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9	<p align="left"><img src="../../boost.png" alt="Boost" width="277" height="86"></p>
10	<h1 align="center">Boost Configuration Reference</h1>
11	<h2>Contents</h2>
12	<pre><a href="#configuring">Configuring Boost for Your Platform</a>
13	<a href="#default_config">Using the default boost configuration
14	</a> <a href="#header">The <boost\config.hpp> header</a>
15	<a href="#config_script">Using the configure script</a>
16	<a href="#user_settable">User settable options</a>
17	<a href="#advanced_config">Advanced configuration usage</a>
18	<a href="#testing">Testing the boost configuration</a>
19	<a href="#macro_ref">Boost Macro Reference</a>
20	<a href="#defects">Macros that describe defects</a>
21	<a href="#features">Macros that describe optional features</a>
22	<a href="#helpers">Boost Helper Macros</a>
23	<a href="#info_macros">Boost Informational Macros</a>
24	<a href="#source">Macros for libraries with separate source code</a>
25	<a href="#guidelines">Guidelines for Boost Authors</a>
26	<a href="#defect_guidelines">Adding New Defect Macros</a>
27	<a href="#feature_guidelines">Adding New Feature Test Macros</a>
28	<a href="#modify_guidelines">Modifying the Boost Configuration Headers</a>
29	<a href="#rationale">Rationale</a>
30	<a href="#Acknowledgements">Acknowledgements</a></pre>
31	<h2><a name="configuring"></a>Configuring Boost for Your Platform</h2>
32	<h4><a name="default_config"></a>Using the default boost configuration</h4>
33	<p>Boost is comes already configured for most common compilers and platforms; you
34	should be able to use boost "as is". Since the compiler is configured
35	separately from the standard library, the default configuration should work
36	even if you replace the compiler's standard library with a third-party standard
37	library (like <a href="http://www.stlport.org">STLport</a>).
38	</p>
39	<p>Using boost "as is" without trying to reconfigure is the recommended method for
40	using boost. You can, however, run the configure script if you want to, and
41	there are regression tests provided that allow you to test the current boost
42	configuration with your particular compiler setup.</p>
43	<p>Boost library users can request support for additional compilers or platforms
44	by visiting our <a href="http://sourceforge.net/tracker/?group_id=7586">Tracker</a>
45	and submitting a support request.
46	</p>
47	<h4>The <a href="../../boost/config.hpp"><boost/config.hpp></a> <a name="header">
48	header</a></h4>
49	<p>Boost library implementations access configuration macros via <code>#include
50	<boost/config.hpp></code>.
51	</p>
52	<P>While Boost library users are not required to include that file directly, or
53	use those configuration macros, such use is acceptable.  The configuration
54	macros are documented as to their purpose, usage, and limitations which makes
55	them usable by both Boost library and user code.
56	</P>
57	<P>Boost <A href="#info_macros">informational</A> or <A href="#helpers">helper</A>
58	macros are designed for use by Boost users as well as for our own internal
59	use.  Note however, that the <A href="#features">feature test</A> and <A href="#defects">
60	defect test</A> macros were designed for internal use by Boost libraries,
61	not user code, so they can change at any time (though no gratuitous changes are
62	made to them). Boost library problems resulting from changes to the
63	configuration macros are caught by the Boost regression tests, so the Boost
64	libraries are updated to account for those changes. By contrast, Boost library
65	user code can be adversely affected by changes to the macros without warning.
66	The best way to keep abreast of changes to the macros used in user code is to
67	monitor the discussions on the Boost developers list.</P>
68	<h4><a name="config_script">Using the configure script</a></h4>
69	<p>If you know that boost is incorrectly configured for your particular setup, and
70	you are on a UNIX like platform, then you may want to try and improve things by
71	running the boost configure script. From a shell command prompt you will need
72	to cd into <boost-root>/libs/config/ and type:</p>
73	<pre>sh ./configure</pre>
74	<p>you will see a list of the items being checked as the script works it way
75	through the regression tests. Note that the configure script only really
76	auto-detects your compiler if it's called g++, c++ or CC. If you are using some
77	other compiler then you will need to set one or more of the following
78	environment variables:</p>
79	<table border="1" cellpadding="7" cellspacing="1" width="624">
80	<tr>
81	<td valign="top" width="50%"><p align="center"><b>Variable</b></p>
82	</td>
83	<td valign="top" width="50%"><p align="center"><b>Description</b></p>
84	</td>
85	</tr>
86	<tr>
87	<td valign="top" width="50%">CXX</td>
88	<td valign="top" width="50%">The name of the compiler, for example "c++".</td>
89	</tr>
90	<tr>
91	<td valign="top" width="50%">CXXFLAGS</td>
92	<td valign="top" width="50%">The compiler flags to use, for example "-O2".</td>
93	</tr>
94	<tr>
95	<td valign="top" width="50%">LDFLAGS</td>
96	<td valign="top" width="50%">The linker flags to use, for example "-L/mypath".</td>
97	</tr>
98	<tr>
99	<td valign="top" width="50%">LIBS</td>
100	<td valign="top" width="50%">Any libraries to link in, for example -lpthread.</td>
101	</tr>
102	</table>
103	<p>For example to run the configure script with HP aCC, you might use something
104	like:</p>
105	<pre>export CXX="aCC"
106	export CXXFLAGS="-Aa
107	-DAportable -D__HPACC_THREAD_SAFE_RB_TREE -DRWSTD_MULTI_THREAD -DRW_MULTI_THREAD -D_REENTRANT -D_THREAD_SAFE" export LDFLAGS="-DAportable"
108	export LIBS=
109	"-lpthread" sh
110	./configure</pre>
111	<p>However you run the configure script, when it finishes you will find a new
112	header - user.hpp - located in the <boost-root/libs/config/> directory. <b><i>Note
113	that configure does not install this header into your boost include path by
114	default.</i></b> This header contains all the options generated by the
115	configure script, plus a header-section that contains the user settable options
116	from the default version of <a href="../../boost/config/user.hpp">user.hpp</a> (located
117	under <boost-root>/boost/config/). There are two ways you can use this
118	header:</p>
119	<p>Option 1: copy the header into <boost-root>/boost/config/ so that it
120	replaces the default <a href="../../boost/config/user.hpp">user.hpp</a> provided
121	by boost. This option allows only one configure-generated setup; boost
122	developers should avoid this option, as it incurs the danger of accidentally
123	committing a configure-modified user.hpp to the cvs repository (something you
124	will not be thanked for!).</p>
125	<p>Option 2: give the header a more memorable name, and place it somewhere
126	convenient, then define the macro BOOST_USER_CONFIG to point to it. For example
127	create a new sub-directory <boost-root>/boost/config/user/, and copy the
128	header there; for example as "multithread-gcc-config.hpp". Then when compiling
129	add the command line option:
130	-DBOOST_USER_CONFIG="boost/config/user/multithread-gcc-config.hpp", and boost
131	will use the new configuration header. This option allows you to generate more
132	than one configuration header, and to keep them separate from the boost source
133	- so that updates to the source do not interfere with your configuration.</p>
134	<h4><a name="user_settable"></a>User settable options</h4>
135	<p>There are some configuration-options that represent user choices, rather than
136	compiler defects or platform specific options. These are listed in
137	<boost/config/user.hpp> and at the start of a configure-generated
138	user.hpp header. You can define these on the command line, or by editing
139	<boost/config/user.hpp>, they are listed in the following table: </p>
140	<table border="1" cellpadding="7" cellspacing="1" width="100%">
141	<tr>
142	<td valign="top" width="48%"><p align="center"><b>Macro</b></p>
143	</td>
144	<td valign="top" width="52%"><p align="center"><b>Description</b></p>
145	</td>
146	</tr>
147	<tr>
148	<td valign="top" width="48%">BOOST_USER_CONFIG</td>
149	<td valign="top" width="52%">When defined, it should point to the name of the user
150	configuration file to include prior to any boost configuration files. When not
151	defined, defaults to <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>.</td>
152	</tr>
153	<tr>
154	<td valign="top" width="48%">BOOST_COMPILER_CONFIG</td>
155	<td valign="top" width="52%">When defined, it should point to the name of the
156	compiler configuration file to use. Defining this cuts out the compiler
157	selection logic, and eliminates the dependency on the header containing that
158	logic. For example if you are using gcc, then you could define
159	BOOST_COMPILER_CONFIG to "<a href="../../boost/config/compiler/gcc.hpp">boost/config/compiler/gcc.hpp</a>".</td>
160	</tr>
161	<tr>
162	<td valign="top" width="48%">BOOST_STDLIB_CONFIG</td>
163	<td valign="top" width="52%">When defined, it should point to the name of the
164	standard library configuration file to use. Defining this cuts out the standard
165	library selection logic, and eliminates the dependency on the header containing
166	that logic. For example if you are using STLport, then you could define
