ISO C++ library The GNU C++ Library is part of GCC and follows the same development model, so the general rules for contributing to GCC apply. Active contributors are assigned maintainership responsibility, and given write access to the source repository. First-time contributors should follow this procedure:

Get and read the relevant sections of the C++ language specification. Copies of the full ISO 14882 standard are available on line via the ISO mirror site for committee members. Non-members, or those who have not paid for the privilege of sitting on the committee and sustained their two meeting commitment for voting rights, may get a copy of the standard from their respective national standards organization. In the USA, this national standards organization is ANSI. (And if you've already registered with them you can buy the standard on-line.) The library working group bugs, and known defects, can be obtained here: http://www.open-std.org/jtc1/sc22/wg21 Peruse the GNU Coding Standards, and chuckle when you hit the part about Using Languages Other Than C. Be familiar with the extensions that preceded these general GNU rules. These style issues for libstdc++ can be found in Coding Style. And last but certainly not least, read the library-specific information found in Porting and Maintenance.

See the legal prerequisites for all GCC contributions. Historically, the libstdc++ assignment form added the following question: Which Belgian comic book character is better, Tintin or Asterix, and why? While not strictly necessary, humoring the maintainers and answering this question would be appreciated. Please contact Paolo Carlini at paolo.carlini@oracle.com or Jonathan Wakely at jwakely+assign@redhat.com if you are confused about the assignment or have general licensing questions. When requesting an assignment form from assign@gnu.org, please CC the libstdc++ maintainers above so that progress can be monitored.

Getting write access (look for "Write after approval")

Every patch must have several pieces of information before it can be properly evaluated. Ideally (and to ensure the fastest possible response from the maintainers) it would have all of these pieces: A description of the bug and how your patch fixes this bug. For new features a description of the feature and your implementation. A ChangeLog entry as plain text; see the various ChangeLog files for format and content. If you are using emacs as your editor, simply position the insertion point at the beginning of your change and hit CX-4a to bring up the appropriate ChangeLog entry. See--magic! Similar functionality also exists for vi. A testsuite submission or sample program that will easily and simply show the existing error or test new functionality. The patch itself. If you are accessing the SVN repository use svn update; svn diff NEW; else, use diff -cp OLD NEW ... If your version of diff does not support these options, then get the latest version of GNU diff. The SVN Tricks wiki page has information on customising the output of svn diff. When you have all these pieces, bundle them up in a mail message and send it to libstdc++@gcc.gnu.org. All patches and related discussion should be sent to the libstdc++ mailing list. In common with the rest of GCC, patches should also be sent to the gcc-patches mailing list.

The libstdc++-v3 directory in the GCC sources contains the files needed to create the GNU C++ Library. It has subdirectories: doc Files in HTML and text format that document usage, quirks of the implementation, and contributor checklists. include All header files for the C++ library are within this directory, modulo specific runtime-related files that are in the libsupc++ directory. include/std Files meant to be found by #include <name> directives in standard-conforming user programs. include/c Headers intended to directly include standard C headers. [NB: this can be enabled via --enable-cheaders=c] include/c_global Headers intended to include standard C headers in the global namespace, and put select names into the std:: namespace. [NB: this is the default, and is the same as --enable-cheaders=c_global] include/c_std Headers intended to include standard C headers already in namespace std, and put select names into the std:: namespace. [NB: this is the same as --enable-cheaders=c_std] include/bits Files included by standard headers and by other files in the bits directory. include/backward Headers provided for backward compatibility, such as <backward/hash_map>. They are not used in this library. include/ext Headers that define extensions to the standard library. No standard header refers to any of them, in theory (there are some exceptions). scripts Scripts that are used during the configure, build, make, or test process. src Files that are used in constructing the library, but are not installed. src/c++98 Source files compiled using -std=gnu++98. src/c++11 Source files compiled using -std=gnu++11. src/filesystem Source files for the Filesystem TS. src/shared Source code included by other files under both src/c++98 and src/c++11 testsuites/[backward, demangle, ext, performance, thread, 17_* to 30_*] Test programs are here, and may be used to begin to exercise the library. Support for "make check" and "make check-install" is complete, and runs through all the subdirectories here when this command is issued from the build directory. Please note that "make check" requires DejaGNU 1.4 or later to be installed. Other subdirectories contain variant versions of certain files that are meant to be copied or linked by the configure script. Currently these are: config/abi config/allocator config/cpu config/io config/locale config/os In addition, a subdirectory holds the convenience library libsupc++. libsupc++ Contains the runtime library for C++, including exception handling and memory allocation and deallocation, RTTI, terminate handlers, etc. Note that glibc also has a bits/ subdirectory. We need to be careful not to collide with names in its bits/ directory. For example <bits/std_mutex.h> has to be renamed from <bits/mutex.h>. Another solution would be to rename bits to (e.g.) cppbits. In files throughout the system, lines marked with an "XXX" indicate a bug or incompletely-implemented feature. Lines marked "XXX MT" indicate a place that may require attention for multi-thread safety.

