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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 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 its 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 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 -DAportable -D__HPACC_THREAD_SAFE_RB_TREE -DRWSTD_MULTI_THREAD -DRW_MULTI_THREAD -D_REENTRANT -D_THREAD_SAFE"
107	export LDFLAGS="-DAportable"
108	export LIBS="-lpthread"
109	sh ./configure</pre>
110	<p>However you run the configure script, when it finishes you will find a new
111	header - user.hpp - located in the <boost-root/libs/config/> directory. <b><i>Note
112	that configure does not install this header into your boost include path by
113	default.</i></b> This header contains all the options generated by the
114	configure script, plus a header-section that contains the user settable options
115	from the default version of <a href="../../boost/config/user.hpp">user.hpp</a> (located
116	under <boost-root>/boost/config/). There are two ways you can use this
117	header:</p>
118	<p>Option 1: copy the header into <boost-root>/boost/config/ so that it
119	replaces the default <a href="../../boost/config/user.hpp">user.hpp</a> provided
120	by boost. This option allows only one configure-generated setup; boost
121	developers should avoid this option, as it incurs the danger of accidentally
122	committing a configure-modified user.hpp to the cvs repository (something you
123	will not be thanked for!).</p>
124	<p>Option 2: give the header a more memorable name, and place it somewhere
125	convenient; then, define the macro BOOST_USER_CONFIG to point to it. For example
126	create a new sub-directory <boost-root>/boost/config/user/, and copy the
127	header there; for example as "multithread-gcc-config.hpp". Then, when compiling
128	add the command line option:
129	-DBOOST_USER_CONFIG="<boost/config/user/multithread-gcc-config.hpp>", and boost
130	will use the new configuration header. This option allows you to generate more
131	than one configuration header, and to keep them separate from the boost source
132	- so that updates to the source do not interfere with your configuration.</p>
133	<h4><a name="user_settable"></a>User settable options</h4>
134	<p>There are some configuration-options that represent user choices, rather than
135	compiler defects or platform specific options. These are listed in
136	<boost/config/user.hpp> and at the start of a configure-generated
137	user.hpp header. You can define these on the command line, or by editing
138	<boost/config/user.hpp>, they are listed in the following table: </p>
139	<table border="1" cellpadding="7" cellspacing="1" width="100%">
140	<tr>
141	<td valign="top" width="48%"><p align="center"><b>Macro</b></p>
142	</td>
143	<td valign="top" width="52%"><p align="center"><b>Description</b></p>
144	</td>
145	</tr>
146	<tr>
147	<td valign="top" width="48%">BOOST_USER_CONFIG</td>
148	<td valign="top" width="52%">When defined, it should point to the name of the user
149	configuration file to include prior to any boost configuration files. When not
150	defined, defaults to <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>.</td>
151	</tr>
152	<tr>
153	<td valign="top" width="48%">BOOST_COMPILER_CONFIG</td>
154	<td valign="top" width="52%">When defined, it should point to the name of the
155	compiler configuration file to use. Defining this cuts out the compiler
156	selection logic, and eliminates the dependency on the header containing that
157	logic. For example if you are using gcc, then you could define
158	BOOST_COMPILER_CONFIG to "<a href="../../boost/config/compiler/gcc.hpp">boost/config/compiler/gcc.hpp</a>".</td>
159	</tr>
160	<tr>
161	<td valign="top" width="48%">BOOST_STDLIB_CONFIG</td>
162	<td valign="top" width="52%">When defined, it should point to the name of the
163	standard library configuration file to use. Defining this cuts out the standard
164	library selection logic, and eliminates the dependency on the header containing
165	that logic. For example if you are using STLport, then you could define
166	BOOST_STDLIB_CONFIG to "<a href="../../boost/config/stdlib/stlport.hpp">boost/config/stdlib/stlport.hpp</a>".</td>
167	</tr>
168	<tr>
169	<td valign="top" width="48%">BOOST_PLATFORM_CONFIG</td>
170	<td valign="top" width="52%">When defined, it should point to the name of the
171	platform configuration file to use. Defining this cuts out the platform
172	selection logic, and eliminates the dependency on the header containing that
173	logic. For example if you are compiling on linux, then you could define
174	BOOST_PLATFORM_CONFIG to "<a href="../../boost/config/platform/linux.hpp">boost/config/platform/linux.hpp</a>".</td>
175	</tr>
176	<tr>
177	<td valign="top" width="48%">BOOST_NO_COMPILER_CONFIG</td>
