[article Predef [quickbook 1.7] [version 1.0] [authors [Rivera, Rene]] [copyright 2005 Rene Rivera, 2008-2013 Redshift Software Inc] [purpose Identification and specification of predefined macros.] [license Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at [@http://www.boost.org/LICENSE_1_0.txt]) ] [source-mode c++] ] [section Introduction] This library defines a set of compiler, architecture, operating system, library, and other version numbers from the information it can gather of C, C++, Objective C, and Objective C++ predefined macros or those defined in generally available headers. The idea for this library grew out of a proposal to extend the Boost Config library to provide more, and consistent, information than the feature definitions it supports. What follows is an edited version of that brief proposal. [heading Proposal] The idea is to define a set of macros to identify compilers and consistently represent their version. This includes: * A unique BOOST_VERSION_NUMBER(major,minor,patch) macro to specify version numbers (unfortunately, the name BOOST_VERSION is already taken to designate the version number of boost itself). * A compiler identification macro, suitable for use in `#if`/`#elif` directives, for each of the supported compilers. All macros would be defined, regardless of the compiler. The one macro corresponding to the compiler being used would be defined, in terms of BOOST_VERSION_NUMBER, to carry the exact compiler version. All other macros would expand to an expression evaluating to false (for instance, the token 0) to indicate that the corresponding compiler is not present. * "Null values" could be set, for all macros, in boost/config/select_compiler.hpp; then, for each compiler the corresponding identification macro would be #undef and re-#defined in the corresponding boost/compiler/(cc).hpp; however in the context of the Boost.Config infrastructure using a "prefix" header (to be introduced) or boost/config/suffix.hpp is a better solution. [heading Current Library] The current Predef library is now, both an independent library, and expanded in scope. It includes detection and definition of architectures, compilers, languages, libraries, operating systems, and endianness. The key benefits are: * Version numbers that are always defined so that one doesn't have to guard with `#ifdef`. * Guard macros that can be used for `#ifdef` checks. * All possible definitions are included with the single `#include ` so that it's friendly to precompiled header usage. * Specific definitions can be included, ex. `#include ` for single checks. * Predefs can be directly used in both preprocessor and compiler expressions for comparison to other similarly defined values. * The headers are usable from multiple languages, that support the C preprocessor. In particular C++, C, Objective C, and Objective C++. [heading Design choices] An important design choice concerns how to represent compiler versions by means of a single integer number suitable for use in preprocessing directives. Let's do some calculation. The "basic" signed type for preprocessing constant-expressions is long in C90 (and C++, as of 2006) and intmax_t in C99. The type long shall at least be able to represent the number [^+2 147 483 647]. This means the most significant digit can only be 0, 1 or 2; and if we want all decimal digits to be able to vary between 0 and 9, the largest range we can consider is [^\[0, 999 999 999\]]. Distributing evenly, this means 3 decimal digits for each version number part. So we can: # use an uneven distribution or # use more bits (a larger type) or # use 3/3/3 and have the particular compiler/platform/stdlib deal with setting the numbers within the 3-digit range. It appears relatively safe to go for the first option and set it at 2/2/5. That covers CodeWarrior and others, which are up to and past 10 for the major number. Some compilers use the build number in lieu of the patch one; five digits (which is already reached by VC++ 8) seems a reasonable limit even in this case. [note A 2/2/6 scheme would allow for bigger patch/build numbers at the cost, for instance, of limiting the major version number to 20 (or, with further constraints, to 21).] It might reassure the reader that this decision is actually encoded in one place in the code; the definition of `BOOST_VERSION_NUMBER`. [heading Future work] Even though the basics of this library are done, there is much work that can be done: * Right now we limit the detection of libraries to known built-in predefined macros, and to guaranteed to exist system and library headers. It might be interesting to add something like auto-configuration predefs. This way we can add definitions for user specific libraries and features. * Along with the above, it might be good to add some user control as to which headers are included with the top-level header. Although in the current form of the library this is less of an issue as one can include the specific headers one needs. * Additionally, even if there is no auto-configure style option.. It would be good to add optionally included headers so that user can get consistent version number definitions for libraries they use. * Having a consistent set of version number definitions opens the door to improving the user level syntax of libraries that do checks against version numbers. Specifically Boost Config's `BOOST_WORKAROUND` macro would benefit from a more readable syntax. As would the `BOOST_TESTED_AT` detail macro. * And obviously there's lots of work to do in reformulating the