[article Boost.Predef
    [quickbook 1.7]
    [version 1.6]
    [authors [Rivera, Rene]]
    [copyright 2005-2016 Rene Rivera]
    [copyright 2015 Charly Chevalier]
    [copyright 2015 Joel Falcou]
    [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++]
    [category miscellaneous]
    [id predef]
    [dirname predef]
]

[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 <boost/predef.h>`
  so that it's friendly to precompiled header usage.
* Specific definitions can be included, ex. `#include <boost/predef/os/windows.h>`
  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.
* 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.

[endsect] [/Introduction]

[section Using the predefs]

To use the automatically defined predefs one needs to only include the
single top-level header:

``
  #include <boost/predef.h>
``

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_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 C++ compiler one would:

``
  #include <boost/predef.h>
  #include <iostream>

  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 <boost/predef.h>
  #include <iostream>

  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 <boost/predef.h>
  #include <iostream>

  #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:

``
  #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 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_COM_MSVC`, and it will also set `BOOST_COMP_MSVC_AVAILABLE`.

[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_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.

[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
``

If the detection depends on the detection of another predef you should
include those headers here.

``
#include <boost/predef/CATEGORY_TAG/DEPENDENCY.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 <boost/predef/version_number.h>
#include <boost/predef/make.h>
``

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_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 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_AVAILABLE`" 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_AVAILABLE
#    endif
#endif
``

We also need to provide the `*_AVAILABLE` versions of the predef.

``
#if BOOST_category_tag
#   define BOOST_category_tag_AVAILABLE
#endif
``

And for convenience we also want to provide a `*_NAME` macro:

``
#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/detail/test.h>
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 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:

``
#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:

``
#if BOOST_category_tag
#   define BOOST_category_tag_AVAILABLE
#   include <boost/predef/detail/CATEGORY_detected.h>
#endif
``

Everything else about the header is the same as the basic detection header.

[heading 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:

``
#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:

``
#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 <boost/predef/detail/comp_detected.h>
#endif
``

[heading Using utility pattern macros]

By including:

``
#include <boost/predef/make.h>
``

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 `BOOST_PLAT` platform macros]
[include ../include/boost/predef/platform/*.h]
[endsect]

[section `BOOST_HW` hardware macros]
[include ../include/boost/predef/hardware/*.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 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.

[heading [^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:

[teletype]
``
predef-definition [ relational-operator version-value ]
``
[c++]

[~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 ommited. In
which case it is equivalent to:

[teletype]
``
predef-definition > 0.0.0
``
[c++]

[heading 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:

[teletype]
``
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" ] ;
``
[c++]

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 mutiple expressions using relational comparisons.
For example:

[teletype]
``
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"] ;
``
[c++]

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:

[teletype]
``
rule require ( expressions + : language ? )
{
    return [ check $(expressions) : $(language) : : <build>no ] ;
}
``
[c++]

The expression can also use explicit "and", "or" logical operators
to for more complex checks:


[teletype]
``
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"] ;
``
[c++]

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:

[teletype]
``
rule check ( expressions + : language ? : true-properties * : false-properties * )
``
[c++]

When invoked as a reuirement 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:

[teletype]
``
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"
        : : <define>ENABLE_WMF=0
        : <define>ENABLE_WMF=1 ] ;
``
[c++]

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".

[endsect]

[include history.qbk]
[include todo.qbk]

[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]
