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+ Contributing
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+</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.
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