= Boost.Predef :author: Rene Rivera :toc: left :toclevels: 3 :sectanchors: :sectnums: :nofooter: :source-highlighter: pygments :source-language: cpp :caution-caption: ⚑ :important-caption: ‼ :note-caption: ℹ :tip-caption: ☀ :warning-caption: ⚠ :CPP: C++ :predef_symbol: Symbol :predef_version: Version :predef_detection: pass:q[*detection*] ifdef::backend-html5[] ++++ ++++ endif::[] == Introduction This library defines a set of compiler, architecture, operating system, library, and other version numbers from the information it can gather of C, {CPP}, Objective C, and Objective {CPP} 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. === 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. === 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 pre-compiled 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 {CPP}, C, Objective C, and Objective {CPP}. === 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 {CPP}, 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 V{CPP} 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`. === 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. * And obviously there's lots of work to do in reformulating the existing Boost libraries to use the Predef library. * And there's the continuing work of adding definitions for present and future compilers, platforms, architectures, languages, and libraries. == Using the predefs To use the automatically defined predefs one needs to only include the single top-level header: [source] ---- #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 {CPP} standard library in use. * `BOOST_OS_` for the operating system we are compiling to. * `BOOST_PLAT_` for platforms on top of operating system or compilers. * `BOOST_ENDIAN_` for endianness of the os and architecture combination. * `BOOST_HW_` for hardware specific features. * `BOOST_HW_SIMD` for SIMD (Single Instruction Multiple Data) detection. 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 {CPP} compiler one would: [source] ---- #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: [source] ---- #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: [source] ---- #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 an `*_AVAILABLE` macro defined only when the particular aspect is detected. I.e. a definition equivalent to: [source] ---- #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. === The `*_EMULATED` macros Predef definitions are guaranteed to be uniquely detected within one category. But there are contexts under which multiple underlying detections are possible. The well known example of this is detection of GCC and MSVC compilers which are commonly emulated by other compilers by defining the same base macros. To account for this detection headers are allowed to define `*_EMULATED` predefs when this situation is detected. The emulated predefs will be set to the version number of the detection instead of the regular predef macro for that detection. For example MSVC will set `BOOST_COMP_MSVC_EMULATED` but not set `BOOST_COMP_MSVC`, and it will also set `BOOST_COMP_MSVC_AVAILABLE`. === 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: [source] ---- #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: [source] ---- #define MY_APPLICATION_VERSION_NUMBER BOOST_VERSION_NUMBER(2,0xA,015) ---- Is equivalent to: [source] ---- #define MY_APPLICATION_VERSION_NUMBER BOOST_VERSION_NUMBER(2,10,13) ---- == 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. === 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_NOT_AVAILABLE`. * The predef must be redefined to a non-zero value once detected. * The predef must, by default, be defined to `BOOST_VERSION_NUMBER_AVAILABLE` when the predef is detected. * If possible, the predef will be defined as the version number detected. * The predef must define `*_AVAILABLE` macros as needed. * The predef must define a symbolic constant string name macro. * The predef must declare itself, after being defined, for the testing system. * The predef must guarantee that it is the only one defined as detected per category. * But a predef can define `*_EMULATED` macros to indicate that it was previously detected by another header and is being "emulated" by the system. Note that the `*_AVAILABLE` macros must still be defined in this situation. 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. === 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: [source] ---- /* 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 ---- If the detection depends on the detection of another predef you should include those headers here. [source] ---- #include ---- 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: [source] ---- #include #include ---- The Predef library uses https://asciidoctor.org/[Asciidoctor] 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: [source] ---- /* tag::reference[] = `BOOST_category_tag` Documentation about what is detected. */ #define BOOST_category_tag BOOST_VERSION_NUMBER_NOT_AVAILABLE ---- 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 removes 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_AVAILABLE`" as the fallback to minimally indicate that the predef was detected: [source] ---- #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_AVAILABLE # endif #endif ---- We also need to provide the `*_AVAILABLE` versions of the predef. [source] ---- #if BOOST_category_tag # define BOOST_category_tag_AVAILABLE #endif ---- And for convenience we also want to provide a `*_NAME` macro: [source] ---- #define BOOST_category_tag_NAME "Name" ---- We close out the include guard at this point. We do whis before the test declaration as the testing system includes the headers multiple times to generate the needed testing code. [source] ---- #endif ---- 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: [source] ---- #include BOOST_PREDEF_DECLARE_TEST(BOOST_category_tag,BOOST_category_tag_NAME) ---- === Adding exclusive predefs For headers of predefs that need to be mutually exclusive in the detection we need to add checks and definitions to detect when the predef is detected by multiple headers. Internally compiler, operating system, and platforms define `BOOST_PREDEF_DETAIL_COMP_DETECTED`, `BOOST_PREDEF_DEFAIL_OS_DETECTED`, and `BOOST_PREDEF_DETAIL_PLAT_DETECTED` respectively when the predef is first detected. This is used to guard against multiple definition of the detection in later included headers. In those cases the detection would instead be written as: [source] ---- #if !BOOST_PREDEF_DETAIL_category_DETECTED && (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 ---- And we also include a header that defines the `*_DETECTED` macro when we have the detection: [source] ---- #if BOOST_category_tag # define BOOST_category_tag_AVAILABLE # include #endif ---- Everything else about the header is the same as the basic detection header. === Adding an exclusive but emulated predef Because compilers are frequently emulated by other compilers we both want to have exclusive detection of the compiler and also provide information that we detected the emulation of the compiler. To accomplish this we define a local `*_DETECTION` macro for the compiler detection. And conditionally define either the base compiler predef `BOOST_COMP_compiler` or the alternate `BOOST_COMP_compiler_EMULATED` predef. The initial detection would look like: [source] ---- #if (condition_a) # if (condition_b) # define BOOST_COMP_tag_DETECTION BOOST_VERSION_NUMBER(major,minor,patch) # else # define BOOST_COMP_tag_DETECTION BOOST_VERSION_NUMBER_AVAILABLE # endif #endif ---- And then we can conditionally define the base or emulated predefs: [source] ---- #ifdef BOOST_COMP_tag_DETECTION # if defined(BOOST_PREDEF_DETAIL_COMP_DETECTED) # define BOOST_COMP_tag_EMULATED BOOST_COMP_tag_DETECTION # else # undef BOOST_COMP_tag # define BOOST_COMP_tag BOOST_COMP_tag_DETECTION # endif # define BOOST_category_tag_AVAILABLE # include #endif ---- === Using utility pattern macros By including: [source] ---- #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: [source] ---- #define BOOST_COMP_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. == Reference === `BOOST_ARCH` architecture macros :leveloffset: +3 include::../include/boost/predef/architecture/alpha.h[tag=reference] include::../include/boost/predef/architecture/arm.h[tag=reference] include::../include/boost/predef/architecture/blackfin.h[tag=reference] include::../include/boost/predef/architecture/convex.h[tag=reference] include::../include/boost/predef/architecture/ia64.h[tag=reference] include::../include/boost/predef/architecture/m68k.h[tag=reference] include::../include/boost/predef/architecture/mips.h[tag=reference] include::../include/boost/predef/architecture/parisc.h[tag=reference] include::../include/boost/predef/architecture/ppc.h[tag=reference] include::../include/boost/predef/architecture/ptx.h[tag=reference] include::../include/boost/predef/architecture/pyramid.h[tag=reference] include::../include/boost/predef/architecture/riscv.h[tag=reference] include::../include/boost/predef/architecture/rs6k.h[tag=reference] include::../include/boost/predef/architecture/sparc.h[tag=reference] include::../include/boost/predef/architecture/superh.h[tag=reference] include::../include/boost/predef/architecture/sys370.h[tag=reference] include::../include/boost/predef/architecture/sys390.h[tag=reference] include::../include/boost/predef/architecture/x86.h[tag=reference] include::../include/boost/predef/architecture/z.h[tag=reference] include::../include/boost/predef/architecture/x86/32.h[tag=reference] include::../include/boost/predef/architecture/x86/64.h[tag=reference] :leveloffset: -3 === `BOOST_COMP` compiler macros :leveloffset: +3 