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Porting and Maintenance" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Design Notes</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="documentation_style.html">Prev</a> </td><th width="60%" align="center">Appendix A. Contributing </th><td width="20%" align="right"> <a accesskey="n" href="appendix_porting.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="contrib.design_notes"></a>Design Notes</h2></div></div></div><p> </p><div class="literallayout"><p><br /> <br /> The Library<br /> -----------<br /> <br /> This paper is covers two major areas:<br /> <br /> - Features and policies not mentioned in the standard that<br /> the quality of the library implementation depends on, including<br /> extensions and "implementation-defined" features;<br /> <br /> - Plans for required but unimplemented library features and<br /> optimizations to them.<br /> <br /> Overhead<br /> --------<br /> <br /> The standard defines a large library, much larger than the standard<br /> C library. A naive implementation would suffer substantial overhead<br /> in compile time, executable size, and speed, rendering it unusable<br /> in many (particularly embedded) applications. The alternative demands<br /> care in construction, and some compiler support, but there is no<br /> need for library subsets.<br /> <br /> What are the sources of this overhead? There are four main causes:<br /> <br /> - The library is specified almost entirely as templates, which<br /> with current compilers must be included in-line, resulting in<br /> very slow builds as tens or hundreds of thousands of lines<br /> of function definitions are read for each user source file.<br /> Indeed, the entire SGI STL, as well as the dos Reis valarray,<br /> are provided purely as header files, largely for simplicity in<br /> porting. Iostream/locale is (or will be) as large again.<br /> <br /> - The library is very flexible, specifying a multitude of hooks<br /> where users can insert their own code in place of defaults.<br /> When these hooks are not used, any time and code expended to<br /> support that flexibility is wasted.<br /> <br /> - Templates are often described as causing to "code bloat". In<br /> practice, this refers (when it refers to anything real) to several<br /> independent processes. First, when a class template is manually<br /> instantiated in its entirely, current compilers place the definitions<br /> for all members in a single object file, so that a program linking<br /> to one member gets definitions of all. Second, template functions<br /> which do not actually depend on the template argument are, under<br /> current compilers, generated anew for each instantiation, rather<br /> than being shared with other instantiations. Third, some of the<br /> flexibility mentioned above comes from virtual functions (both in<br /> regular classes and template classes) which current linkers add<br /> to the executable file even when they manifestly cannot be called.<br /> <br /> - The library is specified to use a language feature, exceptions,<br /> which in the current gcc compiler ABI imposes a run time and<br /> code space cost to handle the possibility of exceptions even when<br /> they are not used. Under the new ABI (accessed with -fnew-abi),<br /> there is a space overhead and a small reduction in code efficiency<br /> resulting from lost optimization opportunities associated with<br /> non-local branches associated with exceptions.<br /> <br /> What can be done to eliminate this overhead? A variety of coding<br /> techniques, and compiler, linker and library improvements and<br /> extensions may be used, as covered below. Most are not difficult,<br /> and some are already implemented in varying degrees.<br /> <br /> Overhead: Compilation Time<br /> --------------------------<br /> <br /> Providing "ready-instantiated" template code in object code archives<br /> allows us to avoid generating and optimizing template instantiations<br /> in each compilation unit which uses them. However, the number of such<br /> instantiations that are useful to provide is limited, and anyway this<br /> is not enough, by itself, to minimize compilation time. In particular,<br /> it does not reduce time spent parsing conforming headers.<br /> <br /> Quicker header parsing will depend on library extensions and compiler<br /> improvements. One approach is some variation on the techniques<br /> previously marketed as "pre-compiled headers", now standardized as<br /> support for the "export" keyword. "Exported" template definitions<br /> can be placed (once) in a "repository" -- really just a library, but<br /> of template definitions rather than object code -- to be drawn upon<br /> at link time when an instantiation is needed, rather than placed in<br /> header files to be parsed along with every compilation unit.<br /> <br /> Until "export" is implemented we can put some of the lengthy template<br /> definitions in #if guards or alternative headers so that users can skip<br /> over the full definitions when they need only the ready-instantiated<br /> specializations.<br /> <br /> To be precise, this means that certain headers which define<br /> templates which users normally use only for certain arguments<br /> can be instrumented to avoid exposing the template definitions<br /> to the compiler unless a macro is defined. For example, in<br /> <string>, we might have:<br /> <br /> template <class _CharT, ... > class basic_string {<br /> ... // member declarations<br /> };<br /> ... // operator declarations<br /> <br /> #ifdef _STRICT_ISO_<br /> # if _G_NO_TEMPLATE_EXPORT<br /> # include <bits/std_locale.h> // headers needed by definitions<br /> # ...