167	BOOST_STDLIB_CONFIG to "<a href="../../boost/config/stdlib/stlport.hpp">boost/config/stdlib/stlport.hpp</a>".</td>
168	</tr>
169	<tr>
170	<td valign="top" width="48%">BOOST_PLATFORM_CONFIG</td>
171	<td valign="top" width="52%">When defined, it should point to the name of the
172	platform configuration file to use. Defining this cuts out the platform
173	selection logic, and eliminates the dependency on the header containing that
174	logic. For example if you are compiling on linux, then you could define
175	BOOST_PLATFORM_CONFIG to "<a href="../../boost/config/platform/linux.hpp">boost/config/platform/linux.hpp</a>".</td>
176	</tr>
177	<tr>
178	<td valign="top" width="48%">BOOST_NO_COMPILER_CONFIG</td>
179	<td valign="top" width="52%">When defined, no compiler configuration file is
180	selected or included, define when the compiler is fully conformant with the
181	standard, or where the user header (see BOOST_USER_CONFIG), has had any options
182	necessary added to it, for example by an autoconf generated configure script.</td>
183	</tr>
184	<tr>
185	<td valign="top" width="48%">BOOST_NO_STDLIB_CONFIG</td>
186	<td valign="top" width="52%">When defined, no standard library configuration file
187	is selected or included, define when the standard library is fully conformant
188	with the standard, or where the user header (see BOOST_USER_CONFIG), has had
189	any options necessary added to it, for example by an autoconf generated
190	configure script.</td>
191	</tr>
192	<tr>
193	<td valign="top" width="48%">BOOST_NO_PLATFORM_CONFIG</td>
194	<td valign="top" width="52%">When defined, no platform configuration file is
195	selected or included, define when the platform is fully conformant with the
196	standard (and has no useful extra features), or where the user header (see
197	BOOST_USER_CONFIG), has had any options necessary added to it, for example by
198	an autoconf generated configure script.</td>
199	</tr>
200	<tr>
201	<td valign="top" width="48%">BOOST_NO_CONFIG</td>
202	<td valign="top" width="52%">Equivalent to defining all of
203	BOOST_NO_COMPILER_CONFIG, BOOST_NO_STDLIB_CONFIG and BOOST_NO_PLATFORM_CONFIG.</td>
204	</tr>
205	<tr>
206	<td valign="top">BOOST_STRICT_CONFIG</td>
207	<td>The normal behavior for compiler versions that are newer than the last known
208	version, is to assume that they have all the same defects as the last known
209	version. By setting this define, then compiler versions that are newer than the
210	last known version are assumed to be fully conforming with the standard. This
211	is probably most useful for boost developers or testers, and for those who want
212	to use boost to test beta compiler versions.</td>
213	</tr>
214	<tr>
215	<td valign="top">BOOST_ASSERT_CONFIG</td>
216	<td>When this flag is set, if the config finds anything unknown, then it will stop
217	with a #error rather than continue. Boost regression testers should set this
218	define, as should anyone who wants to quickly check whether boost is supported
219	on their platform.</td>
220	</tr>
221	<tr>
222	<td valign="top" width="48%">BOOST_DISABLE_THREADS</td>
223	<td valign="top" width="52%">When defined, disables threading support, even if the
224	compiler in its current translation mode supports multiple threads.</td>
225	</tr>
226	<tr>
227	<td valign="top">BOOST_DISABLE_WIN32</td>
228	<td>When defined, disables the use of Win32 specific API's, even when these are
229	available. Also has the effect of setting BOOST_DISABLE_THREADS unless
230	BOOST_HAS_PTHREADS is set. This option may be set automatically by the config
231	system when it detects that the compiler is in "strict mode".</td>
232	</tr>
233	<TR>
234	<TD vAlign="top">BOOST_DISABLE_ABI_HEADERS</TD>
235	<TD>Stops boost headers from including any prefix/suffix headers that normally
236	control things like struct packing and alignment.</TD>
237	</TR>
238	<TR>
239	<TD vAlign="top">BOOST_ABI_PREFIX</TD>
240	<TD>A prefix header to include in place of whatever boost.config would normally
241	select, any replacement should set up struct packing and alignment options as
242	required.</TD>
243	</TR>
244	<TR>
245	<TD vAlign="top">BOOST_ABI_SUFFIX</TD>
246	<TD>A suffix header to include in place of whatever boost.config would normally
247	select, any replacement should undo the effects of the prefix header.</TD>
248	</TR>
249	<TR>
250	<TD vAlign="top">BOOST_ALL_DYN_LINK</TD>
251	<TD>
252	<P>Forces all libraries that have separate source, to be linked as dll's rather
253	than static libraries on Microsoft Windows (this macro is used to turn on
254	__declspec(dllimport) modifiers, so that the compiler knows which symbols to
255	look for in a dll rather than in a static library).
256	</P>
257	<P>Note that there may be some libraries that can only be statically linked
258	(Boost.Test for example) and others which may only be dynamically linked
259	(Boost.Threads for example), in these cases this macro has no effect.</P>
260	</TD>
261	</TR>
262	<TR>
263	<TD vAlign="top">BOOST_WHATEVER_DYN_LINK</TD>
264	<TD>
265	<P>Forces library "whatever" to be linked as a dll rather than a static library on
266	Microsoft Windows: replace the WHATEVER part of the macro name with the name of
267	the library that you want to dynamically link to, for example use
268	BOOST_DATE_TIME_DYN_LINK or BOOST_REGEX_DYN_LINK etc (this macro is used
269	to turn on __declspec(dllimport) modifiers, so that the compiler knows which
270	symbols to look for in a dll rather than in a static library).
271	</P>
272	<P>Note that there may be some libraries that can only be statically linked
273	(Boost.Test for example) and others which may only be dynamically linked
274	(Boost.Threads for example), in these cases this macro is unsupported.</P>
275	</TD>
276	</TR>
277	<TR>
278	<TD vAlign="top">BOOST_ALL_NO_LIB</TD>
279	<TD>
280	<P>Tells the config system not to automatically select which libraries to link
281	against.
282	</P>
283	<P>Normally if a compiler supports #pragma lib, then the correct library build
284	variant will be automatically selected and linked against, simply by the act of
285	including one of that library's headers.  This macro turns that feature
286	off.</P>
287	</TD>
288	</TR>
289	<TR>
290	<TD vAlign="top">BOOST_WHATEVER_NO_LIB</TD>
291	<TD>
292	<P>Tells the config system not to automatically select which library to link
293	against for library "whatever", replace WHATEVER in the macro name with the
294	name of the library; for example BOOST_DATE_TIME_NO_LIB or
295	BOOST_REGEX_NO_LIB.
296	</P>
297	<P>Normally if a compiler supports #pragma lib, then the correct library build
298	variant will be automatically selected and linked against, simply by the act of
299	including one of that library's headers.  This macro turns that feature
300	off.</P>
301	</TD>
302	</TR>
303	<TR>
304	<TD vAlign="top">BOOST_LIB_DIAGNOSTIC</TD>
305	<TD>Causes the auto-linking code to output diagnostic messages indicating the name
306	of the library that is selected for linking.</TD>
307	</TR>
308	<TR>
309	<TD vAlign="top">BOOST_LIB_TOOLSET</TD>
310	<TD>Overrides the name of the toolset part of the name of library being linked to;
311	note if defined this must be defined to a quoted string literal, for example
312	"abc".</TD>
313	</TR>
314	</table>
315	<h4><a name="advanced_config"></a>Advanced configuration usage</h4>
316	<p>By setting various macros on the compiler command line or by editing <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>,
317	the boost configuration setup can be optimised in a variety of ways.
318	</p>
319	<p>Boost's configuration is structured so that the user-configuration is included
320	first (defaulting to <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>
321	if BOOST_USER_CONFIG is not defined). This sets up any user-defined policies,
322	and gives the user-configuration a chance to influence what happens next.
323	</p>
324	<p>Next the compiler, standard library, and platform configuration files are
325	included. These are included via macros (BOOST_COMPILER_CONFIG etc, <a href="#user_settable">
326	see user settable macros</a>), and if the corresponding macro is undefined
327	then a separate header that detects which compiler/standard library/platform is
328	in use is included in order to set these. The config can be told to ignore
329	these headers altogether if the corresponding BOOST_NO_XXX macro is set (for
330	example BOOST_NO_COMPILER_CONFIG to disable including any compiler
331	configuration file - <a href="#user_settable">see user settable macros</a>).
332	</p>
333	<p>Finally the boost configuration header, includes <<a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>>;
334	this header contains any boiler plate configuration code - for example where
335	one boost macro being set implies that another must be set also.</p>
336	<p>The following usage examples represent just a few of the possibilities:</p>
337	<p><u>Example 1, creating our own frozen configuration.</u></p>
338	<p>Lets suppose that we're building boost with Visual C++ 6, and STLport 4.0. Lets
339	suppose also that we don't intend to update our compiler or standard library
340	any time soon. In order to avoid breaking dependencies when we update boost, we
341	may want to "freeze" our configuration headers, so that we only have to rebuild
342	our project if the boost code itself has changed, and not because the boost
343	config has been updated for more recent versions of Visual C++ or STLport.