178	<td valign="top" width="52%">When defined, no compiler configuration file is
179	selected or included, define when the compiler is fully conformant with the
180	standard, or where the user header (see BOOST_USER_CONFIG), has had any options
181	necessary added to it, for example by an autoconf generated configure script.</td>
182	</tr>
183	<tr>
184	<td valign="top" width="48%">BOOST_NO_STDLIB_CONFIG</td>
185	<td valign="top" width="52%">When defined, no standard library configuration file
186	is selected or included, define when the standard library is fully conformant
187	with the standard, or where the user header (see BOOST_USER_CONFIG), has had
188	any options necessary added to it, for example by an autoconf generated
189	configure script.</td>
190	</tr>
191	<tr>
192	<td valign="top" width="48%">BOOST_NO_PLATFORM_CONFIG</td>
193	<td valign="top" width="52%">When defined, no platform configuration file is
194	selected or included, define when the platform is fully conformant with the
195	standard (and has no useful extra features), or where the user header (see
196	BOOST_USER_CONFIG), has had any options necessary added to it, for example by
197	an autoconf generated configure script.</td>
198	</tr>
199	<tr>
200	<td valign="top" width="48%">BOOST_NO_CONFIG</td>
201	<td valign="top" width="52%">Equivalent to defining all of
202	BOOST_NO_COMPILER_CONFIG, BOOST_NO_STDLIB_CONFIG and BOOST_NO_PLATFORM_CONFIG.</td>
203	</tr>
204	<tr>
205	<td valign="top">BOOST_STRICT_CONFIG</td>
206	<td>The normal behavior for compiler versions that are newer than the last known
207	version, is to assume that they have all the same defects as the last known
208	version. By setting this define, then compiler versions that are newer than the
209	last known version are assumed to be fully conforming with the standard. This
210	is probably most useful for boost developers or testers, and for those who want
211	to use boost to test beta compiler versions.</td>
212	</tr>
213	<tr>
214	<td valign="top">BOOST_ASSERT_CONFIG</td>
215	<td>When this flag is set, if the config finds anything unknown, then it will stop
216	with a #error rather than continue. Boost regression testers should set this
217	define, as should anyone who wants to quickly check whether boost is supported
218	on their platform.</td>
219	</tr>
220	<tr>
221	<td valign="top" width="48%">BOOST_DISABLE_THREADS</td>
222	<td valign="top" width="52%">When defined, disables threading support, even if the
223	compiler in its current translation mode supports multiple threads.</td>
224	</tr>
225	<tr>
226	<td valign="top">BOOST_DISABLE_WIN32</td>
227	<td>When defined, disables the use of Win32 specific API's, even when these are
228	available. Also has the effect of setting BOOST_DISABLE_THREADS unless
229	BOOST_HAS_PTHREADS is set. This option may be set automatically by the config
230	system when it detects that the compiler is in "strict mode".</td>
231	</tr>
232	<TR>
233	<TD vAlign="top">BOOST_DISABLE_ABI_HEADERS</TD>
234	<TD>Stops boost headers from including any prefix/suffix headers that normally
235	control things like struct packing and alignment.</TD>
236	</TR>
237	<TR>
238	<TD vAlign="top">BOOST_ABI_PREFIX</TD>
239	<TD>A prefix header to include in place of whatever boost.config would normally
240	select, any replacement should set up struct packing and alignment options as
241	required.</TD>
242	</TR>
243	<TR>
244	<TD vAlign="top">BOOST_ABI_SUFFIX</TD>
245	<TD>A suffix header to include in place of whatever boost.config would normally
246	select, any replacement should undo the effects of the prefix header.</TD>
247	</TR>
248	<TR>
249	<TD vAlign="top">BOOST_ALL_DYN_LINK</TD>
250	<TD>
251	<P>Forces all libraries that have separate source, to be linked as dll's rather
252	than static libraries on Microsoft Windows (this macro is used to turn on
253	__declspec(dllimport) modifiers, so that the compiler knows which symbols to
254	look for in a dll rather than in a static library).
255	</P>
256	<P>Note that there may be some libraries that can only be statically linked
257	(Boost.Test for example) and others which may only be dynamically linked
258	(Boost.Threads for example), in these cases this macro has no effect.</P>
259	</TD>
260	</TR>
261	<TR>
262	<TD vAlign="top">BOOST_WHATEVER_DYN_LINK</TD>
263	<TD>
264	<P>Forces library "whatever" to be linked as a dll rather than a static library on
265	Microsoft Windows: replace the WHATEVER part of the macro name with the name of
266	the library that you want to dynamically link to, for example use
267	BOOST_DATE_TIME_DYN_LINK or BOOST_REGEX_DYN_LINK etc (this macro is used
268	to turn on __declspec(dllimport) modifiers, so that the compiler knows which
269	symbols to look for in a dll rather than in a static library).