<br /> # include <bits/string.tcc> // member and global template definitions.<br /> # endif<br /> #endif<br /> <br /> Users who compile without specifying a strict-ISO-conforming flag<br /> would not see many of the template definitions they now see, and rely<br /> instead on ready-instantiated specializations in the library. This<br /> technique would be useful for the following substantial components:<br /> string, locale/iostreams, valarray. It would *not* be useful or<br /> usable with the following: containers, algorithms, iterators,<br /> allocator. Since these constitute a large (though decreasing)<br /> fraction of the library, the benefit the technique offers is<br /> limited.<br /> <br /> The language specifies the semantics of the "export" keyword, but<br /> the gcc compiler does not yet support it. When it does, problems<br /> with large template inclusions can largely disappear, given some<br /> minor library reorganization, along with the need for the apparatus<br /> described above.<br /> <br /> Overhead: Flexibility Cost<br /> --------------------------<br /> <br /> The library offers many places where users can specify operations<br /> to be performed by the library in place of defaults. Sometimes<br /> this seems to require that the library use a more-roundabout, and<br /> possibly slower, way to accomplish the default requirements than<br /> would be used otherwise.<br /> <br /> The primary protection against this overhead is thorough compiler<br /> optimization, to crush out layers of inline function interfaces.<br /> Kuck & Associates has demonstrated the practicality of this kind<br /> of optimization.<br /> <br /> The second line of defense against this overhead is explicit<br /> specialization. By defining helper function templates, and writing<br /> specialized code for the default case, overhead can be eliminated<br /> for that case without sacrificing flexibility. This takes full<br /> advantage of any ability of the optimizer to crush out degenerate<br /> code.<br /> <br /> The library specifies many virtual functions which current linkers<br /> load even when they cannot be called. Some minor improvements to the<br /> compiler and to ld would eliminate any such overhead by simply<br /> omitting virtual functions that the complete program does not call.<br /> A prototype of this work has already been done. For targets where<br /> GNU ld is not used, a "pre-linker" could do the same job.<br /> <br /> The main areas in the standard interface where user flexibility<br /> can result in overhead are:<br /> <br /> - Allocators: Containers are specified to use user-definable<br /> allocator types and objects, making tuning for the container<br /> characteristics tricky.<br /> <br /> - Locales: the standard specifies locale objects used to implement<br /> iostream operations, involving many virtual functions which use<br /> streambuf iterators.<br /> <br /> - Algorithms and containers: these may be instantiated on any type,<br /> frequently duplicating code for identical operations.<br /> <br /> - Iostreams and strings: users are permitted to use these on their<br /> own types, and specify the operations the stream must use on these<br /> types.<br /> <br /> Note that these sources of overhead are _avoidable_. The techniques<br /> to avoid them are covered below.<br /> <br /> Code Bloat<br /> ----------<br /> <br /> In the SGI STL, and in some other headers, many of the templates<br /> are defined "inline" -- either explicitly or by their placement<br /> in class definitions -- which should not be inline. This is a<br /> source of code bloat. Matt had remarked that he was relying on<br /> the compiler to recognize what was too big to benefit from inlining,<br /> and generate it out-of-line automatically. However, this also can<br /> result in code bloat except where the linker can eliminate the extra<br /> copies.<br /> <br /> Fixing these cases will require an audit of all inline functions<br /> defined in the library to determine which merit inlining, and moving<br /> the rest out of line. This is an issue mainly in chapters 23, 25, and<br /> 27. Of course it can be done incrementally, and we should generally<br /> accept patches that move large functions out of line and into ".tcc"<br /> files, which can later be pulled into a repository. Compiler/linker<br /> improvements to recognize very large inline functions and move them<br /> out-of-line, but shared among compilation units, could make this<br /> work unnecessary.<br /> <br /> Pre-instantiating template specializations currently produces large<br /> amounts of dead code which bloats statically linked programs. The<br /> current state of the static library, libstdc++.a, is intolerable on<br /> this account, and will fuel further confused speculation about a need<br /> for a library "subset". A compiler improvement that treats each<br /> instantiated function as a separate object file, for linking purposes,<br /> would be one solution to this problem. An alternative would be to<br /> split up the manual instantiation files into dozens upon dozens of<br /> little files, each compiled separately, but an abortive attempt at<br /> this was done for <string> and, though it is far from complete, it<br /> is already a nuisance. A better interim solution (just until we have<br /> "export") is badly needed.<br /> <br /> When building a shared library, the current compiler/linker cannot<br /> automatically generate the instantiations needed. This creates a<br /> miserable situation; it means any time something is changed in the<br /> library, before a shared library can be built someone must manually<br /> copy the declarations of all templates that are needed by other parts<br /> of the library to an "instantiation" file, and add it to the build<br /> system to be compiled and linked to the library. This process is<br /> readily automated, and should be automated as soon as possible.