Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

PrevUpHomeNext

Guidelines for Boost Authors

Disabling Compiler Warnings
Adding New Defect Macros
Adding New Feature Test Macros
Modifying the Boost Configuration Headers

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:

The header <boost/config/warning_disable.hpp> can be used to disable certain compiler warnings 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++.

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.

    The xxx_pass_test and the xxx_fail_test should both report **passed**.

    If MACRONAME is not defined when it should be defined, xxx_pass_test will not report **passed**. If MACRONAME is defined when it should not be defined, xxx_fail_test will not report **passed**.

  • 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.

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>).

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!


PrevUpHomeNext