existing Boost libraries to use the Predef library once it's accepted. * And there's the continuing work of adding definitions for present and future compilers, platforms, architectures, languages, and libraries. [endsect] [/Introduction] [section Using the predefs] To use the automatically defined predefs one needs to only include the single top-level header: `` #include `` This defines [*all] the version macros known to the library. For each macro it will be defined to either a /zero/ valued expression for when the particular item is not detected, and to a /positive/ value if it is detected. The predef macros fall onto five categories each with macros of a particular prefix: * `BOOST_ARCH_`for system/CPU architecture one is compiling for. * `BOOST_COMP_` for the compiler one is using. * `BOOST_LANG_` for language standards one is compiling against. * `BOOST_LIB_C_` and `BOOST_LIB_STD_` for the C and C++ standard library in use. * `BOOST_OS_` for the operating system we are compiling to. * `BOOST_ENDIAN_` for endianness of the os and architecture combination. [note The detected definitions are for the configuration one is targeting during the compile. In particular in a cross-compile this means the target system, and not the host system.] One uses the individual definitions to compare against specific versions by comparing against the `BOOST_VERSION_NUMBER` macro. For example, to make a choice based on the version of the GCC C++ compiler one would: `` #include #include int main() { if (BOOST_COMP_GNUC >= BOOST_VERSION_NUMBER(4,0,0)) std::cout << "GCC compiler is at least version 4.0.0" << std::endl; else std::cout << "GCC compiler is at older than version 4.0.0, or not a GCC compiler" << std::endl; return 0; } `` As you might notice above the `else` clause also covers the case where the particular compiler is not detected. But one can make the test also test for the detection. All predef definitions are defined as a zero (0) expression when not detected. Hence one could use the detection with a natural single condition. For example: `` #include #include int main() { if (BOOST_COMP_GNUC) std::cout << "This is GNU GCC!" << std::endl; else std::cout << "Not GNU GCC." << std::endl; return 0; } `` And since the predef's are preprocessor definitions the same is possible from the preprocessor: `` #include #include #if BOOST_COMP_GNUC #if BOOST_COMP_GNUC >= BOOST_VERSION_NUMBER(4,0,0) const char * the_compiler = "GNU GCC, of at least version 4." #else const char * the_compiler = "GNU GCC, less than version 4." #endif #else const char * the_compiler = "Not GNU GCC." #endif int main() { std::cout << the_compiler << std::endl; return 0; } `` In addition, for each version macro defined there is a `*_AVAILABLE` macro defined only when the particular aspect is detected. I.e. a definition equivalent to: `` #if BOOST_PREDEF_ABC #define BOOST_PREDEF_ABC_AVAILABLE #endif `` Also for each aspect there is a macro defined with a descriptive name of what the detection is. [heading Using the `BOOST_VERSION_NUMBER` macro] All the predefs are defined to be a use of the `BOOST_VERSION_NUMBER` macro. The macro takes individual major, minor, and patch value expressions: `` #define BOOST_VERSION_NUMBER( major, minor, patch ) ... `` The arguments are: # Major version number, as a constant value expression in the range [0,99]. # Minor version number, as a constant value expression in the range [0,99]. # Patch-level version number, as a constant value expression in the range [0,99999]. The ranges for each are "enforced" by the use of a modulo ("%"), i.e. truncation, as opposed to a clamp. And hence this means that the limits are enforced only enough to keep from having out-of-range problems. But not enough to prevent other kinds of problems. Like exceeding the range and getting false detections, or non-detections. It is up to the individual predefs to ensure correct usage beyond the range guarantee. The values for the arguments can be any preprocessor valid constant value expression. Only constant value arithmetic is used in the definition of the `BOOST_VERSION_NUMBER` macro and in any of the other predef macros. This means that any allowed base is possible, i.e. binary, octal, decimal, and hexadecimal. For example: `` #define MY_APPLICATION_VERSION_NUMBER BOOST_VERSION_NUMBER(2,0xA,015) `` Is equivalent to: `` #define MY_APPLICATION_VERSION_NUMBER BOOST_VERSION_NUMBER(2,10,13) `` [endsect] [section Adding new predefs] We know that a library like this one will be an eternal work-in-progress. And as such we expect, and look forward to, others contributing corrections and additions to the predefs. With that in mind we need to keep a consistent way of defining the new predefs. Hence all current, and future, predefs follow the same structure and requirements. [heading Requirements of the header] All predefs need to follow a set of requirements: * The headers must use the Boost Software License. * The predef must, by default, be defined as `BOOST_VERSION_NUMBER(0,0,0)`. * The predef must be redefined to a non-zero value once detected. * The predef must, by default, be defined to `BOOST_VERSION_NUMBER(0,0,1)` when the predef is detected. * If possible, the predef will be defined as the version number detected. * The predef must define `*_AVAILABLE` macros. * The predef must define a symbolic constant string name macro. * The predef must declare itself, after being defined, for the testing system. And there are some extra guidelines that predef headers should follow: * The detection should avoid including