<br /> Users building their own shared libraries experience identical<br /> frustrations.<br /> <br /> Sharing common aspects of template definitions among instantiations<br /> can radically reduce code bloat. The compiler could help a great<br /> deal here by recognizing when a function depends on nothing about<br /> a template parameter, or only on its size, and giving the resulting<br /> function a link-name "equate" that allows it to be shared with other<br /> instantiations. Implementation code could take advantage of the<br /> capability by factoring out code that does not depend on the template<br /> argument into separate functions to be merged by the compiler.<br /> <br /> Until such a compiler optimization is implemented, much can be done<br /> manually (if tediously) in this direction. One such optimization is<br /> to derive class templates from non-template classes, and move as much<br /> implementation as possible into the base class. Another is to partial-<br /> specialize certain common instantiations, such as vector<T*>, to share<br /> code for instantiations on all types T. While these techniques work,<br /> they are far from the complete solution that a compiler improvement<br /> would afford.<br /> <br /> Overhead: Expensive Language Features<br /> -------------------------------------<br /> <br /> The main "expensive" language feature used in the standard library<br /> is exception support, which requires compiling in cleanup code with<br /> static table data to locate it, and linking in library code to use<br /> the table. For small embedded programs the amount of such library<br /> code and table data is assumed by some to be excessive. Under the<br /> "new" ABI this perception is generally exaggerated, although in some<br /> cases it may actually be excessive.<br /> <br /> To implement a library which does not use exceptions directly is<br /> not difficult given minor compiler support (to "turn off" exceptions<br /> and ignore exception constructs), and results in no great library<br /> maintenance difficulties. To be precise, given "-fno-exceptions",<br /> the compiler should treat "try" blocks as ordinary blocks, and<br /> "catch" blocks as dead code to ignore or eliminate. Compiler<br /> support is not strictly necessary, except in the case of "function<br /> try blocks"; otherwise the following macros almost suffice:<br /> <br /> #define throw(X)<br /> #define try if (true)<br /> #define catch(X) else if (false)<br /> <br /> However, there may be a need to use function try blocks in the<br /> library implementation, and use of macros in this way can make<br /> correct diagnostics impossible. Furthermore, use of this scheme<br /> would require the library to call a function to re-throw exceptions<br /> from a try block. Implementing the above semantics in the compiler<br /> is preferable.<br /> <br /> Given the support above (however implemented) it only remains to<br /> replace code that "throws" with a call to a well-documented "handler"<br /> function in a separate compilation unit which may be replaced by<br /> the user. The main source of exceptions that would be difficult<br /> for users to avoid is memory allocation failures, but users can<br /> define their own memory allocation primitives that never throw.<br /> Otherwise, the complete list of such handlers, and which library<br /> functions may call them, would be needed for users to be able to<br /> implement the necessary substitutes. (Fortunately, they have the<br /> source code.)<br /> <br /> Opportunities<br /> -------------<br /> <br /> The template capabilities of C++ offer enormous opportunities for<br /> optimizing common library operations, well beyond what would be<br /> considered "eliminating overhead". In particular, many operations<br /> done in Glibc with macros that depend on proprietary language<br /> extensions can be implemented in pristine Standard C++. For example,<br /> the chapter 25 algorithms, and even C library functions such as strchr,<br /> can be specialized for the case of static arrays of known (small) size.<br /> <br /> Detailed optimization opportunities are identified below where<br /> the component where they would appear is discussed. Of course new<br /> opportunities will be identified during implementation.<br /> <br /> Unimplemented Required Library Features<br /> ---------------------------------------<br /> <br /> The standard specifies hundreds of components, grouped broadly by<br /> chapter. These are listed in excruciating detail in the CHECKLIST<br /> file.<br /> <br /> 17 general<br /> 18 support<br /> 19 diagnostics<br /> 20 utilities<br /> 21 string<br /> 22 locale<br /> 23 containers<br /> 24 iterators<br /> 25 algorithms<br /> 26 numerics<br /> 27 iostreams<br /> Annex D backward compatibility<br /> <br /> Anyone participating in implementation of the library should obtain<br /> a copy of the standard, ISO 14882. People in the U.S. can obtain an<br /> electronic copy for US$18 from ANSI's web site. Those from other<br /> countries should visit http://www.iso.ch/ to find out the location<br /> of their country's representation in ISO, in order to know who can<br /> sell them a copy.<br /> <br /> The emphasis in the following sections is on unimplemented features<br /> and optimization opportunities.<br /> <br /> Chapter 17 General<br /> -------------------<br /> <br /> Chapter 17 concerns overall library requirements.