344	We'll start by realising that the configuration files in use are: <<a href="../../boost/config/compiler/visualc.hpp">boost/config/compiler/visualc.hpp</a>>
345	for the compiler, <<a href="../../boost/config/stdlib/stlport.hpp">boost/config/stdlib/stlport.hpp</a>>
346	for the standard library, and <<a href="../../boost/config/platform/win32.hpp">boost/config/platform/win32.hpp</a>>
347	for the platform. Next we'll create our own private configuration directory:
348	boost/config/mysetup/, and copy the configuration files into there. Finally,
349	open up <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>
350	and edit the following defines:</p>
351	<pre>#define BOOST_COMPILER_CONFIG "boost/config/mysetup/visualc.hpp"
352	#define BOOST_STDLIB_CONFIG "boost/config/mysetup/stlport.hpp"
353	#define BOOST_USER_CONFIG "boost/config/mysetup/win32.hpp"</pre>
354	<p>Now when you use boost, its configuration header will go straight to our
355	"frozen" versions, and ignore the default versions, you will now be insulated
356	from any configuration changes when you update boost. This technique is also
357	useful if you want to modify some of the boost configuration files; for example
358	if you are working with a beta compiler release not yet supported by boost.</p>
359	<p><u>Example 2: skipping files that you don't need.</u></p>
360	<p>Lets suppose that you're using boost with a compiler that is fully conformant
361	with the standard; you're not interested in the fact that older versions of
362	your compiler may have had bugs, because you know that your current version
363	does not need any configuration macros setting. In a case like this, you can
364	define BOOST_NO_COMPILER_CONFIG either on the command line, or in
365	<boost/config/user.hpp>, and miss out the compiler configuration header
366	altogether (actually you miss out two headers, one which works out what the
367	compiler is, and one that configures boost for it). This has two consequences:
368	the first is that less code has to be compiled, and the second that you have
369	removed a dependency on two boost headers.</p>
370	<p><u>Example 3: using configure script to freeze the boost configuration.</u></p>
371	<p>If you are working on a unix-like platform then you can use the configure
372	script to generate a "frozen" configuration based on your current compiler
373	setup - <a href="#config_script">see using the configure script</a> for more
374	details.</p>
375	<h4><a name="testing"></a>Testing the boost configuration</h4>
376	<p>The boost configuration library provides a full set of regression test programs
377	under the <boost-root>/libs/config/test/ sub-directory:</p>
378	<table border="1" cellpadding="7" cellspacing="1" width="100%">
379	<tr>
380	<td valign="top" width="50%"><p align="center"><b>File</b></p>
381	</td>
382	<td valign="top" width="50%"><p align="center"><b>Description</b></p>
383	</td>
384	</tr>
385	<tr>
386	<td valign="top" width="50%">config_info.cpp</td>
387	<td valign="top" width="50%">Prints out a detailed description of your
388	compiler/standard library/platform setup, plus your current boost
389	configuration. The information provided by this program useful in setting up
390	the boost configuration files. If you report that boost is incorrectly
391	configured for your compiler/library/platform then please include the output
392	from this program when reporting the changes required.</td>
393	</tr>
394	<tr>
395	<td valign="top" width="50%">config_test.cpp</td>
396	<td valign="top" width="50%">A monolithic test program that includes most of the
397	individual test cases. This provides a quick check to see if boost is correctly
398	configured for your compiler/library/platform.</td>
399	</tr>
400	<tr>
401	<td valign="top" width="50%">limits_test.cpp</td>
402	<td valign="top" width="50%">Tests your standard library's std::numeric_limits
403	implementation (or its boost provided replacement if BOOST_NO_LIMITS is
404	defined). This test file fails with most versions of numeric_limits, mainly due
405	to the way that some compilers treat NAN's and infinity.</td>
406	</tr>
407	<tr>
408	<td valign="top" width="50%">no_*pass.cpp</td>
409	<td valign="top" width="50%">Individual compiler defect test files. Each of these
410	should compile, if one does not then the corresponding BOOST_NO_XXX macro needs
411	to be defined - see each test file for specific details.</td>
412	</tr>
413	<tr>
414	<td valign="top" width="50%">no_*fail.cpp</td>
415	<td valign="top" width="50%">Individual compiler defect test files. Each of these
416	should <i>not</i> compile, if one does then the corresponding BOOST_NO_XXX
417	macro is defined when it need not be - see each test file for specific details.</td>
418	</tr>
419	<tr>
420	<td valign="top" width="50%">has_*pass.cpp</td>
421	<td valign="top" width="50%">Individual feature test files. If one of these does <i>not</i>
422	compile then the corresponding BOOST_HAS_XXX macro is defined when it should
423	not be - see each test file for specific details.</td>
424	</tr>
425	<tr>
426	<td valign="top" width="50%">has_*fail.cpp</td>
427	<td valign="top" width="50%">Individual feature test files. If one of these does
428	compile then the corresponding BOOST_HAS_XXX macro can be safely defined - see
429	each test file for specific details.</td>
430	</tr>
431	</table>
432	<p>Although you can run the configuration regression tests as individual test
433	files, there are rather a lot of them, so there are a couple of shortcuts to
434	help you out:</p>
435	<p>If you have built the <a href="../../more/regression.html">boost regression test
436	driver</a>, then you can use this to produce a nice html formatted report of
437	the results using the supplied test file.</p>
438	<p>Alternatively you can run the configure script like this:</p>
439	<pre>./configure --enable-test</pre>
440	<p>in which case the script will test the current configuration rather than
441	creating a new one from scratch.</p>
442	<p>If you are reporting the results of these tests for a new
443	platform/library/compiler then please include a log of the full compiler
444	output, the output from config_info.cpp, and the pass/fail test results.</p>
445	<h2><a name="macro_ref"></a>Boost Macro Reference</h2>
446	<h4><a name="defects"></a>Macros that describe defects:</h4>
447	<p>The following macros all describe features that are required by the C++
448	standard, if one of the following macros is defined, then it represents a
449	defect in the compiler's conformance with the standard.</p>
450	<table border="1" cellpadding="7" cellspacing="1" width="100%">
451	<tr>
452	<td valign="top" width="51%"><p align="center"><b>Macro</b></p>
453	</td>
454	<td valign="top" width="16%"><p align="center"><b>Section</b></p>
455	</td>
456	<td valign="top" width="33%"><p align="center"><b>Description</b></p>
457	</td>
458	</tr>
459	<tr>
460	<td>BOOST_BCB_PARTIAL_SPECIALIZATION_BUG</td>
461	<td>Compiler</td>
462	<td>The compiler exibits certain partial specialisation bug - probably Borland C++
463	Builder specific.</td>
464	</tr>
465	<TR>
466	<TD vAlign="top" width="51%">BOOST_FUNCTION_SCOPE_USING_DECLARATION_BREAKS_ADL</TD>
467	<TD vAlign="top" width="16%">Compiler</TD>
468	<TD vAlign="top" width="33%">Argument dependent lookup fails if there is a using
469	declaration for the symbol being looked up in the current scope.  For
470	example, <code>using boost::get_pointer;</code> prevents ADL from finding
471	overloads of <code>get_pointer</code> in namespaces nested inside boost (but
472	not elsewhere).  Probably Borland specific.</TD>
473	</TR>
474	<tr>
475	<td valign="top" width="51%">BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</td>
476	<td valign="top" width="16%">Compiler</td>
477	<td valign="top" width="33%">Compiler does not implement argument-dependent lookup
478	(also named Koenig lookup); see std::3.4.2 [basic.koenig.lookup]</td>
479	</tr>
480	<tr>
481	<td valign="top" width="51%">BOOST_NO_AUTO_PTR</td>
482	<td valign="top" width="16%">Standard library</td>
483	<td valign="top" width="33%">If the compiler / library supplies non-standard or
484	broken std::auto_ptr.</td>
485	</tr>
486	<tr>
487	<td valign="top" width="51%">BOOST_NO_CTYPE_FUNCTIONS</td>
488	<td valign="top" width="16%">Platform</td>
489	<td valign="top" width="33%">The Platform does not provide functions for the
490	character-classifying operations <ctype.h> and <cctype>, only
491	macros.</td>
492	</tr>
493	<tr>
494	<td valign="top" width="51%">BOOST_NO_CV_SPECIALIZATIONS</td>
495	<td valign="top" width="16%">Compiler</td>
496	<td valign="top" width="33%">If template specialisations for cv-qualified types
497	conflict with a specialisation for a cv-unqualififed type.</td>
498	</tr>
499	<tr>
500	<td valign="top" width="51%">BOOST_NO_CV_VOID_SPECIALIZATIONS</td>
501	<td valign="top" width="16%">Compiler</td>
502	<td valign="top" width="33%">If template specialisations for cv-void types
503	conflict with a specialisation for void.</td>
504	</tr>
505	<tr>
506	<td valign="top" width="51%">BOOST_NO_CWCHAR</td>
507	<td valign="top" width="16%">Platform</td>
508	<td valign="top" width="33%">The Platform does not provide <wchar.h> and
509	<cwchar>.</td>
510	</tr>
511	<tr>
512	<td valign="top" width="51%">BOOST_NO_CWCTYPE</td>
513	<td valign="top" width="16%">Platform</td>
514	<td valign="top" width="33%">The Platform does not provide <wctype.h> and
515	<cwctype>.</td>
516	</tr>
517	<tr>
518	<td valign="top" width="51%">BOOST_NO_DEPENDENT_NESTED_DERIVATIONS</td>
519	<td valign="top" width="16%">Compiler</td>
520	<td valign="top" width="33%">The compiler fails to compile a nested class that has
521	a dependent base class:<pre>template<typename T>
522	struct foo : {
523	template<typename U>
524	struct bar : public U {};
525	};</pre>
526	</td>
527	</tr>
528	<tr>
529	<td valign="top" width="51%">BOOST_NO_DEPENDENT_TYPES_IN_TEMPLATE_VALUE_PARAMETERS</td>
530	<td valign="top" width="16%">Compiler</td>
531	<td valign="top" width="33%">Template value parameters cannot have a dependent
532	type, for example:<pre>template<class T, typename T::type value>
533	class X { ... };</pre>
534	</td>
535	</tr>
536	<tr>
537	<td valign="top">BOOST_NO_EXCEPTION_STD_NAMESPACE</td>
538	<td valign="top">Standard Library</td>
539	<td>The standard library does not put some or all of the contents of
540	<exception> in namespace std.</td>
541	</tr>
542	<tr>
543	<td valign="top">BOOST_NO_EXCEPTIONS</td>
544	<td valign="top">Compiler</td>
545	<td>The compiler does not support exception handling (this setting is typically
546	required by many C++ compilers for embedded platforms). Note that there is no
547	requirement for boost libraries to honor this configuration setting - indeed
548	doing so may be impossible in some cases. Those libraries that do honor this
549	will typically abort if a critical error occurs - you have been warned!</td>
550	</tr>
551	<tr>
552	<td valign="top" width="51%">BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS</td>
553	<td valign="top" width="16%">Compiler</td>
554	<td valign="top" width="33%">Can only use deduced template arguments when calling
555	function template instantiations.</td>
556	</tr>
557	<tr>
558	<td valign="top" width="51%">BOOST_NO_FUNCTION_TEMPLATE_ORDERING</td>
559	<td valign="top" width="16%">Compiler</td>
560	<td valign="top" width="33%">The compiler does not perform function template
561	ordering or its function template ordering is incorrect.