include::../include/boost/predef/compiler/borland.h[tag=reference] include::../include/boost/predef/compiler/clang.h[tag=reference] include::../include/boost/predef/compiler/comeau.h[tag=reference] include::../include/boost/predef/compiler/compaq.h[tag=reference] include::../include/boost/predef/compiler/diab.h[tag=reference] include::../include/boost/predef/compiler/digitalmars.h[tag=reference] include::../include/boost/predef/compiler/dignus.h[tag=reference] include::../include/boost/predef/compiler/edg.h[tag=reference] include::../include/boost/predef/compiler/ekopath.h[tag=reference] include::../include/boost/predef/compiler/gcc.h[tag=reference] include::../include/boost/predef/compiler/gcc_xml.h[tag=reference] include::../include/boost/predef/compiler/greenhills.h[tag=reference] include::../include/boost/predef/compiler/hp_acc.h[tag=reference] include::../include/boost/predef/compiler/iar.h[tag=reference] include::../include/boost/predef/compiler/ibm.h[tag=reference] include::../include/boost/predef/compiler/intel.h[tag=reference] include::../include/boost/predef/compiler/kai.h[tag=reference] include::../include/boost/predef/compiler/llvm.h[tag=reference] include::../include/boost/predef/compiler/metaware.h[tag=reference] include::../include/boost/predef/compiler/metrowerks.h[tag=reference] include::../include/boost/predef/compiler/microtec.h[tag=reference] include::../include/boost/predef/compiler/mpw.h[tag=reference] include::../include/boost/predef/compiler/nvcc.h[tag=reference] include::../include/boost/predef/compiler/palm.h[tag=reference] include::../include/boost/predef/compiler/pgi.h[tag=reference] include::../include/boost/predef/compiler/sgi_mipspro.h[tag=reference] include::../include/boost/predef/compiler/sunpro.h[tag=reference] include::../include/boost/predef/compiler/tendra.h[tag=reference] include::../include/boost/predef/compiler/visualc.h[tag=reference] include::../include/boost/predef/compiler/watcom.h[tag=reference] :leveloffset: -3 === `BOOST_LANG` language standards macros :leveloffset: +3 include::../include/boost/predef/language/cuda.h[tag=reference] include::../include/boost/predef/language/objc.h[tag=reference] include::../include/boost/predef/language/stdc.h[tag=reference] include::../include/boost/predef/language/stdcpp.h[tag=reference] :leveloffset: -3 === `BOOST_LIB` library macros :leveloffset: +3 include::../include/boost/predef/library/c/cloudabi.h[tag=reference] include::../include/boost/predef/library/c/gnu.h[tag=reference] include::../include/boost/predef/library/c/uc.h[tag=reference] include::../include/boost/predef/library/c/vms.h[tag=reference] include::../include/boost/predef/library/c/zos.h[tag=reference] include::../include/boost/predef/library/std/cxx.h[tag=reference] include::../include/boost/predef/library/std/dinkumware.h[tag=reference] include::../include/boost/predef/library/std/libcomo.h[tag=reference] include::../include/boost/predef/library/std/modena.h[tag=reference] include::../include/boost/predef/library/std/msl.h[tag=reference] include::../include/boost/predef/library/std/roguewave.h[tag=reference] include::../include/boost/predef/library/std/sgi.h[tag=reference] include::../include/boost/predef/library/std/stdcpp3.h[tag=reference] include::../include/boost/predef/library/std/stlport.h[tag=reference] include::../include/boost/predef/library/std/vacpp.h[tag=reference] :leveloffset: -3 === `BOOST_OS` operating system macros :leveloffset: +3 include::../include/boost/predef/os/aix.h[tag=reference] include::../include/boost/predef/os/amigaos.h[tag=reference] include::../include/boost/predef/os/beos.h[tag=reference] include::../include/boost/predef/os/bsd.h[tag=reference] include::../include/boost/predef/os/cygwin.h[tag=reference] include::../include/boost/predef/os/haiku.h[tag=reference] include::../include/boost/predef/os/hpux.h[tag=reference] include::../include/boost/predef/os/ios.h[tag=reference] include::../include/boost/predef/os/irix.h[tag=reference] include::../include/boost/predef/os/linux.h[tag=reference] include::../include/boost/predef/os/macos.h[tag=reference] include::../include/boost/predef/os/os400.h[tag=reference] include::../include/boost/predef/os/qnxnto.h[tag=reference] include::../include/boost/predef/os/solaris.h[tag=reference] include::../include/boost/predef/os/unix.h[tag=reference] include::../include/boost/predef/os/vms.h[tag=reference] include::../include/boost/predef/os/windows.h[tag=reference] include::../include/boost/predef/os/bsd/bsdi.h[tag=reference] include::../include/boost/predef/os/bsd/dragonfly.h[tag=reference] include::../include/boost/predef/os/bsd/free.h[tag=reference] include::../include/boost/predef/os/bsd/net.h[tag=reference] include::../include/boost/predef/os/bsd/open.h[tag=reference] :leveloffset: -3 === `BOOST_PLAT` platform macros :leveloffset: +3 include::../include/boost/predef/platform/android.h[tag=reference] include::../include/boost/predef/platform/cloudabi.h[tag=reference] include::../include/boost/predef/platform/ios.h[tag=reference] include::../include/boost/predef/platform/mingw.h[tag=reference] include::../include/boost/predef/platform/mingw32.h[tag=reference] include::../include/boost/predef/platform/mingw64.h[tag=reference] include::../include/boost/predef/platform/windows_desktop.h[tag=reference] include::../include/boost/predef/platform/windows_phone.h[tag=reference] include::../include/boost/predef/platform/windows_runtime.h[tag=reference] include::../include/boost/predef/platform/windows_server.h[tag=reference] include::../include/boost/predef/platform/windows_store.h[tag=reference] include::../include/boost/predef/platform/windows_system.h[tag=reference] include::../include/boost/predef/platform/windows_uwp.h[tag=reference] :leveloffset: -3 === `BOOST_HW` hardware macros :leveloffset: +3 