<br /> <br /> The standard doesn't mention threads. A multi-thread (MT) extension<br /> primarily affects operators new and delete (18), allocator (20),<br /> string (21), locale (22), and iostreams (27). The common underlying<br /> support needed for this is discussed under chapter 20.<br /> <br /> The standard requirements on names from the C headers create a<br /> lot of work, mostly done. Names in the C headers must be visible<br /> in the std:: and sometimes the global namespace; the names in the<br /> two scopes must refer to the same object. More stringent is that<br /> Koenig lookup implies that any types specified as defined in std::<br /> really are defined in std::. Names optionally implemented as<br /> macros in C cannot be macros in C++. (An overview may be read at<br /> <http://www.cantrip.org/cheaders.html>). The scripts "inclosure"<br /> and "mkcshadow", and the directories shadow/ and cshadow/, are the<br /> beginning of an effort to conform in this area.<br /> <br /> A correct conforming definition of C header names based on underlying<br /> C library headers, and practical linking of conforming namespaced<br /> customer code with third-party C libraries depends ultimately on<br /> an ABI change, allowing namespaced C type names to be mangled into<br /> type names as if they were global, somewhat as C function names in a<br /> namespace, or C++ global variable names, are left unmangled. Perhaps<br /> another "extern" mode, such as 'extern "C-global"' would be an<br /> appropriate place for such type definitions. Such a type would<br /> affect mangling as follows:<br /> <br /> namespace A {<br /> struct X {};<br /> extern "C-global" { // or maybe just 'extern "C"'<br /> struct Y {};<br /> };<br /> }<br /> void f(A::X*); // mangles to f__FPQ21A1X<br /> void f(A::Y*); // mangles to f__FP1Y<br /> <br /> (It may be that this is really the appropriate semantics for regular<br /> 'extern "C"', and 'extern "C-global"', as an extension, would not be<br /> necessary.) This would allow functions declared in non-standard C headers<br /> (and thus fixable by neither us nor users) to link properly with functions<br /> declared using C types defined in properly-namespaced headers. The<br /> problem this solves is that C headers (which C++ programmers do persist<br /> in using) frequently forward-declare C struct tags without including<br /> the header where the type is defined, as in<br /> <br /> struct tm;<br /> void munge(tm*);<br /> <br /> Without some compiler accommodation, munge cannot be called by correct<br /> C++ code using a pointer to a correctly-scoped tm* value.<br /> <br /> The current C headers use the preprocessor extension "#include_next",<br /> which the compiler complains about when run "-pedantic".<br /> (Incidentally, it appears that "-fpedantic" is currently ignored,<br /> probably a bug.) The solution in the C compiler is to use<br /> "-isystem" rather than "-I", but unfortunately in g++ this seems<br /> also to wrap the whole header in an 'extern "C"' block, so it's<br /> unusable for C++ headers. The correct solution appears to be to<br /> allow the various special include-directory options, if not given<br /> an argument, to affect subsequent include-directory options additively,<br /> so that if one said<br /> <br /> -pedantic -iprefix $(prefix) \<br /> -idirafter -ino-pedantic -ino-extern-c -iwithprefix -I g++-v3 \<br /> -iwithprefix -I g++-v3/ext<br /> <br /> the compiler would search $(prefix)/g++-v3 and not report<br /> pedantic warnings for files found there, but treat files in<br /> $(prefix)/g++-v3/ext pedantically. (The undocumented semantics<br /> of "-isystem" in g++ stink. Can they be rescinded? If not it<br /> must be replaced with something more rationally behaved.)<br /> <br /> All the C headers need the treatment above; in the standard these<br /> headers are mentioned in various chapters. Below, I have only<br /> mentioned those that present interesting implementation issues.<br /> <br /> The components identified as "mostly complete", below, have not been<br /> audited for conformance. In many cases where the library passes<br /> conformance tests we have non-conforming extensions that must be<br /> wrapped in #if guards for "pedantic" use, and in some cases renamed<br /> in a conforming way for continued use in the implementation regardless<br /> of conformance flags.<br /> <br /> The STL portion of the library still depends on a header<br /> stl/bits/stl_config.h full of #ifdef clauses. This apparatus<br /> should be replaced with autoconf/automake machinery.<br /> <br /> The SGI STL defines a type_traits<> template, specialized for<br /> many types in their code including the built-in numeric and<br /> pointer types and some library types, to direct optimizations of<br /> standard functions. The SGI compiler has been extended to generate<br /> specializations of this template automatically for user types,<br /> so that use of STL templates on user types can take advantage of<br /> these optimizations. Specializations for other, non-STL, types<br /> would make more optimizations possible, but extending the gcc<br /> compiler in the same way would be much better. Probably the next<br /> round of standardization will ratify this, but probably with<br /> changes, so it probably should be renamed to place it in the<br /> implementation namespace.