562	<pre>template<typename T> void f(T); // #1
563	template<typename T, typename U> void f(T (*)(U)); // #2
564
565	void bar(int);
566
567	f(&bar); // should choose #2.</pre>
568	</td>
569	</tr>
570	<tr>
571	<td valign="top" width="51%">BOOST_NO_INCLASS_MEMBER_INITIALIZATION</td>
572	<td valign="top" width="16%">Compiler</td>
573	<td valign="top" width="33%">Compiler violates std::9.4.2/4.</td>
574	</tr>
575	<tr>
576	<td valign="top" width="51%">BOOST_NO_INTRINSIC_WCHAR_T</td>
577	<td valign="top" width="16%">Compiler</td>
578	<td valign="top" width="33%">The C++ implementation does not provide wchar_t, or
579	it is really a synonym for another integral type. Use this symbol to decide
580	whether it is appropriate to explicitly specialize a template on wchar_t if
581	there is already a specialization for other integer types.</td>
582	</tr>
583	<TR>
584	<TD vAlign="top" width="51%">BOOST_NO_IS_ABSTRACT</TD>
585	<TD vAlign="top" width="16%">Compiler</TD>
586	<TD vAlign="top" width="33%">The C++ compiler does not support SFINAE with
587	abstract types, this is covered by <A href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#337">
588	Core Language DR337</A>, but is not part of the current standard.
589	Fortunately most compilers that support SFINAE also support this DR.</TD>
590	</TR>
591	<tr>
592	<td valign="top" width="51%">BOOST_NO_LIMITS</td>
593	<td valign="top" width="16%">Standard library</td>
594	<td valign="top" width="33%">The C++ implementation does not provide the
595	<limits> header. Never check for this symbol in library code; always
596	include <boost/limits.hpp>, which guarantees to provide <code>std::numeric_limits</code>.</td>
597	</tr>
598	<tr>
599	<td valign="top" width="51%">BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS</td>
600	<td valign="top" width="16%">Standard library</td>
601	<td valign="top" width="33%">Constants such as numeric_limits<T>::is_signed
602	are not available for use at compile-time.</td>
603	</tr>
604	<tr>
605	<td>BOOST_NO_LONG_LONG_NUMERIC_LIMITS</td>
606	<td>Standard library</td>
607	<td>There is no specialization for numeric_limits<long long> and
608	numeric_limits<unsigned long long>. <boost/limits.hpp> will then
609	add these specializations as a standard library "fix" only if the compiler
610	supports the long long datatype.</td>
611	</tr>
612	<tr>
613	<td>BOOST_NO_MEMBER_FUNCTION_SPECIALIZATIONS</td>
614	<td>Compiler</td>
615	<td>The compiler does not support the specialization of individual member
616	functions of template classes.</td>
617	</tr>
618	<tr>
619	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATE_KEYWORD</td>
620	<td valign="top" width="16%">Compiler</td>
621	<td valign="top" width="33%">If the compiler supports member templates, but not
622	the template keyword when accessing member template classes.</td>
623	</tr>
624	<tr>
625	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATE_FRIENDS</td>
626	<td valign="top" width="16%">Compiler</td>
627	<td valign="top" width="33%">Member template friend syntax ("template<class
628	P> friend class frd;") described in the C++ Standard, 14.5.3, not supported.</td>
629	</tr>
630	<tr>
631	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATES</td>
632	<td valign="top" width="16%">Compiler</td>
633	<td valign="top" width="33%">Member template functions not fully supported.</td>
634	</tr>
635	<tr>
636	<td>BOOST_NO_MS_INT64_NUMERIC_LIMITS</td>
637	<td>Standard library</td>
638	<td>There is no specialization for numeric_limits<__int64> and
639	numeric_limits<unsigned __int64>. <boost/limits.hpp> will then add
640	these specializations as a standard library "fix", only if the compiler
641	supports the __int64 datatype.</td>
642	</tr>
643	<tr>
644	<td valign="top" width="51%">BOOST_NO_OPERATORS_IN_NAMESPACE</td>
645	<td valign="top" width="16%">Compiler</td>
646	<td valign="top" width="33%">Compiler requires inherited operator friend functions
647	to be defined at namespace scope, then using'ed to boost. Probably GCC
648	specific. See <a href="../../boost/operators.hpp">boost/operators.hpp</a> for
649	example.</td>
650	</tr>
651	<tr>
652	<td valign="top" width="51%">BOOST_NO_POINTER_TO_MEMBER_CONST</td>
653	<td valign="top" width="16%">Compiler</td>
654	<td valign="top" width="33%">The compiler does not correctly handle pointers to
655	const member functions, preventing use of these in overloaded function
656	templates. See <a href="../../boost/functional.hpp">boost/functional.hpp</a> for
657	example.</td>
658	</tr>
659	<TR>
660	<TD vAlign="top" width="51%">BOOST_NO_POINTER_TO_MEMBER_TEMPLATE_PARAMETERS</TD>
661	<TD vAlign="top" width="16%">Compiler</TD>
662	<TD vAlign="top" width="33%">Pointers to members don't work when used as template
663	parameters.</TD>
664	</TR>
665	<tr>
666	<td valign="top" width="51%">BOOST_NO_PRIVATE_IN_AGGREGATE</td>
667	<td valign="top" width="16%">Compiler</td>
668	<td valign="top" width="33%">The compiler misreads 8.5.1, treating classes as
669	non-aggregate if they contain private or protected member functions.</td>
670	</tr>
671	<TR>
672	<TD vAlign="top" width="51%">BOOST_NO_SFINAE</TD>
673	<TD vAlign="top" width="16%">Compiler</TD>
674	<TD vAlign="top" width="33%">The compiler does not support the "Substitution
675	Failure Is Not An Error" meta-programming idiom.</TD>
676	</TR>
677	<tr>
678	<td valign="top" width="51%">BOOST_NO_STD_ALLOCATOR</td>
679	<td valign="top" width="16%">Standard library</td>
680	<td valign="top" width="33%">The C++ standard library does not provide a standards
681	conforming std::allocator.</td>
682	</tr>
683	<tr>
684	<td valign="top" width="51%">BOOST_NO_STD_DISTANCE</td>
685	<td valign="top" width="16%">Standard library</td>
686	<td valign="top" width="33%">The platform does not have a conforming version of
687	std::distance.</td>
688	</tr>
689	<tr>
690	<td valign="top" width="51%">BOOST_NO_STD_ITERATOR</td>
691	<td valign="top" width="16%">Standard library</td>
692	<td valign="top" width="33%">The C++ implementation fails to provide the
693	std::iterator class.</td>
694	</tr>
695	<tr>
696	<td valign="top" width="51%">BOOST_NO_STD_ITERATOR_TRAITS</td>
697	<td valign="top" width="16%">Standard library</td>
698	<td valign="top" width="33%">The compiler does not provide a standard compliant
699	implementation of std::iterator_traits. Note that the compiler may still have a
700	non-standard implementation.</td>
701	</tr>
702	<tr>
703	<td valign="top" width="51%">BOOST_NO_STD_LOCALE</td>
704	<td valign="top" width="16%">Standard library</td>
705	<td valign="top" width="33%">The standard library lacks std::locale.</td>
706	</tr>
707	<tr>
708	<td valign="top" width="51%">BOOST_NO_STD_MESSAGES</td>
709	<td valign="top" width="16%">Standard library</td>
710	<td valign="top" width="33%">The standard library lacks a conforming std::messages
711	facet.</td>
712	</tr>
713	<tr>
714	<td valign="top" width="51%">BOOST_NO_STD_MIN_MAX</td>
715	<td valign="top" width="16%">Standard library</td>
716	<td valign="top" width="33%">The C++ standard library does not provide the min()
717	and max() template functions that should be in <algorithm>.</td>
718	</tr>
719	<tr>
720	<td valign="top">BOOST_NO_STD_OUTPUT_ITERATOR_ASSIGN</td>
721	<td valign="top">Standard library</td>
722	<td valign="top">Defined if the standard library's output iterators are not
723	assignable.</td>
724	</tr>
725	<tr>
726	<td valign="top" width="51%">BOOST_NO_STD_USE_FACET</td>
727	<td valign="top" width="16%">Standard library</td>
728	<td valign="top" width="33%">The standard library lacks a conforming
729	std::use_facet.</td>
730	</tr>
731	<tr>
732	<td>BOOST_NO_STD_WSTREAMBUF</td>
733	<td>Standard library</td>
734	<td>The standard library's implementation of std::basic_streambuf<wchar_t>
735	is either missing, incomplete, or buggy.</td>
736	</tr>
737	<tr>
738	<td valign="top" width="51%">BOOST_NO_STD_WSTRING</td>
739	<td valign="top" width="16%">Standard library</td>
740	<td valign="top" width="33%">The standard library lacks std::wstring.</td>
741	</tr>
742	<tr>
743	<td valign="top" width="51%">BOOST_NO_STDC_NAMESPACE</td>
744	<td valign="top" width="16%">Compiler/Platform</td>
745	<td valign="top" width="33%">The contents of C++ standard headers for C library
746	functions (the <c...> headers) have not been placed in namespace std.
