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## Guidelines for Boost Authors

The <boost/config.hpp> header is used to pass configuration information to other boost files, allowing them to cope with platform dependencies such as arithmetic byte ordering, compiler pragmas, or compiler shortcomings. Without such configuration information, many current compilers would not work with the Boost libraries.

Centralizing configuration information in this header reduces the number of files that must be modified when porting libraries to new platforms, or when compilers are updated. Ideally, no other files would have to be modified when porting to a new platform.

Configuration headers are controversial because some view them as condoning broken compilers and encouraging non-standard subsets. Adding settings for additional platforms and maintaining existing settings can also be a problem. In other words, configuration headers are a necessary evil rather than a desirable feature. The boost config.hpp policy is designed to minimize the problems and maximize the benefits of a configuration header.

Note that:

- Boost library implementers are not required to "`#include <boost/config.hpp>`", and are not required in any way to support compilers that do not comply with the C++ Standard (ISO/IEC 14882).
- If a library implementer wishes to support some non-conforming compiler, or to support some platform specific feature, "`#include <boost/config.hpp>`" is the preferred way to obtain configuration information not available from the standard headers such as `<climits>`, etc.
- If configuration information can be deduced from standard headers such as `<climits>`, use those standard headers rather than `<boost/config.hpp>`.
- Boost files that use macros defined in `<boost/config.hpp>` should have sensible, standard conforming, default behavior if the macro is not defined. This means that the starting point for porting `<boost/config.hpp>` to a new platform is simply to define nothing at all specific to that platform. In the rare case where there is no sensible default behavior, an #error message should describe the problem.
- If a Boost library implementer wants something added to `config.hpp`, post a request on the Boost mailing list. There is no guarantee such a request will be honored; the intent is to limit the complexity of config.hpp.
- The intent is to support only compilers which appear on their way to becoming C++ Standard compliant, and only recent releases of those compilers at that.
- The intent is not to disable mainstream features now well-supported by the majority of compilers, such as namespaces, exceptions, RTTI, or templates.

### Disabling Compiler Warnings

The header `<boost/config/warning_disable.hpp>` can be used to disable certain compiler warings that are hard or impossible to otherwise remove.

Note that:

- This header should never be included by another Boost header, it should only ever be used by a library source file or a test case.
- The header should be included before you include any other header.
- This header only disables warnings that are hard or impossible to otherwise deal with, and which are typically emitted by one compiler only, or in one compilers own standard library headers.

Currently it disables the following warnings:

Compiler | Warning
--- | ---
Visual C++ 8 and later | C4996: Error 'function': was declared deprecated
Intel C++ | Warning 1786: relates to the use of "deprecated" standard library functions rather like C4996 in Visual C++.

### Adding New Defect Macros

When you need to add a new defect macro - either to fix a problem with an existing library, or when adding a new library - distil the issue down to a simple test case; often, at this point other (possibly better) workarounds may become apparent. Secondly always post the test case code to the boost mailing list and invite comments; remember that C++ is complex and that sometimes what may appear a defect, may in fact turn out to be a problem with the authors understanding of the standard.

When you name the macro, follow the `BOOST_NO_`SOMETHING naming convention, so that it's obvious that this is a macro reporting a defect.

Finally, add the test program to the regression tests. You will need to place the test case in a `.ipp` file with the following comments near the top:

    //  MACRO:         BOOST_NO_FOO
    //  TITLE:         foo
    //  DESCRIPTION:   If the compiler fails to support foo

These comments are processed by the autoconf script, so make sure the format follows the one given. The file should be named "`boost_no_foo.ipp`", where foo is the defect description - try and keep the file name under the Mac 30 character filename limit though. You will also need to provide a function prototype "`int test()`" that is declared in a namespace with the same name as the macro, but in all lower case, and which returns zero on success:

    namespace boost_no_foo {

    int test()
    {
        // test code goes here:
        //
        return 0;
    }

    }

Once the test code is in place in libs/config/test, updating the configuration test system proceeds as:

- cd into `libs/config/tools` and run `bjam` : this generates the `.cpp` file test cases from the `.ipp` file, updates the libs/config/test/all/Jamfile.v2, `config_test.cpp` and `config_info.cpp`.
- cd into `libs/config/test/all` and run `bjam` MACRONAME`compiler-list` : where MACRONAME is the name of the new macro, and `compiler-list` is a space separated list of compilers to test with. You should see the tests pass with those compilers that don't have the defect, and fail with those that do.
- cd into `libs/config/test` and run `bjam config_info config_test` `compiler-list` : `config_info` should build and run cleanly for all the compilers in `compiler-list` while `config_test` should fail for those that have the defect, and pass for those that do not.

Then you should:

- Define the defect macro in those config headers that require it.
- Document the macro in this documentation (please do not forget this step!!)
- Commit everything.
- Keep an eye on the regression tests for new failures in Boost.Config caused by the addition.
- Start using the macro.

### Adding New Feature Test Macros

When you need to add a macro that describes a feature that the standard does not require, follow the convention for adding a new defect macro (above), but call the macro `BOOST_HAS_FOO`, and name the test file "`boost_has_foo.ipp`". Try not to add feature test macros unnecessarily, if there is a platform specific macro that can already be used (for example `_WIN32`, `__BEOS__`, or `__linux`) to identify the feature then use that. Try to keep the macro to a feature group, or header name, rather than one specific API (for example `BOOST_HAS_NL_TYPES_H` rather than `BOOST_HAS_CATOPEN`). If the macro describes a POSIX feature group, then add boilerplate code to <boost/config/suffix.hpp> to auto-detect the feature where possible (if you are wondering why we can't use POSIX feature test macro directly, remember that many of these features can be added by third party libraries, and are not therefore identified inside `<unistd.h>`).

### Modifying the Boost Configuration Headers

The aim of boost's configuration setup is that the configuration headers should be relatively stable - a boost user should not have to recompile their code just because the configuration for some compiler that they're not interested in has changed. Separating the configuration into separate compiler/standard library/platform sections provides for part of this stability, but boost authors require some amount of restraint as well, in particular:

<boost/config.hpp> should never change, don't alter this file.

<boost/config/user.hpp> is included by default, don't add extra code to this file unless you have to. If you do, please remember to update libs/config/tools/configure.in as well.

<boost/config/suffix.hpp> is always included so be careful about modifying this file as it breaks dependencies for everyone. This file should include only "boilerplate" configuration code, and generally should change only when new macros are added.

<boost/config/select_compiler_config.hpp>, <boost/config/select_platform_config.hpp> and <boost/config/select_stdlib_config.hpp> are included by default and should change only if support for a new compiler/standard library/platform is added.

The compiler/platform/standard library selection code is set up so that unknown platforms are ignored and assumed to be fully standards compliant - this gives unknown platforms a "sporting chance" of working "as is" even without running the configure script.

When adding or modifying the individual mini-configs, assume that future, as yet unreleased versions of compilers, have all the defects of the current version. Although this is perhaps unnecessarily pessimistic, it cuts down on the maintenance of these files, and experience suggests that pessimism is better placed than optimism here!