<br /> <br /> The SGI STL also defines a large number of extensions visible in<br /> standard headers. (Other extensions that appear in separate headers<br /> have been sequestered in subdirectories ext/ and backward/.) All<br /> these extensions should be moved to other headers where possible,<br /> and in any case wrapped in a namespace (not std!), and (where kept<br /> in a standard header) girded about with macro guards. Some cannot be<br /> moved out of standard headers because they are used to implement<br /> standard features. The canonical method for accommodating these<br /> is to use a protected name, aliased in macro guards to a user-space<br /> name. Unfortunately C++ offers no satisfactory template typedef<br /> mechanism, so very ad-hoc and unsatisfactory aliasing must be used<br /> instead.<br /> <br /> Implementation of a template typedef mechanism should have the highest<br /> priority among possible extensions, on the same level as implementation<br /> of the template "export" feature.<br /> <br /> Chapter 18 Language support<br /> ----------------------------<br /> <br /> Headers: <limits> <new> <typeinfo> <exception><br /> C headers: <cstddef> <climits> <cfloat> <cstdarg> <csetjmp><br /> <ctime> <csignal> <cstdlib> (also 21, 25, 26)<br /> <br /> This defines the built-in exceptions, rtti, numeric_limits<>,<br /> operator new and delete. Much of this is provided by the<br /> compiler in its static runtime library.<br /> <br /> Work to do includes defining numeric_limits<> specializations in<br /> separate files for all target architectures. Values for integer types<br /> except for bool and wchar_t are readily obtained from the C header<br /> <limits.h>, but values for the remaining numeric types (bool, wchar_t,<br /> float, double, long double) must be entered manually. This is<br /> largely dog work except for those members whose values are not<br /> easily deduced from available documentation. Also, this involves<br /> some work in target configuration to identify the correct choice of<br /> file to build against and to install.<br /> <br /> The definitions of the various operators new and delete must be<br /> made thread-safe, which depends on a portable exclusion mechanism,<br /> discussed under chapter 20. Of course there is always plenty of<br /> room for improvements to the speed of operators new and delete.<br /> <br /> <cstdarg>, in Glibc, defines some macros that gcc does not allow to<br /> be wrapped into an inline function. Probably this header will demand<br /> attention whenever a new target is chosen. The functions atexit(),<br /> exit(), and abort() in cstdlib have different semantics in C++, so<br /> must be re-implemented for C++.<br /> <br /> Chapter 19 Diagnostics<br /> -----------------------<br /> <br /> Headers: <stdexcept><br /> C headers: <cassert> <cerrno><br /> <br /> This defines the standard exception objects, which are "mostly complete".<br /> Cygnus has a version, and now SGI provides a slightly different one.<br /> It makes little difference which we use.<br /> <br /> The C global name "errno", which C allows to be a variable or a macro,<br /> is required in C++ to be a macro. For MT it must typically result in<br /> a function call.<br /> <br /> Chapter 20 Utilities<br /> ---------------------<br /> Headers: <utility> <functional> <memory><br /> C header: <ctime> (also in 18)<br /> <br /> SGI STL provides "mostly complete" versions of all the components<br /> defined in this chapter. However, the auto_ptr<> implementation<br /> is known to be wrong. Furthermore, the standard definition of it<br /> is known to be unimplementable as written. A minor change to the<br /> standard would fix it, and auto_ptr<> should be adjusted to match.<br /> <br /> Multi-threading affects the allocator implementation, and there must<br /> be configuration/installation choices for different users' MT<br /> requirements. Anyway, users will want to tune allocator options<br /> to support different target conditions, MT or no.<br /> <br /> The primitives used for MT implementation should be exposed, as an<br /> extension, for users' own work. We need cross-CPU "mutex" support,<br /> multi-processor shared-memory atomic integer operations, and single-<br /> processor uninterruptible integer operations, and all three configurable<br /> to be stubbed out for non-MT use, or to use an appropriately-loaded<br /> dynamic library for the actual runtime environment, or statically<br /> compiled in for cases where the target architecture is known.<br /> <br /> Chapter 21 String<br /> ------------------<br /> Headers: <string><br /> C headers: <cctype> <cwctype> <cstring> <cwchar> (also in 27)<br /> <cstdlib> (also in 18, 25, 26)<br /> <br /> We have "mostly-complete" char_traits<> implementations. Many of the<br /> char_traits<char> operations might be optimized further using existing<br /> proprietary language extensions.<br /> <br /> We have a "mostly-complete" basic_string<> implementation. The work<br /> to manually instantiate char and wchar_t specializations in object<br /> files to improve link-time behavior is extremely unsatisfactory,<br /> literally tripling library-build time with no commensurate improvement<br /> in static program link sizes. It must be redone. (Similar work is<br /> needed for some components in chapters 22 and 27.)<br /> <br /> Other work needed for strings is MT-safety, as discussed under the<br /> chapter 20 heading.<br /> <br /> The standard C type mbstate_t from <cwchar> and used in char_traits<><br /> must be different in C++ than in C, because in C++ the default constructor<br /> value mbstate_t() must be the "base" or "ground" sequence state.<br /> (According to the likely resolution of a recently raised Core issue,<br /> this may become unnecessary. However, there are other reasons to<br /> use a state type not as limited as whatever the C library provides.)