747	This test is difficult - some libraries "fake" the std C functions by adding
748	using declarations to import them into namespace std, unfortunately they don't
749	necessarily catch all of them...</td>
750	</tr>
751	<tr>
752	<td valign="top" width="51%">BOOST_NO_STRINGSTREAM</td>
753	<td valign="top" width="16%">Standard library</td>
754	<td valign="top" width="33%">The C++ implementation does not provide the
755	<sstream> header.</td>
756	</tr>
757	<tr>
758	<td valign="top" width="51%">BOOST_NO_SWPRINTF</td>
759	<td valign="top" width="16%">Platform</td>
760	<td valign="top" width="33%">The platform does not have a conforming version of
761	swprintf.</td>
762	</tr>
763	<tr>
764	<td valign="top" width="51%">BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION</td>
765	<td valign="top" width="16%">Compiler</td>
766	<td valign="top" width="33%">Class template partial specialization (14.5.4
767	[temp.class.spec]) not supported.</td>
768	</tr>
769	<tr>
770	<td valign="top" width="51%">BOOST_NO_TEMPLATED_ITERATOR_CONSTRUCTORS</td>
771	<td valign="top" width="16%">Standard library</td>
772	<td valign="top" width="33%">The standard library does not provide templated
773	iterator constructors for its containers.</td>
774	</tr>
775	<tr>
776	<td>BOOST_NO_TEMPLATE_TEMPLATES</td>
777	<td>Compiler</td>
778	<td>The compiler does not support template template parameters.</td>
779	</tr>
780	<tr>
781	<td>BOOST_NO_UNREACHABLE_RETURN_DETECTION</td>
782	<td>Compiler</td>
783	<td>If a return is unreachable, then no return statement should be required,
784	however some compilers insist on it, while other issue a bunch of warnings if
785	it is in fact present.</td>
786	</tr>
787	<TR>
788	<TD vAlign="top" width="51%">BOOST_NO_USING_DECLARATION_OVERLOADS_FROM_TYPENAME_BASE</TD>
789	<TD vAlign="top" width="16%">Compiler</TD>
790	<TD vAlign="top" width="33%">The compiler will not accept a using
791	declaration that brings a function from a typename used as a base
792	class into a derived class if functions of the same name are present
793	in the derived class.</TD>
794	</TR>
795	<tr>
796	<td valign="top" width="51%">BOOST_NO_USING_TEMPLATE</td>
797	<td valign="top" width="16%">Compiler</td>
798	<td valign="top" width="33%">The compiler will not accept a using declaration that
799	imports a template class or function from another namespace. Originally a
800	Borland specific problem with imports to/from the global namespace, extended to
801	MSVC6 which has a specific issue with importing template classes (but not
802	functions).</td>
803	</tr>
804	<tr>
805	<td valign="top" width="51%">BOOST_NO_VOID_RETURNS</td>
806	<td valign="top" width="16%">Compiler</td>
807	<td valign="top" width="33%">The compiler does not allow a void function to return
808	the result of calling another void function.
809	<pre>void f() {}
810	void g() { return f(); }</pre>
811	</td>
812	</tr>
813	</table>
814	<h4><a name="features"></a>Macros that describe optional features:</h4>
815	<p>The following macros describe features that are not required by the C++
816	standard. The macro is only defined if the feature is present.</p>
817	<table border="1" cellpadding="7" cellspacing="1" width="100%">
818	<tr>
819	<td valign="top" width="48%"><p align="center"><b>Macro</b></p>
820	</td>
821	<td valign="top" width="15%"><p align="center"><b>Section</b></p>
822	</td>
823	<td valign="top" width="37%"><p align="center"><b>Description</b></p>
824	</td>
825	</tr>
826	<tr>
827	<td valign="top" width="48%">BOOST_HAS_BETHREADS</td>
828	<td valign="top" width="15%">Platform</td>
829	<td valign="top" width="37%">The platform supports BeOS style threads.</td>
830	</tr>
831	<tr>
832	<td>BOOST_HAS_CLOCK_GETTIME</td>
833	<td>Platform</td>
834	<td>The platform has the POSIX API clock_gettime.</td>
835	</tr>
836	<TR>
837	<TD>BOOST_HAS_DECLSPEC
838	</TD>
839	<TD>Compiler</TD>
840	<TD>The compiler uses __declspec(dllexport) and __declspec(dllimport) to
841	export/import symbols from dll's.</TD>
842	</TR>
843	<tr>
844	<td>BOOST_HAS_DIRENT_H</td>
845	<td>Platform</td>
846	<td>The platform has the POSIX header <dirent.h>.</td>
847	</tr>
848	<tr>
849	<td>BOOST_HAS_FTIME</td>
850	<td>Platform</td>
851	<td>The platform has the Win32 API GetSystemTimeAsFileTime.</td>
852	</tr>
853	<tr>
854	<td>BOOST_HAS_GETTIMEOFDAY</td>
855	<td>Platform</td>
856	<td>The platform has the POSIX API gettimeofday.</td>
857	</tr>
858	<tr>
859	<td valign="top" width="48%">BOOST_HAS_HASH</td>
860	<td valign="top" width="15%">Standard library</td>
861	<td valign="top" width="37%">The C++ implementation provides the (SGI) hash_set or
862	hash_map classes.</td>
863	</tr>
864	<tr>
865	<td>BOOST_HAS_LONG_LONG</td>
866	<td>Compiler</td>
867	<td>The compiler supports the long long data type.</td>
868	</tr>
869	<tr>
870	<td valign="top" width="48%">BOOST_HAS_MACRO_USE_FACET</td>
871	<td valign="top" width="15%">Standard library</td>
872	<td valign="top" width="37%">The standard library lacks a conforming
873	std::use_facet, but has a macro _USE(loc, Type) that does the job. This is
874	primarily for the Dinkumware std lib.</td>
875	</tr>
876	<tr>
877	<td>BOOST_HAS_MS_INT64</td>
878	<td>Compiler</td>
879	<td>The compiler supports the __int64 data type.</td>
880	</tr>
881	<tr>
882	<td>BOOST_HAS_NANOSLEEP</td>
883	<td>Platform</td>
884	<td>The platform has the POSIX API nanosleep.</td>
885	</tr>
886	<tr>
887	<td valign="top" width="48%">BOOST_HAS_NL_TYPES_H</td>
888	<td valign="top" width="15%">Platform</td>
889	<td valign="top" width="37%">The platform has an <nl_types.h>.</td>
890	</tr>
891	<tr>
892	<td>BOOST_HAS_NRVO</td>
893	<td>Compiler</td>
894	<td>Indicated that the compiler supports the named return value optimization
895	(NRVO). Used to select the most efficient implementation for some function. See <a href="../../boost/operators.hpp">
896	boost/operators.hpp</a> for example.</td>
897	</tr>
898	<tr>
899	<td valign="top">BOOST_HAS_PARTIAL_STD_ALLOCATOR</td>
900	<td>Standard Library</td>
901	<td>The standard library has a partially conforming std::allocator class, but
902	without any of the member templates.</td>
903	</tr>
904	<tr>
905	<td>BOOST_HAS_PTHREAD_DELAY_NP</td>
906	<td>Platform</td>
907	<td>The platform has the POSIX API pthread_delay_np.</td>
908	</tr>
909	<tr>
910	<td>BOOST_HAS_PTHREAD_MUTEXATTR_SETTYPE</td>
911	<td>Platform</td>
912	<td>The platform has the POSIX API pthread_mutexattr_settype.</td>
913	</tr>
914	<tr>
915	<td>BOOST_HAS_PTHREAD_YIELD</td>
916	<td>Platform</td>
917	<td>The platform has the POSIX API pthread_yield.</td>
918	</tr>
919	<tr>
920	<td valign="top" width="48%">BOOST_HAS_PTHREADS</td>
921	<td valign="top" width="15%">Platform</td>
922	<td valign="top" width="37%">The platform support POSIX style threads.</td>
923	</tr>
924	<tr>
925	<td>BOOST_HAS_SCHED_YIELD</td>
926	<td>Platform</td>
927	<td>The platform has the POSIX API sched_yield.</td>
928	</tr>
929	<tr>
930	<td>BOOST_HAS_SGI_TYPE_TRAITS</td>
931	<td>Compiler/standard library</td>
932	<td>The compiler has native support for SGI style type traits.</td>
933	</tr>
934	<tr>
935	<td>BOOST_HAS_STDINT_H</td>
936	<td>Platform</td>
937	<td>The platform has a <stdint.h></td>
938	</tr>
939	<tr>
940	<td valign="top" width="48%">BOOST_HAS_SLIST</td>
941	<td valign="top" width="15%">Standard library</td>
942	<td valign="top" width="37%">The C++ implementation provides the (SGI) slist
943	class.</td>
944	</tr>
945	<tr>
946	<td valign="top" width="48%">BOOST_HAS_STLP_USE_FACET</td>
947	<td valign="top" width="15%">Standard library</td>
948	<td valign="top" width="37%">The standard library lacks a conforming
949	std::use_facet, but has a workaround class-version that does the job. This is
950	primarily for the STLport std lib.</td>
951	</tr>
952	<tr>
953	<td valign="top" width="48%">BOOST_HAS_THREADS</td>
954	<td valign="top" width="15%">Platform/compiler</td>
955	<td valign="top" width="37%">Defined if the compiler, in its current translation
956	mode, supports multiple threads of execution.</td>
957	</tr>
958	<tr>
959	<td valign="top" width="48%">BOOST_HAS_TWO_ARG_USE_FACET</td>
960	<td valign="top" width="15%">Standard library</td>
961	<td valign="top" width="37%">The standard library lacks a conforming
962	std::use_facet, but has a two argument version that does the job. This is
963	primarily for the Rogue Wave std lib.</td>
964	</tr>
965	<tr>
966	<td valign="top" width="48%">BOOST_HAS_UNISTD_H</td>
967	<td valign="top" width="15%">Platform</td>
968	<td valign="top" width="37%">The Platform provides <unistd.h>.</td>
969	</tr>
970	<tr>
971	<td valign="top" width="48%">BOOST_HAS_WINTHREADS</td>
972	<td valign="top" width="15%">Platform</td>
973	<td valign="top" width="37%">The platform supports MS Windows style threads.</td>
974	</tr>