Identifiers that conflict and should be avoided. This is the list of names reserved to the implementation that have been claimed by certain compilers and system headers of interest, and should not be used in the library. It will grow, of course. We generally are interested in names that are not all-caps, except for those like "_T" For Solaris: _B _C _L _N _P _S _U _X _E1 .. _E24 Irix adds: _A _G MS adds: _T BSD adds: __used __unused __inline _Complex __istype __maskrune __tolower __toupper __wchar_t __wint_t _res _res_ext __tg_* SPU adds: __ea For GCC: [Note that this list is out of date. It applies to the old name-mangling; in G++ 3.0 and higher a different name-mangling is used. In addition, many of the bugs relating to G++ interpreting these names as operators have been fixed.] The full set of __* identifiers (combined from gcc/cp/lex.c and gcc/cplus-dem.c) that are either old or new, but are definitely recognized by the demangler, is: __aa __aad __ad __addr __adv __aer __als __alshift __amd __ami __aml __amu __aor __apl __array __ars __arshift __as __bit_and __bit_ior __bit_not __bit_xor __call __cl __cm __cn __co __component __compound __cond __convert __delete __dl __dv __eq __er __ge __gt __indirect __le __ls __lt __max __md __method_call __mi __min __minus __ml __mm __mn __mult __mx __ne __negate __new __nop __nt __nw __oo __op __or __pl __plus __postdecrement __postincrement __pp __pt __rf __rm __rs __sz __trunc_div __trunc_mod __truth_andif __truth_not __truth_orif __vc __vd __vn SGI badnames: __builtin_alloca __builtin_fsqrt __builtin_sqrt __builtin_fabs __builtin_dabs __builtin_cast_f2i __builtin_cast_i2f __builtin_cast_d2ll __builtin_cast_ll2d __builtin_copy_dhi2i __builtin_copy_i2dhi __builtin_copy_dlo2i __builtin_copy_i2dlo __add_and_fetch __sub_and_fetch __or_and_fetch __xor_and_fetch __and_and_fetch __nand_and_fetch __mpy_and_fetch __min_and_fetch __max_and_fetch __fetch_and_add __fetch_and_sub __fetch_and_or __fetch_and_xor __fetch_and_and __fetch_and_nand __fetch_and_mpy __fetch_and_min __fetch_and_max __lock_test_and_set __lock_release __lock_acquire __compare_and_swap __synchronize __high_multiply __unix __sgi __linux__ __i386__ __i486__ __cplusplus __embedded_cplusplus // long double conversion members mangled as __opr // http://gcc.gnu.org/ml/libstdc++/1999-q4/msg00060.html __opr