<br /> If we might want to provide conversions from (e.g.) internally-<br /> represented EUC-wide to externally-represented Unicode, or vice-<br /> versa, the mbstate_t we choose will need to be more accommodating<br /> than what might be provided by an underlying C library.<br /> <br /> There remain some basic_string template-member functions which do<br /> not overload properly with their non-template brethren. The infamous<br /> hack akin to what was done in vector<> is needed, to conform to<br /> 23.1.1 para 10. The CHECKLIST items for basic_string marked 'X',<br /> or incomplete, are so marked for this reason.<br /> <br /> Replacing the string iterators, which currently are simple character<br /> pointers, with class objects would greatly increase the safety of the<br /> client interface, and also permit a "debug" mode in which range,<br /> ownership, and validity are rigorously checked. The current use of<br /> raw pointers as string iterators is evil. vector<> iterators need the<br /> same treatment. Note that the current implementation freely mixes<br /> pointers and iterators, and that must be fixed before safer iterators<br /> can be introduced.<br /> <br /> Some of the functions in <cstring> are different from the C version.<br /> generally overloaded on const and non-const argument pointers. For<br /> example, in <cstring> strchr is overloaded. The functions isupper<br /> etc. in <cctype> typically implemented as macros in C are functions<br /> in C++, because they are overloaded with others of the same name<br /> defined in <locale>.<br /> <br /> Many of the functions required in <cwctype> and <cwchar> cannot be<br /> implemented using underlying C facilities on intended targets because<br /> such facilities only partly exist.<br /> <br /> Chapter 22 Locale<br /> ------------------<br /> Headers: <locale><br /> C headers: <clocale><br /> <br /> We have a "mostly complete" class locale, with the exception of<br /> code for constructing, and handling the names of, named locales.<br /> The ways that locales are named (particularly when categories<br /> (e.g. LC_TIME, LC_COLLATE) are different) varies among all target<br /> environments. This code must be written in various versions and<br /> chosen by configuration parameters.<br /> <br /> Members of many of the facets defined in <locale> are stubs. Generally,<br /> there are two sets of facets: the base class facets (which are supposed<br /> to implement the "C" locale) and the "byname" facets, which are supposed<br /> to read files to determine their behavior. The base ctype<>, collate<>,<br /> and numpunct<> facets are "mostly complete", except that the table of<br /> bitmask values used for "is" operations, and corresponding mask values,<br /> are still defined in libio and just included/linked. (We will need to<br /> implement these tables independently, soon, but should take advantage<br /> of libio where possible.) The num_put<>::put members for integer types<br /> are "mostly complete".<br /> <br /> A complete list of what has and has not been implemented may be<br /> found in CHECKLIST. However, note that the current definition of<br /> codecvt<wchar_t,char,mbstate_t> is wrong. It should simply write<br /> out the raw bytes representing the wide characters, rather than<br /> trying to convert each to a corresponding single "char" value.<br /> <br /> Some of the facets are more important than others. Specifically,<br /> the members of ctype<>, numpunct<>, num_put<>, and num_get<> facets<br /> are used by other library facilities defined in <string>, <istream>,<br /> and <ostream>, and the codecvt<> facet is used by basic_filebuf<><br /> in <fstream>, so a conforming iostream implementation depends on<br /> these.<br /> <br /> The "long long" type eventually must be supported, but code mentioning<br /> it should be wrapped in #if guards to allow pedantic-mode compiling.<br /> <br /> Performance of num_put<> and num_get<> depend critically on<br /> caching computed values in ios_base objects, and on extensions<br /> to the interface with streambufs.<br /> <br /> Specifically: retrieving a copy of the locale object, extracting<br /> the needed facets, and gathering data from them, for each call to<br /> (e.g.) operator<< would be prohibitively slow. To cache format<br /> data for use by num_put<> and num_get<> we have a _Format_cache<><br /> object stored in the ios_base::pword() array. This is constructed<br /> and initialized lazily, and is organized purely for utility. It<br /> is discarded when a new locale with different facets is imbued.<br /> <br /> Using only the public interfaces of the iterator arguments to the<br /> facet functions would limit performance by forbidding "vector-style"<br /> character operations. The streambuf iterator optimizations are<br /> described under chapter 24, but facets can also bypass the streambuf<br /> iterators via explicit specializations and operate directly on the<br /> streambufs, and use extended interfaces to get direct access to the<br /> streambuf internal buffer arrays. These extensions are mentioned<br /> under chapter 27. These optimizations are particularly important<br /> for input parsing.<br /> <br /> Unused virtual members of locale facets can be omitted, as mentioned<br /> above, by a smart linker.<br /> <br /> Chapter 23 Containers<br /> ----------------------<br /> Headers: <deque> <list> <queue> <stack> <vector> <map> <set> <bitset><br /> <br /> All the components in chapter 23 are implemented in the SGI STL.<br /> They are "mostly complete"; they include a large number of<br /> nonconforming extensions which must be wrapped. Some of these<br /> are used internally and must be renamed or duplicated.