975	<tr>
976	<td valign="top" width="48%">BOOST_MSVC_STD_ITERATOR</td>
977	<td valign="top" width="15%">Standard library</td>
978	<td valign="top" width="37%">Microsoft's broken version of std::iterator is being
979	used. This implies that std::iterator takes no more than two template
980	parameters.</td>
981	</tr>
982	<tr>
983	<td valign="top" width="48%">BOOST_MSVC6_MEMBER_TEMPLATES</td>
984	<td valign="top" width="15%">Compiler</td>
985	<td valign="top" width="37%">Microsoft Visual C++ 6.0 has enough member template
986	idiosyncrasies (being polite) that BOOST_NO_MEMBER_TEMPLATES is defined for
987	this compiler. BOOST_MSVC6_MEMBER_TEMPLATES is defined to allow compiler
988	specific workarounds. This macro gets defined automatically if
989	BOOST_NO_MEMBER_TEMPLATES is not defined - in other words this is treated as a
990	strict subset of the features required by the standard.</td>
991	</tr>
992	<tr>
993	<td valign="top" width="48%">BOOST_HAS_STDINT_H</td>
994	<td valign="top" width="15%">Platform</td>
995	<td valign="top" width="37%">There are no 1998 C++ Standard headers
996	<stdint.h> or <cstdint>, although the 1999 C Standard does include
997	<stdint.h>. If <stdint.h> is present, <boost/stdint.h> can
998	make good use of it, so a flag is supplied (signalling presence; thus the
999	default is not present, conforming to the current C++ standard).</td>
1000	</tr>
1001	</table>
1002	<h4><a name="helpers"></a>Boost Helper Macros</h4>
1003	<p>The following macros are either simple helpers, or macros that provide
1004	workarounds for compiler/standard library defects.</p>
1005	<table border="1" cellpadding="7" cellspacing="1" width="100%">
1006	<tr>
1007	<td valign="top" width="50%"><p align="center"><b>Macro</b></p>
1008	</td>
1009	<td valign="top" width="50%"><p align="center"><b>Description</b></p>
1010	</td>
1011	</tr>
1012	<tr>
1013	<td valign="top" width="50%">BOOST_DEDUCED_TYPENAME</td>
1014	<td valign="top" width="50%">Some compilers don't support the use of <code>typename</code>
1015	for dependent types in deduced contexts. This macro expands to nothing on those
1016	compilers, and <code>typename</code> elsewhere. For example, replace:<pre>template <class T> void f(T, typename T::type);</pre>
1017	<p>with:</p>
1018	<pre>template <class T> void f(T, BOOST_DEDUCED_TYPENAME T::type);</pre>
1019	</td>
1020	</tr>
1021	<tr>
1022	<td>BOOST_STD_EXTENSION_NAMESPACE</td>
1023	<td>The namespace used for std library extensions (hashtable classes etc).</td>
1024	</tr>
1025	<tr>
1026	<td valign="top" width="50%">BOOST_STATIC_CONSTANT(Type, assignment)</td>
1027	<td valign="top" width="50%">On compilers which don't allow in-class
1028	initialization of static integral constant members, we must use enums as a
1029	workaround if we want the constants to be available at compile-time. This macro
1030	gives us a convenient way to declare such constants. For example instead of:<pre>struct foo{
1031	static const int value = 2;
1032	};</pre>
1033	<p>use:</p>
1034	<pre>struct foo{
1035	BOOST_STATIC_CONSTANT(int, value = 2);
1036	};</pre>
1037	</td>
1038	</tr>
1039	<tr>
1040	<td>BOOST_UNREACHABLE_RETURN(result)</td>
1041	<td>Normally evaluates to nothing, but evaluates to <font face="Courier New">return
1042	x;</font> if the compiler requires a return, even when it can never be
1043	reached.</td>
1044	</tr>
1045	<tr>
1046	<td>BOOST_EXPLICIT_TEMPLATE_TYPE(t)<br>
1047	BOOST_EXPLICIT_TEMPLATE_NON_TYPE(t, v)<br>
1048	BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE(t)<br>
1049	BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE(t, v)<br>
1050	</td>
1051	<td>Some compilers silently "fold" different function template instantiations if
1052	some of the template parameters don't appear in the function parameter list.
1053	For instance:
1054	<pre> #include <iostream>
1055	#include <ostream>
1056	#include <typeinfo>
1057
1058	template <int n>
1059	void f() { std::cout << n << ' '; }
1060
1061	template <typename T>
1062	void g() { std::cout << typeid(T).name() << ' '; }
1063
1064	int main() {
1065	f<1>();
1066	f<2>();
1067
1068	g<int>();
1069	g<double>();
1070	}
1071	</pre>
1072	incorrectly outputs <tt>"2 2 double double "</tt> on VC++ 6. These macros, to
1073	be used in the function parameter list, fix the problem without effects on the
1074	calling syntax. For instance, in the case above write:
1075	<pre> template <int n>
1076	void f(BOOST_EXPLICIT_TEMPLATE_NON_TYPE(int, n)) { ... }
1077
1078	template <typename T>
1079	void g(BOOST_EXPLICIT_TEMPLATE_TYPE(T)) { ... }
1080	</pre>
1081	Beware that they can declare (for affected compilers) a dummy <i>defaulted</i> parameter,
1082	so they
1083	<br>
1084	<br>
1085	a) should be always invoked at the end of the parameter list
1086	<br>
1087	b) can't be used if your function template is multiply declared.
1088	<br>
1089	<br>
1090	Furthermore, in order to add any needed comma separator, an "APPEND_*" version
1091	must be used when the macro invocation appears after a normal parameter
1092	declaration or after the invocation of another macro of this same group.
1093	</td>
1094	</tr>
1095	<tr>
1096	<td valign="top" width="50%">BOOST_USE_FACET(Type, loc)</td>
1097	<td valign="top" width="50%">When the standard library does not have a comforming
1098	std::use_facet there are various workarounds available, but they differ from
1099	library to library. This macro provides a consistent way to access a locale's
1100	facets. For example, replace:<pre>std::use_facet<Type>(loc);</pre>
1101	<p>with:</p>
1102	<pre>BOOST_USE_FACET(Type, loc);</pre>
1103	<p>Note do not add a std:: prefix to the front of BOOST_USE_FACET.</p>
1104	</td>
1105	</tr>
1106	<tr>
1107	<td valign="top" width="50%">BOOST_HAS_FACET(Type, loc)</td>
1108	<td valign="top" width="50%">When the standard library does not have a comforming
1109	std::has_facet there are various workarounds available, but they differ from
1110	library to library. This macro provides a consistent way to check a locale's
1111	facets. For example, replace:<pre>std::has_facet<Type>(loc);</pre>
1112	<p>with:</p>
1113	<pre>BOOST_HAS_FACET(Type, loc);</pre>
1114	<p>Note do not add a std:: prefix to the front of BOOST_HAS_FACET.</p>
1115	</td>
1116	</tr>
1117	<tr>
1118	<td valign="top" width="50%">BOOST_NESTED_TEMPLATE</td>
1119	<td valign="top" width="50%">Member templates are supported by some compilers even
1120	though they can't use the A::template member<U> syntax, as a workaround
1121	replace:<pre>typedef typename A::template rebind<U> binder;</pre>
1122	<p>with:</p>
1123	<pre>typedef typename A::BOOST_NESTED_TEMPLATE rebind<U> binder;</pre>
1124	</td>
1125	</tr>
1126	<tr>
1127	<td valign="top" width="50%">BOOST_STRINGIZE(X)</td>
1128	<td valign="top" width="50%">Converts the parameter X to a string after macro
1129	replacement on X has been performed.</td>
1130	</tr>
1131	<tr>
1132	<td valign="top" width="50%">BOOST_JOIN(X,Y)</td>
1133	<td valign="top" width="50%">This piece of macro magic joins the two arguments
1134	together, even when one of the arguments is itself a macro (see 16.3.1 in C++
1135	standard). This is normally used to create a mangled name in combination with a
1136	predefined macro such a __LINE__.</td>
1137	</tr>
1138	</table>
1139	<h4><a name="info_macros"></a>Boost Informational Macros</h4>
1140	<p>The following macros describe boost features; these are the generally speaking
1141	the only boost macros that should be tested in user code.</p>
1142	<table border="1" cellpadding="7" cellspacing="1" width="100%">
1143	<tr>
1144	<td valign="top" width="33%"><p align="center"><b>Macro</b></p>
1145	</td>
1146	<td valign="top" width="33%"><p align="center"><b>Header</b></p>
1147	</td>
1148	<td valign="top" width="33%"><p align="center"><b>Description</b></p>
1149	</td>
1150	</tr>
1151	<tr>
1152	<td valign="top" width="33%">BOOST_VERSION</td>
1153	<td valign="top" width="33%"><boost/version.hpp></td>
1154	<td valign="top" width="33%">Describes the boost version number in XXYYZZ format
1155	such that: (BOOST_VERSION % 100) is the sub-minor version, ((BOOST_VERSION /
1156	100) % 1000) is the minor version, and (BOOST_VERSION / 100000) is the major
1157	version.</td>
1158	</tr>
1159	<tr>
1160	<td valign="top" width="33%">BOOST_NO_INT64_T</td>
1161	<td valign="top" width="33%"><boost/cstdint.hpp><p><boost/stdint.h></p>
1162	</td>
1163	<td valign="top" width="33%">Defined if there are no 64-bit integral types:
1164	int64_t, uint64_t etc.</td>
1165	</tr>
1166	<tr>
1167	<td valign="top" width="33%">BOOST_NO_INTEGRAL_INT64_T</td>
1168	<td valign="top" width="33%"><boost/cstdint.hpp><p><boost/stdint.h></p>
1169	</td>
1170	<td valign="top" width="33%">Defined if int64_t as defined by
1171	<boost/cstdint.hpp> is not usable in integral constant expressions.</td>