extra headers that might otherwise not be included by default. * If the detection must include a header, prefer guarding it within the detection if possible. * If the detection must include headers unconditionally, and has a choice of headers to include, prefer the ones with the least impact. I.e. include the one with the minimal set of definitions and other dependencies. [heading Structure of the header] For general consistency it's suggested that new predef headers follow the structure below, as current predef headers do. First we have the copyright and license statement, followed by the include guard: `` /* Copyright Jane Doe YYYY Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) */ #ifndef BOOST_PREDEF_category_tag_H #define BOOST_PREDEF_category_tag_H `` Depending on how you are defining the predef you will at minimum have to include the `version_number.h` header. But you might also want to include the `make.h` header for the version number decomposing utility macros: `` #include #include `` The Predef library uses Quickbook for documentation and for the individual predefs to appear in the reference section we add in-code documentation followed by the zero-value default definition of the predef macro. We strongly recommend this particular placement of the documentation and default definition because some development environments automatically interpret this and provide in-line help for the macro. In particular this works for the popular Eclipse IDE: `` /*` [heading `BOOST_category_tag`] Documentation about what is detected. */ #define BOOST_category_tag BOOST_VERSION_NUMBER(0,0,0) `` Next is the detection and definition of the particular predef. The structure for this is to do a single overall check (`condition_a`) and place further version detection inside this. The first action inside the overall check is to "`#undef BOOST_category_tag`" which undefines the zero-value default. The rest is up to the you how to do the checks for defining the version. But at minimum it must "`#define BOOST_category_tag BOOST_VERSION_NUMBER(0,0,1)`" as the fallback to minimally indicate that the predef was detected: `` #if (condition_a) # undef BOOST_category_tag # if (condition_b) # define BOOST_category_tag BOOST_VERSION_NUMBER(major,minor,patch) # else # define BOOST_category_tag BOOST_VERSION_NUMBER(0,0,1) # endif #endif `` We also need to provide the `*_AVAILABLE` versions of the predef. And for convenience we also want to provide a `*_NAME` macro: `` #if BOOST_category_tag # define BOOST_category_tag_AVAILABLE #endif #define BOOST_catagory_tag_NAME "Name" `` The testing of the predef macros is automated to generate checks for all the defined predefs, whether detected or not. To do this we need to declare the predef to the test system. This declaration is empty for regular use. And during the test programs they expand out specially to create informational output: `` #include BOOST_PREDEF_DECLARE_TEST(BOOST_category_tag,BOOST_category_tag_NAME) `` And, of course, we last need to close out the include guard: `` #endif `` [heading Using utility pattern macros] By including: `` #include `` One will get a set of utility macros to decompose common version macros as defined by compilers. For example the EDG compiler uses a simple 3-digit version macro (M,N,P). It can be decomposed and defined as: `` #define BOOST_CCOMP_EDG BOOST_PREDEF_MAKE_N_N_N(__EDG_VERSION__) `` The decomposition macros are split into three types: decimal decomposition, hexadecimal decomposition, and date decomposition. They follow the format of using "N" for decimal, "F" for hexadecimal, and "Y", "M", "D" for dates. [endsect] [def __predef_symbol__ Symbol] [def __predef_version__ Version] [def __predef_detection__ *detection*] [section Reference] [section `BOOST_ARCH` architecture macros] [include ../include/boost/predef/architecture/*.h] [include ../include/boost/predef/architecture/x86/*.h] [endsect] [section `BOOST_COMP` compiler macros] [include ../include/boost/predef/compiler/*.h] [endsect] [section `BOOST_LANG` language standards macros] [include ../include/boost/predef/language/*.h] [endsect] [section `BOOST_LIB` library macros] [include ../include/boost/predef/library/c/*.h] [include ../include/boost/predef/library/std/*.h] [endsect] [section `BOOST_OS` operating system macros] [include ../include/boost/predef/os/*.h] [include ../include/boost/predef/os/bsd/*.h] [endsect] [section Other macros] [include ../include/boost/predef/other/*.h] [endsect] [section Version definition macros] [include ../include/boost/predef/version_number.h] [include ../include/boost/predef/make.h] [endsect] [endsect] [section Acknoledgements] The comprehensiveness of this library would not be possible without the existence of the indispensable resource that is the [@http://sourceforge.net/p/predef/ Pre-defined C/C++ Compiler Macros] Project. It was, and continues to be, the primary source of the definitions that make up this library. Thanks to Bjorn Reese and all the volunteers that make that resource possible. This library would be an incoherent mess if it weren't for Boost community that provided invaluable feedback for the eight years that it took to polish into a useable form. In particular I would like to thank: Mathias Gaunard, Robert Stewart, Joël Lamotte, Lars Viklund, Nathan Ridge, Artyom Beilis, Joshua Boyce, Gottlob Frege, Thomas Heller, Edward Diener, Dave Abrahams, Iain Denniston, Dan Price, Ioannis Papadopoulos, and Robert Ramey. And thanks to Joel Falcou for managing the review of this library. [endsect]