270	</P>
271	<P>Note that there may be some libraries that can only be statically linked
272	(Boost.Test for example) and others which may only be dynamically linked
273	(Boost.Threads for example), in these cases this macro is unsupported.</P>
274	</TD>
275	</TR>
276	<TR>
277	<TD vAlign="top">BOOST_ALL_NO_LIB</TD>
278	<TD>
279	<P>Tells the config system not to automatically select which libraries to link
280	against.
281	</P>
282	<P>Normally if a compiler supports #pragma lib, then the correct library build
283	variant will be automatically selected and linked against, simply by the act of
284	including one of that library's headers.  This macro turns that feature
285	off.</P>
286	</TD>
287	</TR>
288	<TR>
289	<TD vAlign="top">BOOST_WHATEVER_NO_LIB</TD>
290	<TD>
291	<P>Tells the config system not to automatically select which library to link
292	against for library "whatever", replace WHATEVER in the macro name with the
293	name of the library; for example BOOST_DATE_TIME_NO_LIB or
294	BOOST_REGEX_NO_LIB.
295	</P>
296	<P>Normally if a compiler supports #pragma lib, then the correct library build
297	variant will be automatically selected and linked against, simply by the act of
298	including one of that library's headers.  This macro turns that feature
299	off.</P>
300	</TD>
301	</TR>
302	<TR>
303	<TD vAlign="top">BOOST_LIB_DIAGNOSTIC</TD>
304	<TD>Causes the auto-linking code to output diagnostic messages indicating the name
305	of the library that is selected for linking.</TD>
306	</TR>
307	<TR>
308	<TD vAlign="top">BOOST_LIB_TOOLSET</TD>
309	<TD>Overrides the name of the toolset part of the name of library being linked to;
310	note if defined this must be defined to a quoted string literal, for example
311	"abc".</TD>
312	</TR>
313	</table>
314	<h4><a name="advanced_config"></a>Advanced configuration usage</h4>
315	<p>By setting various macros on the compiler command line or by editing <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>,
316	the boost configuration setup can be optimised in a variety of ways.
317	</p>
318	<p>Boost's configuration is structured so that the user-configuration is included
319	first (defaulting to <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>
320	if BOOST_USER_CONFIG is not defined). This sets up any user-defined policies,
321	and gives the user-configuration a chance to influence what happens next.
322	</p>
323	<p>Next the compiler, standard library, and platform configuration files are
324	included. These are included via macros (BOOST_COMPILER_CONFIG etc, <a href="#user_settable">
325	see user settable macros</a>), and if the corresponding macro is undefined
326	then a separate header that detects which compiler/standard library/platform is
327	in use is included in order to set these. The config can be told to ignore
328	these headers altogether if the corresponding BOOST_NO_XXX macro is set (for
329	example BOOST_NO_COMPILER_CONFIG to disable including any compiler
330	configuration file - <a href="#user_settable">see user settable macros</a>).
331	</p>
332	<p>Finally the boost configuration header, includes <<a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>>;
333	this header contains any boiler plate configuration code - for example where
334	one boost macro being set implies that another must be set also.</p>
335	<p>The following usage examples represent just a few of the possibilities:</p>
336	<p><u>Example 1, creating our own frozen configuration.</u></p>
337	<p>Lets suppose that we're building boost with Visual C++ 6, and STLport 4.0. Lets
338	suppose also that we don't intend to update our compiler or standard library
339	any time soon. In order to avoid breaking dependencies when we update boost, we
340	may want to "freeze" our configuration headers, so that we only have to rebuild
341	our project if the boost code itself has changed, and not because the boost
342	config has been updated for more recent versions of Visual C++ or STLport.