1172	</tr>
1173	<tr>
1174	<td valign="top">BOOST_MSVC</td>
1175	<td valign="top"><boost/config.hpp></td>
1176	<td valign="top">Defined if the compiler is really Microsoft Visual C++, as
1177	opposed to one of the many other compilers that also define _MSC_VER.</td>
1178	</tr>
1179	<tr>
1180	<td valign="top">BOOST_INTEL</td>
1181	<td valign="top"><boost/config.hpp></td>
1182	<td valign="top">Defined if the compiler is an Intel compiler, takes the same
1183	value as the compiler version macro.</td>
1184	</tr>
1185	<TR>
1186	<TD vAlign="top">BOOST_WINDOWS</TD>
1187	<TD vAlign="top"><boost/config.hpp></TD>
1188	<TD vAlign="top">Defined if the Windows platform API is available.</TD>
1189	</TR>
1190	<tr>
1191	<td>BOOST_DINKUMWARE_STDLIB</td>
1192	<td><boost/config.hpp></td>
1193	<td>Defined if the dinkumware standard library is in use, takes the same value as
1194	the Dinkumware library version macro _CPPLIB_VER if defined, otherwise 1.</td>
1195	</tr>
1196	<tr>
1197	<td valign="top" width="33%">BOOST_NO_WREGEX</td>
1198	<td valign="top" width="33%"><boost/regex.hpp></td>
1199	<td valign="top" width="33%">Defined if the regex library does not support wide
1200	character regular expressions.</td>
1201	</tr>
1202	<tr>
1203	<td valign="top" width="33%">BOOST_COMPILER</td>
1204	<td valign="top" width="33%"><boost/config.hpp></td>
1205	<td valign="top" width="33%">Defined as a string describing the name and version
1206	number of the compiler in use. Mainly for debugging the configuration.</td>
1207	</tr>
1208	<tr>
1209	<td valign="top" width="33%">BOOST_STDLIB</td>
1210	<td valign="top" width="33%"><boost/config.hpp></td>
1211	<td valign="top" width="33%">Defined as a string describing the name and version
1212	number of the standard library in use. Mainly for debugging the configuration.</td>
1213	</tr>
1214	<tr>
1215	<td valign="top" width="33%">BOOST_PLATFORM</td>
1216	<td valign="top" width="33%"><boost/config.hpp></td>
1217	<td valign="top" width="33%">Defined as a string describing the name of the
1218	platform. Mainly for debugging the configuration.</td>
1219	</tr>
1220	</table>
1221	<h2><a name="guidelines"></a></h2>
1222	<H4><A name="source"></A>Macros for libraries with separate source code</H4>
1223	<P>The following macros and helper headers are of use to authors whose libraries
1224	include separate source code, and are intended to address two issues: fixing
1225	the ABI of the compiled library, and selecting which compiled library to link
1226	against based upon the compilers settings.</P>
1227	<H5>ABI Fixing</H5>
1228	<P>When linking against a pre-compiled library it vital that the ABI used by the
1229	compiler when building the library <EM>matches</EM> <EM>exactly</EM> the ABI
1230	used by the code using the library.  In this case ABI means things like
1231	the struct packing arrangement used, the name mangling scheme used, or the size
1232	of some types (enum types for example).  This is separate from things like
1233	threading support, or runtime library variations, which have to be dealt with
1234	by build variants.  To put this in perspective there is one compiler
1235	(Borland's) that has so many compiler options that make subtle changes to the
1236	ABI, that at least in theory there 3200 combinations, and that's without
1237	considering runtime library variations.  Fortunately these variations can
1238	be managed by #pragma's that tell the compiler what ABI to use for the types
1239	declared in your library, in order to avoid sprinkling #pragma's all over the
1240	boost headers, there are some prefix and suffix headers that do the job,
1241	typical usage would be:</P>
1242	<PRE>#ifndef MY_INCLUDE_GUARD
1243	#define MY_INCLUDE_GUARD
1244
1245	// all includes go here:
1246	#include <boost/config.hpp>
1247	#include <whatever>
1248
1249	#ifdef BOOST_HAS_ABI_HEADERS
1250	# include BOOST_ABI_PREFIX
1251	#endif
1252
1253	namespace boost{
1254	// your code goes here
1255	}
1256
1257	#ifdef BOOST_HAS_ABI_HEADERS
1258	# include BOOST_ABI_SUFFIX
1259	#endif
1260
1261
1262	#endif // include guard
1263	</PRE>
1264	<P>The user can disable this mechanism by defining BOOST_DISABLE_ABI_HEADERS, or
1265	they can define BOOST_ABI_PREFIX and/or BOOST_ABI_SUFFIX to point to their own
1266	prefix/suffix headers if they so wish.</P>
1267	<H5>Automatic library selection</H5>
1268	<P>It is essential that users link to a build of a library which was built against
1269	the same runtime library that their application will be built against - if this
1270	does not happen then the library will not be binary compatible with their own
1271	code - and there is a high likelihood  that their application will
1272	experience runtime crashes.  These kinds of problems can be extremely
1273	time consuming and difficult to debug, and often lead to frustrated users and
1274	authors alike (simply selecting the right library to link against is not as
1275	easy as it seems when their are 6-8 of them to chose from, and some users seem
1276	to be blissfully unaware that there even are different runtimes available to
1277	them).</P>
1278	<P>To solve this issue, some compilers allow source code to contain #pragma's that
1279	instruct the linker which library to link against, all the user need do is
1280	include the headers they need, place the compiled libraries in their library
1281	search path, and the compiler and linker do the rest.  Boost.config
1282	supports this via the header <boost/config/auto_link.hpp>, before
1283	including this header one or more of the following macros need to be defined:</P>
1284	<P>
1285	<TABLE id="Table1" cellSpacing="1" cellPadding="1" width="100%" border="1">
1286	<TR>
1287	<TD>BOOST_LIB_NAME</TD>
1288	<TD>
1289	Required: An identifier containing the basename of the library, for
1290	example 'boost_regex'.</TD>
1291	</TR>
1292	<TR>
1293	<TD>BOOST_DYN_LINK</TD>
1294	<TD>Optional: when set link to dll rather than static library.</TD>
1295	</TR>
1296	<TR>
1297	<TD>BOOST_LIB_DIAGNOSTIC</TD>
1298	<TD>Optional: when set the header will print out the name of the library selected
1299	(useful for debugging).</TD>
1300	</TR>
1301	</TABLE>
1302	</P>
1303	<P>If the compiler supports this mechanism, then it will be told to link against
1304	the appropriately named library, the actual algorithm used to mangle the name
1305	of the library is documented inside <boost/config/auto_link.hpp> and has
1306	to match that used to create the libraries via bjam 's install rules.</P>
1307	<P>Typical usage would be:</P>
1308	<PRE>//
1309	// Don't include auto-linking code if the user has disabled it by
1310	// defining BOOST_WHATEVER_NO_LIB, or if this is one of our own
1311	// source files (signified by BOOST_WHATEVER_SOURCE):
1312	//
1313	#if !defined(BOOST_WHATEVER_NO_LIB) && !defined(BOOST_WHATEVER_SOURCE)
1314	# define BOOST_LIB_NAME boost_whatever
1315	# ifdef BOOST_WHATEVER_DYN_LINK
1316	# define BOOST_DYN_LINK
1317	# endif
1318	# include <boost/config/auto_link.hpp>
1319	#endif
1320	</PRE>
1321	<H2>Guidelines for Boost Authors</H2>
1322	<p>The <a href="../../boost/config.hpp">boost/config.hpp</a> header is used to
1323	pass configuration information to other boost files, allowing them to cope with
1324	platform dependencies such as arithmetic byte ordering, compiler pragmas, or
1325	compiler shortcomings. Without such configuration information, many current
1326	compilers would not work with the Boost libraries.</p>
1327	<p>Centralizing configuration information in this header reduces the number of
1328	files that must be modified when porting libraries to new platforms, or when
1329	compilers are updated. Ideally, no other files would have to be modified when
1330	porting to a new platform.</p>
1331	<p>Configuration headers are controversial because some view them as condoning
1332	broken compilers and encouraging non-standard subsets. Adding settings for
1333	additional platforms and maintaining existing settings can also be a problem.
1334	In other words, configuration headers are a necessary evil rather than a
1335	desirable feature. The boost config.hpp policy is designed to minimize the
1336	problems and maximize the benefits of a configuration header.</p>
1337	<p>Note that:</p>
1338	<ul>
1339	<li>
1340	Boost library implementers are not required to #include
1341	<boost/config.hpp>, and are not required in any way to support compilers
1342	that do not comply with the C++ Standard (ISO/IEC 14882).
1343	<li>
1344	If a library implementer wishes to support some non-conforming compiler, or to
1345	support some platform specific feature, #include <boost/config.hpp> is
1346	the preferred way to obtain configuration information not available from the
1347	standard headers such as <climits>, etc.