<br /> <br /> The SGI components are optimized for large-memory environments. For<br /> embedded targets, different criteria might be more appropriate. Users<br /> will want to be able to tune this behavior. We should provide<br /> ways for users to compile the library with different memory usage<br /> characteristics.<br /> <br /> A lot more work is needed on factoring out common code from different<br /> specializations to reduce code size here and in chapter 25. The<br /> easiest fix for this would be a compiler/ABI improvement that allows<br /> the compiler to recognize when a specialization depends only on the<br /> size (or other gross quality) of a template argument, and allow the<br /> linker to share the code with similar specializations. In its<br /> absence, many of the algorithms and containers can be partial-<br /> specialized, at least for the case of pointers, but this only solves<br /> a small part of the problem. Use of a type_traits-style template<br /> allows a few more optimization opportunities, more if the compiler<br /> can generate the specializations automatically.<br /> <br /> As an optimization, containers can specialize on the default allocator<br /> and bypass it, or take advantage of details of its implementation<br /> after it has been improved upon.<br /> <br /> Replacing the vector iterators, which currently are simple element<br /> pointers, with class objects would greatly increase the safety of the<br /> client interface, and also permit a "debug" mode in which range,<br /> ownership, and validity are rigorously checked. The current use of<br /> pointers for iterators is evil.<br /> <br /> As mentioned for chapter 24, the deque iterator is a good example of<br /> an opportunity to implement a "staged" iterator that would benefit<br /> from specializations of some algorithms.<br /> <br /> Chapter 24 Iterators<br /> ---------------------<br /> Headers: <iterator><br /> <br /> Standard iterators are "mostly complete", with the exception of<br /> the stream iterators, which are not yet templatized on the<br /> stream type. Also, the base class template iterator<> appears<br /> to be wrong, so everything derived from it must also be wrong,<br /> currently.<br /> <br /> The streambuf iterators (currently located in stl/bits/std_iterator.h,<br /> but should be under bits/) can be rewritten to take advantage of<br /> friendship with the streambuf implementation.<br /> <br /> Matt Austern has identified opportunities where certain iterator<br /> types, particularly including streambuf iterators and deque<br /> iterators, have a "two-stage" quality, such that an intermediate<br /> limit can be checked much more quickly than the true limit on<br /> range operations. If identified with a member of iterator_traits,<br /> algorithms may be specialized for this case. Of course the<br /> iterators that have this quality can be identified by specializing<br /> a traits class.<br /> <br /> Many of the algorithms must be specialized for the streambuf<br /> iterators, to take advantage of block-mode operations, in order<br /> to allow iostream/locale operations' performance not to suffer.<br /> It may be that they could be treated as staged iterators and<br /> take advantage of those optimizations.<br /> <br /> Chapter 25 Algorithms<br /> ----------------------<br /> Headers: <algorithm><br /> C headers: <cstdlib> (also in 18, 21, 26))<br /> <br /> The algorithms are "mostly complete". As mentioned above, they<br /> are optimized for speed at the expense of code and data size.<br /> <br /> Specializations of many of the algorithms for non-STL types would<br /> give performance improvements, but we must use great care not to<br /> interfere with fragile template overloading semantics for the<br /> standard interfaces. Conventionally the standard function template<br /> interface is an inline which delegates to a non-standard function<br /> which is then overloaded (this is already done in many places in<br /> the library). Particularly appealing opportunities for the sake of<br /> iostream performance are for copy and find applied to streambuf<br /> iterators or (as noted elsewhere) for staged iterators, of which<br /> the streambuf iterators are a good example.<br /> <br /> The bsearch and qsort functions cannot be overloaded properly as<br /> required by the standard because gcc does not yet allow overloading<br /> on the extern-"C"-ness of a function pointer.<br /> <br /> Chapter 26 Numerics<br /> --------------------<br /> Headers: <complex> <valarray> <numeric><br /> C headers: <cmath>, <cstdlib> (also 18, 21, 25)<br /> <br /> Numeric components: Gabriel dos Reis's valarray, Drepper's complex,<br /> and the few algorithms from the STL are "mostly done". Of course<br /> optimization opportunities abound for the numerically literate. It<br /> is not clear whether the valarray implementation really conforms<br /> fully, in the assumptions it makes about aliasing (and lack thereof)<br /> in its arguments.<br /> <br /> The C div() and ldiv() functions are interesting, because they are the<br /> only case where a C library function returns a class object by value.<br /> Since the C++ type div_t must be different from the underlying C type<br /> (which is in the wrong namespace) the underlying functions div() and<br /> ldiv() cannot be re-used efficiently. Fortunately they are trivial to<br /> re-implement.<br /> <br /> Chapter 27 Iostreams<br /> ---------------------<br /> Headers: <iosfwd> <streambuf> <ios> <ostream> <istream> <iostream><br /> <iomanip> <sstream> <fstream><br /> C headers: <cstdio> <cwchar> (also in 21)<br /> <br /> Iostream is currently in a very incomplete state. <iosfwd>, <iomanip>,<br /> ios_base, and basic_ios<> are "mostly complete". basic_streambuf<> and<br /> basic_ostream<> are well along, but basic_istream<> has had little work<br /> done. The standard stream objects, <sstream> and <fstream> have been<br /> started; basic_filebuf<> "write" functions have been implemented just<br /> enough to do "hello, world".