This library is written to appropriate C++ coding standards. As such, it is intended to precede the recommendations of the GNU Coding Standard, which can be referenced in full here: http://www.gnu.org/prep/standards/standards.html#Formatting The rest of this is also interesting reading, but skip the "Design Advice" part. The GCC coding conventions are here, and are also useful: http://gcc.gnu.org/codingconventions.html In addition, because it doesn't seem to be stated explicitly anywhere else, there is an 80 column source limit. ChangeLog entries for member functions should use the classname::member function name syntax as follows: 1999-04-15 Dennis Ritchie <dr@att.com> * src/basic_file.cc (__basic_file::open): Fix thinko in _G_HAVE_IO_FILE_OPEN bits. Notable areas of divergence from what may be previous local practice (particularly for GNU C) include: 01. Pointers and references char* p = "flop"; char& c = *p; -NOT- char *p = "flop"; // wrong char &c = *p; // wrong Reason: In C++, definitions are mixed with executable code. Here, p is being initialized, not *p. This is near-universal practice among C++ programmers; it is normal for C hackers to switch spontaneously as they gain experience. 02. Operator names and parentheses operator==(type) -NOT- operator == (type) // wrong Reason: The == is part of the function name. Separating it makes the declaration look like an expression. 03. Function names and parentheses void mangle() -NOT- void mangle () // wrong Reason: no space before parentheses (except after a control-flow keyword) is near-universal practice for C++. It identifies the parentheses as the function-call operator or declarator, as opposed to an expression or other overloaded use of parentheses. 04. Template function indentation template<typename T> void template_function(args) { } -NOT- template<class T> void template_function(args) {}; Reason: In class definitions, without indentation whitespace is needed both above and below the declaration to distinguish it visually from other members. (Also, re: "typename" rather than "class".) T often could be int, which is not a class. ("class", here, is an anachronism.) 05. Template class indentation template<typename _CharT, typename _Traits> class basic_ios : public ios_base { public: // Types: }; -NOT- template<class _CharT, class _Traits> class basic_ios : public ios_base { public: // Types: }; -NOT- template<class _CharT, class _Traits> class basic_ios : public ios_base { public: // Types: }; 06. Enumerators enum { space = _ISspace, print = _ISprint, cntrl = _IScntrl }; -NOT- enum { space = _ISspace, print = _ISprint, cntrl = _IScntrl }; 07. Member initialization lists All one line, separate from class name. gribble::gribble() : _M_private_data(0), _M_more_stuff(0), _M_helper(0) { } -NOT- gribble::gribble() : _M_private_data(0), _M_more_stuff(0), _M_helper(0) { } 08. Try/Catch blocks try { // } catch (...) { // } -NOT- try { // } catch(...) { // } 09. Member functions declarations and definitions Keywords such as extern, static, export, explicit, inline, etc go on the line above the function name. Thus virtual int foo() -NOT- virtual int foo() Reason: GNU coding conventions dictate return types for functions are on a separate line than the function name and parameter list for definitions. For C++, where we have member functions that can be either inline definitions or declarations, keeping to this standard allows all member function names for a given class to be aligned to the same margin, increasing readability. 10. Invocation of member functions with "this->" For non-uglified names, use this->name to call the function. this->sync() -NOT- sync() Reason: Koenig lookup. 11. Namespaces namespace std { blah blah blah; } // namespace std -NOT- namespace std { blah blah blah; } // namespace std 12. Spacing under protected and private in class declarations: space above, none below i.e. public: int foo; -NOT- public: int foo; 13. Spacing WRT return statements. no extra spacing before returns, no parenthesis i.e. } return __ret; -NOT- } return __ret; -NOT- } return (__ret); 14. Location of global variables. All global variables of class type, whether in the "user visible" space (e.g., cin) or the implementation namespace, must be defined as a character array with the appropriate alignment and then later re-initialized to the correct value. This is due to startup issues on certain platforms, such as AIX. For more explanation and examples, see src/globals.cc. All such variables should be contained in that file, for simplicity. 15. Exception abstractions Use the exception abstractions found in functexcept.h, which allow C++ programmers to use this library with -fno-exceptions. (Even if that is rarely advisable, it's a necessary evil for backwards compatibility.) 16. Exception error messages All start with the name of the function where the exception is thrown, and then (optional) descriptive text is added. Example: __throw_logic_error(__N("basic_string::_S_construct NULL not valid")); Reason: The verbose terminate handler prints out exception::what(), as well as the typeinfo for the thrown exception. As this is the default terminate handler, by putting location info into the exception string, a very useful error message is printed out for uncaught exceptions. So useful, in fact, that non-programmers can give useful error messages, and programmers can intelligently speculate what went wrong without even using a debugger. 17. The doxygen style guide to comments is a separate document, see index. The library currently has a mixture of GNU-C and modern C++ coding styles. The GNU C usages will be combed out gradually. Name patterns: For nonstandard names appearing in Standard headers, we are constrained to use names that begin with underscores. This is called "uglification". The convention is: Local and argument names: __[a-z].* Examples: __count __ix __s1 Type names and template formal-argument names: _[A-Z][^_].* Examples: _Helper _CharT _N Member data and function names: _M_.* Examples: _M_num_elements _M_initialize () Static data members, constants, and enumerations: _S_.* Examples: _S_max_elements _S_default_value Don't use names in the same scope that differ only in the prefix, e.g. _S_top and _M_top. See BADNAMES for a list of forbidden names. (The most tempting of these seem to be and "_T" and "__sz".) Names must never have "__" internally; it would confuse name unmanglers on some targets. Also, never use "__[0-9]", same reason. -------------------------- [BY EXAMPLE] #ifndef _HEADER_ #define _HEADER_ 1 namespace std { class gribble { public: gribble() throw(); gribble(const gribble&); explicit gribble(int __howmany); gribble& operator=(const gribble&); virtual ~gribble() throw (); // Start with a capital letter, end with a period. inline void public_member(const char* __arg) const; // In-class function definitions should be restricted to one-liners. int one_line() { return 0 } int two_lines(const char* arg) { return strchr(arg, 'a'); } inline int three_lines(); // inline, but defined below. // Note indentation. template<typename _Formal_argument> void public_template() const throw(); template<typename _Iterator> void other_template(); private: class _Helper; int _M_private_data; int _M_more_stuff; _Helper* _M_helper; int _M_private_function(); enum _Enum { _S_one, _S_two }; static void _S_initialize_library(); }; // More-or-less-standard language features described by lack, not presence. # ifndef _G_NO_LONGLONG extern long long _G_global_with_a_good_long_name; // avoid globals! # endif // Avoid in-class inline definitions, define separately; // likewise for member class definitions: inline int gribble::public_member() const { int __local = 0; return __local; } class gribble::_Helper { int _M_stuff; friend class gribble; }; } // Names beginning with "__": only for arguments and // local variables; never use "__" in a type name, or // within any name; never use "__[0-9]". #endif /* _HEADER_ */ namespace std { template<typename T> // notice: "typename", not "class", no space long_return_value_type<with_many, args> function_name(char* pointer, // "char *pointer" is wrong. char* argument, const Reference& ref) { // int a_local; /* wrong; see below. */ if (test) { nested code } int a_local = 0; // declare variable at first use. // char a, b, *p; /* wrong */ char a = 'a'; char b = a + 1; char* c = "abc"; // each variable goes on its own line, always. // except maybe here... for (unsigned i = 0, mask = 1; mask; ++i, mask <<= 1) { // ... } } gribble::gribble() : _M_private_data(0), _M_more_stuff(0), _M_helper(0) { } int gribble::three_lines() { // doesn't fit in one line. } } // namespace std

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