constant string objects:: * compatibility_alias::  File: gcc.info, Node: Executing code before main, Next: Type encoding, Prev: Objective-C, Up: Objective-C `+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 `main' function. The code is executed on a per-class and a per-category basis, through a special class method `+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 `+initialize' method, might not be useful because `+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 `FileStream' class that declares `Stdin', `Stdout' and `Stderr' as global variables, like below: 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]; } /* Other methods here */ @end In this example, the initialization of `Stdin', `Stdout' and `Stderr' in `+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 `nil' object. The `+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 `main'. The correct solution of the above problem is to use the `+load' method instead of `+initialize': @implementation FileStream + (void)load { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end The `+load' is a method that is not overridden by categories. If a class and a category of it both implement `+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 `+initialize'. You should be aware of its limitations when you decide to use it instead of `+initialize'. * Menu: * What you can and what you cannot do in +load::  File: gcc.info, Node: What you can and what you cannot do in +load, Prev: Executing code before main, Up: Executing code before main What you can and what you cannot do in `+load' ---------------------------------------------- The `+load' implementation in the GNU runtime guarantees you the following things: * you can write whatever C code you like; * you can send messages to Objective-C constant strings (`@"this is a constant string"'); * you can allocate and send messages to objects whose class is implemented in the same file; * the `+load' implementation of all super classes of a class are executed before the `+load' of that class is executed; * the `+load' implementation of a class is executed before the `+load' implementation of any category. In particular, the following things, even if they can work in a particular case, are not guaranteed: * allocation of or sending messages to arbitrary objects; * allocation of or sending messages to objects whose classes have a category implemented in the same file; You should make no assumptions about receiving `+load' in sibling classes when you write `+load' of a class. The order in which sibling classes receive `+load' is not guaranteed. The order in which `+load' and `+initialize' are called could be problematic if this matters. If you don't allocate objects inside `+load', it is guaranteed that `+load' is called before `+initialize'. If you create an object inside `+load' the `+initialize' method of object's class is invoked even if `+load' was not invoked. Note if you explicitly call `+load' on a class, `+initialize' will be called first. To avoid possible problems try to implement only one of these methods. The `+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 `+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.  File: gcc.info, Node: Type encoding, Next: Garbage Collection, Prev: Executing code before main, Up: Objective-C 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: `char' `c' `unsigned char' `C' `short' `s' `unsigned short' `S' `int' `i' `unsigned int' `I' `long' `l' `unsigned long' `L' `long long' `q' `unsigned long `Q' long' `float' `f' `double' `d' `void' `v' `id' `@' `Class' `#' `SEL' `:' `char*' `*' unknown type `?' bit-fields `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) 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: pointers `^' followed by the pointed type. arrays `[' followed by the number of elements in the array followed by the type of the elements followed by `]' structures `{' followed by the name of the structure (or `?' if the structure is unnamed), the `=' sign, the type of the members and by `}' unions `(' followed by the name of the structure (or `?' if the union is unnamed), the `=' sign, the type of the members followed by `)' Here are some types and their encodings, as they are generated by the compiler on an i386 machine: Objective-C type Compiler encoding int a[10]; `[10i]' struct { `{?=i[3f]b128i3b131i2c}' int i; float f[3]; int a:3; int b:2; char c; } 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: Specifier Encoding `const' `r' `in' `n' `inout' `N' `out' `o' `bycopy' `O' `oneway' `V' 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.  File: gcc.info, Node: Garbage Collection, Next: Constant string objects, Prev: Type encoding, Up: Objective-C 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 `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, `--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, `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 `class_ivar_set_gcinvisible()' which can be used to declare a so-called "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 `id', `Class' and `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: @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 Weak pointers are supported through a new type character specifier represented by the `!' character. The `class_ivar_set_gcinvisible()' function adds or removes this specifier to the string type description of the instance variable named as argument.