1348	<li>
1349	If configuration information can be deduced from standard headers such as
1350	<climits>, use those standard headers rather than
1351	<boost/config.hpp>.
1352	<li>
1353	Boost files that use macros defined in <boost/config.hpp> should have
1354	sensible, standard conforming, default behavior if the macro is not defined.
1355	This means that the starting point for porting <boost/config.hpp> to a
1356	new platform is simply to define nothing at all specific to that platform. In
1357	the rare case where there is no sensible default behavior, an #error message
1358	should describe the problem.
1359	<li>
1360	If a Boost library implementer wants something added to config.hpp, post a
1361	request on the Boost mailing list. There is no guarantee such a request will be
1362	honored; the intent is to limit the complexity of config.hpp.
1363	<li>
1364	The intent is to support only compilers which appear on their way to becoming
1365	C++ Standard compliant, and only recent releases of those compilers at that.
1366	<li>
1367	The intent is not to disable mainstream features now well-supported by the
1368	majority of compilers, such as namespaces, exceptions, RTTI, or templates.
1369	</li>
1370	</ul>
1371	<h4><a name="defect_guidelines"></a>Adding New Defect Macros</h4>
1372	<p>When you need to add a new defect macro - either to fix a problem with an
1373	existing library, or when adding a new library - distil the issue down to a
1374	simple test case, often at this point other (possibly better) workarounds may
1375	become apparent. Secondly always post the test case code to the boost mailing
1376	list and invite comments; remember that C++ is complex and that sometimes what
1377	may appear a defect, may in fact turn out to be a problem with the authors
1378	understanding of the standard.</p>
1379	<p>When you name the macro, follow the BOOST_NO_SOMETHING naming convention, so
1380	that it's obvious that this is a macro reporting a defect.</p>
1381	<p>Finally, add the test program to the regression tests. You will need to place
1382	the test case in a .ipp file with the following comments near the top:</p>
1383	<pre>// MACRO: BOOST_NO_FOO
1384	// TITLE: foo
1385	// DESCRIPTION: If the compiler fails to support foo</pre>
1386	<p>These comments are processed by the autoconf script, so make sure the format
1387	follows the one given. The file should be named "boost_no_foo.ipp", where foo
1388	is the defect description - try and keep the file name under the Mac 30
1389	character filename limit though. You will also need to provide a function
1390	prototype "int test()" that is declared in a namespace with the same name as
1391	the macro, but in all lower case, and which returns zero on success:</p>
1392	<pre>namespace boost_no_foo{
1393
1394	int test()
1395	{
1396	// test code goes here:
1397	//
1398	return 0;
1399	}
1400
1401	}</pre>
1402	<p>
1403	Once the test code is in place, build and run the program "generate.cpp" that
1404	you will find in the boost-root/libs/config/tools/ directory. This generates
1405	two .cpp test files from the new test code, and adds the tests to the
1406	regression test Jamfile, and the config_test.cpp test program. Finally add a
1407	new entry to config_info.cpp so that the new macro gets printed out when that
1408	program is run.</p>
1409	<h4><a name="feature_guidelines"></a>Adding New Feature Test Macros</h4>
1410	<p>When you need to add a macro that describes a feature that the standard does
1411	not require, follow the convention for adding a new defect macro (above), but
1412	call the macro BOOST_HAS_FOO, and name the test file "boost_has_foo.ipp". Try
1413	not to add feature test macros unnecessarily, if there is a platform specific
1414	macro that can already be used (for example _WIN32, __BEOS__, or __linux) to
1415	identify the feature then use that. Try to keep the macro to a feature group,
1416	or header name, rather than one specific API (for example BOOST_HAS_NL_TYPES_H
1417	rather than BOOST_HAS_CATOPEN). If the macro describes a POSIX feature group,
1418	then add boilerplate code to <a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>
1419	to auto-detect the feature where possible (if you are wondering why we can't
1420	use POSIX feature test macro directly, remember that many of these features can
1421	be added by third party libraries, and are not therefore identified inside
1422	<unistd.h>).</p>
1423	<h4><a name="modify_guidelines"></a>Modifying the Boost Configuration Headers</h4>
1424	<p>The aim of boost's configuration setup is that the configuration headers should
1425	be relatively stable - a boost user should not have to recompile their code
1426	just because the configuration for some compiler that they're not interested in
1427	has changed. Separating the configuration into separate compiler/standard
1428	library/platform sections provides for part of this stability, but boost
1429	authors require some amount of restraint as well, in particular:</p>
1430	<p><<a href="../../boost/config.hpp">boost/config.hpp</a>> should never
1431	change, don't alter this file.</p>
1432	<p><<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>> is
1433	included by default, don't add extra code to this file unless you have to. If
1434	you do, please remember to update <a href="tools/configure.in">libs/config/tools/configure.in</a>
1435	as well.</p>
1436	<p><<a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>> is
1437	always included so be careful about modifying this file as it breaks
1438	dependencies for everyone. This file should include only "boilerplate"
1439	configuration code, and generally should change only when new macros are added.</p>
1440	<p><<a href="../../boost/config/select_compiler_config.hpp">boost/config/select_compiler_config.hpp</a>>,
1441	<<a href="../../boost/config/select_platform_config.hpp">boost/config/select_platform_config.hpp</a>>
1442	and <<a href="../../boost/config/select_stdlib_config.hpp">boost/config/select_stdlib_config.hpp</a>>
1443	are included by default and should change only if support for a new
1444	compiler/standard library/platform is added.</p>
1445	<p>The compiler/platform/standard library selection code is set up so that unknown
1446	platforms are ignored and assumed to be fully standards compliant - this gives
1447	unknown platforms a "sporting chance" of working "as is" even without running
1448	the configure script.</p>
1449	<p>When adding or modifying the individual mini-configs, assume that future, as
1450	yet unreleased versions of compilers, have all the defects of the current
1451	version. Although this is perhaps unnecessarily pessimistic, it cuts down on
1452	the maintenance of these files, and experience suggests that pessimism is
1453	better placed than optimism here!</p>
1454	<h2><a name="rationale"></a>Rationale</h2>
1455	<p>The problem with many traditional "textbook" implementations of configuration
1456	headers (where all the configuration options are in a single "monolithic"
1457	header) is that they violate certain fundamental software engineering
1458	principles which would have the effect of making boost more fragile, more
1459	difficult to maintain and more difficult to use safely. You can find a
1460	description of the principles from the <a href="http://www.objectmentor.com/publications/Principles%20and%20Patterns.PDF">
1461	following article</a>.</p>
1462	<h4>The problem</h4>
1463	<p>Consider a situation in which you are concurrently developing on multiple
1464	platforms. Then consider adding a new platform or changing the platform
1465	definitions of an existing platform. What happens? Everything, and this does
1466	literally mean everything, recompiles.. Isn't it quite absurd that adding a new
1467	platform, which has absolutely nothing to do with previously existing
1468	platforms, means that all code on all existing platforms needs to be
1469	recompiled?</p>
1470	<p>Effectively, there is an imposed physical dependency between platforms that
1471	have nothing to do with each other. Essentially, the traditional solution
1472	employed by configuration headers does not conform to the Open-Closed
1473	Principle:</p>
1474	<p><b><i>"A module should be open for extension but closed for modification."</i></b></p>
1475	<p>Extending a traditional configuration header implies modifying existing code.</p>
1476	<p>Furthermore, consider the complexity and fragility of the platform detection
1477	code. What if a simple change breaks the detection on some minor platform? What
1478	if someone accidentally or on purpose (as a workaround for some other problem)
1479	defines some platform dependent macros that are used by the detection code? A
1480	traditional configuration header is one of the most volatile headers of the
1481	entire library, and more stable elements of Boost would depend on it. This
1482	violates the Stable Dependencies Principle:</p>
1483	<p><b><i>"Depend in the direction of stability."</i></b></p>
1484	<p>After even a minor change to a traditional configuration header on one minor
1485	platform, almost everything on every platform should be tested if we follow
1486	sound software engineering practice.</p>
1487	<p>Another important issue is that it is not always possible to submit changes to
1488	<boost/config.hpp>. Some boost users are currently working on platforms
1489	using tools and libraries that are under strict Non-Disclosure Agreements. In
1490	this situation it is impossible to submit changes to a traditional monolithic
1491	configuration header, instead some method by which the user can insert their
1492	own configuration code must be provided.</p>
1493	<h4>The solution</h4>
1494	<p>The approach taken by boost's configuration headers is to separate
1495	configuration into three orthogonal parts: the compiler, the standard library
1496	and the platform. Each compiler/standard library/platform gets its own
1497	mini-configuration header, so that change to one compiler's configuration (for
1498	example) does not effect other compilers. In addition there are measures that
1499	can be taken both to omit the compiler/standard library/platform detection code
1500	(so that adding support to a new platform does not break dependencies), or to
1501	freeze the configuration completely; providing almost complete protection
1502	against dependency changes.</p>
1503	<h2><a name="Acknowledgements"></a>Acknowledgements</h2>
1504	<p>Beman Dawes provided the original config.hpp and part of this document. Vesa
1505	Karvonen provided a description of the principles (see <a href="#rationale">rationale</a>)
1506	and put together an early version of the current configuration setup. John
1507	Maddock put together the configuration current code, the test programs, the
1508	configuration script and the reference section of this document. Numerous boost
1509	members, past and present, have contributed fixes to boost's configuration.</p>
1510	<p> </p>
1511	<hr>
1512	<p>© Beman Dawes 2001</p>
1513	<p>© Vesa Karvonen 2001</p>
1514	<p>© John Maddock 2001</p>
1515	<p> </p>
1516	<p> </p>
1517	<p> </p>
1518	<p> </p>
1519	<p> </p>
1520	</body>
1521	</html>

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