<br /> <br /> Most of the istream and ostream operators << and >> (with the exception<br /> of the op<<(integer) ones) have not been changed to use locale primitives,<br /> sentry objects, or char_traits members.<br /> <br /> All these templates should be manually instantiated for char and<br /> wchar_t in a way that links only used members into user programs.<br /> <br /> Streambuf is fertile ground for optimization extensions. An extended<br /> interface giving iterator access to its internal buffer would be very<br /> useful for other library components.<br /> <br /> Iostream operations (primarily operators << and >>) can take advantage<br /> of the case where user code has not specified a locale, and bypass locale<br /> operations entirely. The current implementation of op<</num_put<>::put,<br /> for the integer types, demonstrates how they can cache encoding details<br /> from the locale on each operation. There is lots more room for<br /> optimization in this area.<br /> <br /> The definition of the relationship between the standard streams<br /> cout et al. and stdout et al. requires something like a "stdiobuf".<br /> The SGI solution of using double-indirection to actually use a<br /> stdio FILE object for buffering is unsatisfactory, because it<br /> interferes with peephole loop optimizations.<br /> <br /> The <sstream> header work has begun. stringbuf can benefit from<br /> friendship with basic_string<> and basic_string<>::_Rep to use<br /> those objects directly as buffers, and avoid allocating and making<br /> copies.<br /> <br /> The basic_filebuf<> template is a complex beast. It is specified to<br /> use the locale facet codecvt<> to translate characters between native<br /> files and the locale character encoding. In general this involves<br /> two buffers, one of "char" representing the file and another of<br /> "char_type", for the stream, with codecvt<> translating. The process<br /> is complicated by the variable-length nature of the translation, and<br /> the need to seek to corresponding places in the two representations.<br /> For the case of basic_filebuf<char>, when no translation is needed,<br /> a single buffer suffices. A specialized filebuf can be used to reduce<br /> code space overhead when no locale has been imbued. Matt Austern's<br /> work at SGI will be useful, perhaps directly as a source of code, or<br /> at least as an example to draw on.<br /> <br /> Filebuf, almost uniquely (cf. operator new), depends heavily on<br /> underlying environmental facilities. In current releases iostream<br /> depends fairly heavily on libio constant definitions, but it should<br /> be made independent. It also depends on operating system primitives<br /> for file operations. There is immense room for optimizations using<br /> (e.g.) mmap for reading. The shadow/ directory wraps, besides the<br /> standard C headers, the libio.h and unistd.h headers, for use mainly<br /> by filebuf. These wrappings have not been completed, though there<br /> is scaffolding in place.<br /> <br /> The encapsulation of certain C header <cstdio> names presents an<br /> interesting problem. It is possible to define an inline std::fprintf()<br /> implemented in terms of the 'extern "C"' vfprintf(), but there is no<br /> standard vfscanf() to use to implement std::fscanf(). It appears that<br /> vfscanf but be re-implemented in C++ for targets where no vfscanf<br /> extension has been defined. This is interesting in that it seems<br /> to be the only significant case in the C library where this kind of<br /> rewriting is necessary. (Of course Glibc provides the vfscanf()<br /> extension.) (The functions related to exit() must be rewritten<br /> for other reasons.)<br /> <br /> <br /> Annex D<br /> -------<br /> Headers: <strstream><br /> <br /> Annex D defines many non-library features, and many minor<br /> modifications to various headers, and a complete header.<br /> It is "mostly done", except that the libstdc++-2 <strstream><br /> header has not been adopted into the library, or checked to<br /> verify that it matches the draft in those details that were<br /> clarified by the committee. Certainly it must at least be<br /> moved into the std namespace.<br /> <br /> We still need to wrap all the deprecated features in #if guards<br /> so that pedantic compile modes can detect their use.<br /> <br /> Nonstandard Extensions<br /> ----------------------<br /> Headers: <iostream.h> <strstream.h> <hash> <rbtree><br /> <pthread_alloc> <stdiobuf> (etc.)<br /> <br /> User code has come to depend on a variety of nonstandard components<br /> that we must not omit. Much of this code can be adopted from<br /> libstdc++-v2 or from the SGI STL. This particularly includes<br /> <iostream.h>, <strstream.h>, and various SGI extensions such<br /> as <hash_map.h>. Many of these are already placed in the<br /> subdirectories ext/ and backward/. (Note that it is better to<br /> include them via "<backward/hash_map.h>" or "<ext/hash_map>" than<br /> to search the subdirectory itself via a "-I" directive.<br /> </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="documentation_style.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendix_contributing.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="appendix_porting.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Documentation Style </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Appendix B. Porting and Maintenance </td></tr></table></div></body></html>