include::../include/boost/predef/hardware/simd.h[tag=reference] include::../include/boost/predef/hardware/simd/arm.h[tag=reference] include::../include/boost/predef/hardware/simd/arm/versions.h[tag=reference] include::../include/boost/predef/hardware/simd/ppc.h[tag=reference] include::../include/boost/predef/hardware/simd/ppc/versions.h[tag=reference] include::../include/boost/predef/hardware/simd/x86.h[tag=reference] include::../include/boost/predef/hardware/simd/x86/versions.h[tag=reference] include::../include/boost/predef/hardware/simd/x86_amd.h[tag=reference] include::../include/boost/predef/hardware/simd/x86_amd/versions.h[tag=reference] :leveloffset: -3 === Other macros :leveloffset: +3 include::../include/boost/predef/other/endian.h[tag=reference] include::../include/boost/predef/other/workaround.h[tag=reference] :leveloffset: -3 === Version definition macros :leveloffset: +3 include::../include/boost/predef/version_number.h[tag=reference] include::../include/boost/predef/make.h[tag=reference] :leveloffset: -3 == Check Utilities The `predef_check` utility provides a facility for building a program that will check a given set of expressions against the definitions it detected when it was built. === `predef_check` programs Even though there is only one `predef_check` program, there are variations for each of the languages that are detected by Predef to match the convention for sources files. For all of them one invokes with a list of expression arguments. The expressions are evaluated within the context of the particular `predef_check` program and if they all are true zero (0) is returned. Otherwise the index of the first false expression is returned. The expression syntax is simple: [source,jam] ---- predef-definition [ relational-operator version-value ] ---- _predef-definition_ can be any of the Predef definitions. For example `BOOST_COMP_GCC`. _relational-operator_ can be any of: `>`, `<`, `>=`, `<=`, `==` and `!=`. _version-number_ can be a full or partial version triplet value. If it's a partial version triple it is completed with zeros. That is `x.y` is equivalent to `x.y.0` and `x` is equivalent to `x.0.0`. The _relations-operator_ and _version-number_ can be omitted. In which case it is equivalent to: [source,jam] ---- predef-definition > 0.0.0 ---- === Using with Boost.Build You can use the `predef_check` programs directly from Boost Build to configure target requirements. This is useful for controlling what gets built as part of your project based on the detailed version information available in Predef. The basic use is simple: [source,jam] ---- import path-to-predef-src/tools/check/predef : check require : predef-check predef-require ; exe my_windows_program : windows_source.cpp : [ predef-require "BOOST_OS_WINDOWS" ] ; ---- That simple use case will skip building the `my_windows_program` unless one is building for Windows. Like the direct `predef_check` you can pass multiple expressions using relational comparisons. For example: [source,jam] ---- import path-to-predef-src/tools/check/predef : check require : predef-check predef-require ; lib my_special_lib : source.cpp : [ predef-require "BOOST_OS_WINDOWS != 0" "BOOST_OS_VMS != 0"] ; ---- And in that case the `my_special_lib` is built only when the OS is not Windows or VMS. The `requires` rule is a special case of the `check` rule. And is defined in terms of it: [source,jam] ---- rule require ( expressions + : language ? ) { return [ check $(expressions) : $(language) : : no ] ; } ---- The expression can also use explicit "and", "or" logical operators to for more complex checks: [source,jam] ---- import path-to-predef-src/tools/check/predef : check require : predef-check predef-require ; lib my_special_lib : source.cpp : [ predef-require "BOOST_OS_WINDOWS" or "BOOST_OS_VMS"] ; ---- You can use the `check` rule for more control and to implement something other than control of what gets built. The definition for the `check` rule is: [source,jam] ---- rule check ( expressions + : language ? : true-properties * : false-properties * ) ---- When invoked as a requirement of a Boost Build target this rule will add the `true-properties` to the target if all the `expressions` evaluate to true. Otherwise the `false-properties` get added as requirements. For example you could use it to enable or disable features in your programs: [source,jam] ---- import path-to-predef-src/tools/check/predef : check require : predef-check predef-require ; exe my_special_exe : source.cpp : [ predef-check "BOOST_OS_WINDOWS == 0" : : ENABLE_WMF=0 : ENABLE_WMF=1 ] ; ---- For both `check` and `require` the `language` argument controls which variant of the `predef_check` program is used to check the expressions. It defaults to "c++", but can be any of: "c", "cpp", "objc", and "objcpp". :leveloffset: +1 include::history.adoc[] include::todo.adoc[] :leveloffset: -1 == Acknowledgements 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/{CPP} 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. [colophon] == Colophon Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt) Copyright 2005-2020 Rene Rivera; Copyright 2015 Charly Chevalier; Copyright 2015 Joel Falcou