343	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>>
344	for the compiler, <<a href="../../boost/config/stdlib/stlport.hpp">boost/config/stdlib/stlport.hpp</a>>
345	for the standard library, and <<a href="../../boost/config/platform/win32.hpp">boost/config/platform/win32.hpp</a>>
346	for the platform. Next we'll create our own private configuration directory:
347	boost/config/mysetup/, and copy the configuration files into there. Finally,
348	open up <<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>>
349	and edit the following defines:</p>
350	<pre>#define BOOST_COMPILER_CONFIG "boost/config/mysetup/visualc.hpp"
351	#define BOOST_STDLIB_CONFIG "boost/config/mysetup/stlport.hpp"
352	#define BOOST_USER_CONFIG "boost/config/mysetup/win32.hpp"</pre>
353	<p>Now when you use boost, its configuration header will go straight to our
354	"frozen" versions, and ignore the default versions, you will now be insulated
355	from any configuration changes when you update boost. This technique is also
356	useful if you want to modify some of the boost configuration files; for example
357	if you are working with a beta compiler release not yet supported by boost.</p>
358	<p><u>Example 2: skipping files that you don't need.</u></p>
359	<p>Lets suppose that you're using boost with a compiler that is fully conformant
360	with the standard; you're not interested in the fact that older versions of
361	your compiler may have had bugs, because you know that your current version
362	does not need any configuration macros setting. In a case like this, you can
363	define BOOST_NO_COMPILER_CONFIG either on the command line, or in
364	<boost/config/user.hpp>, and miss out the compiler configuration header
365	altogether (actually you miss out two headers, one which works out what the
366	compiler is, and one that configures boost for it). This has two consequences:
367	the first is that less code has to be compiled, and the second that you have
368	removed a dependency on two boost headers.</p>
369	<p><u>Example 3: using configure script to freeze the boost configuration.</u></p>
370	<p>If you are working on a unix-like platform then you can use the configure
371	script to generate a "frozen" configuration based on your current compiler
372	setup - <a href="#config_script">see using the configure script</a> for more
373	details.</p>
374	<h4><a name="testing"></a>Testing the boost configuration</h4>
375	<p>The boost configuration library provides a full set of regression test programs
376	under the <boost-root>/libs/config/test/ sub-directory:</p>
377	<table border="1" cellpadding="7" cellspacing="1" width="100%">
378	<tr>
379	<td valign="top" width="50%"><p align="center"><b>File</b></p>
380	</td>
381	<td valign="top" width="50%"><p align="center"><b>Description</b></p>
382	</td>
383	</tr>
384	<tr>
385	<td valign="top" width="50%">config_info.cpp</td>
386	<td valign="top" width="50%">Prints out a detailed description of your
387	compiler/standard library/platform setup, plus your current boost
388	configuration. The information provided by this program useful in setting up
389	the boost configuration files. If you report that boost is incorrectly
390	configured for your compiler/library/platform then please include the output
391	from this program when reporting the changes required.</td>
392	</tr>
393	<tr>
394	<td valign="top" width="50%">config_test.cpp</td>
395	<td valign="top" width="50%">A monolithic test program that includes most of the
396	individual test cases. This provides a quick check to see if boost is correctly
397	configured for your compiler/library/platform.</td>
398	</tr>
399	<tr>
400	<td valign="top" width="50%">limits_test.cpp</td>
401	<td valign="top" width="50%">Tests your standard library's std::numeric_limits
402	implementation (or its boost provided replacement if BOOST_NO_LIMITS is
403	defined). This test file fails with most versions of numeric_limits, mainly due
404	to the way that some compilers treat NAN's and infinity.</td>
405	</tr>
406	<tr>
407	<td valign="top" width="50%">no_*pass.cpp</td>
408	<td valign="top" width="50%">Individual compiler defect test files. Each of these
409	should compile, if one does not then the corresponding BOOST_NO_XXX macro needs
410	to be defined - see each test file for specific details.</td>
411	</tr>
412	<tr>
413	<td valign="top" width="50%">no_*fail.cpp</td>
414	<td valign="top" width="50%">Individual compiler defect test files. Each of these
415	should <i>not</i> compile, if one does then the corresponding BOOST_NO_XXX
416	macro is defined when it need not be - see each test file for specific details.</td>
417	</tr>
418	<tr>
419	<td valign="top" width="50%">has_*pass.cpp</td>
420	<td valign="top" width="50%">Individual feature test files. If one of these does <i>not</i>
421	compile then the corresponding BOOST_HAS_XXX macro is defined when it should
422	not be - see each test file for specific details.</td>
423	</tr>
424	<tr>
425	<td valign="top" width="50%">has_*fail.cpp</td>
426	<td valign="top" width="50%">Individual feature test files. If one of these does
427	compile then the corresponding BOOST_HAS_XXX macro can be safely defined - see
428	each test file for specific details.</td>
429	</tr>
430	</table>
431	<p>Although you can run the configuration regression tests as individual test
432	files, there are rather a lot of them, so there are a couple of shortcuts to
433	help you out:</p>
434	<p>If you have built the <a href="../../more/regression.html">boost regression test
435	driver</a>, then you can use this to produce a nice html formatted report of
436	the results using the supplied test file.</p>
437	<p>Alternatively you can run the configure script like this:</p>
438	<pre>./configure --enable-test</pre>
439	<p>in which case the script will test the current configuration rather than
440	creating a new one from scratch.</p>
441	<p>If you are reporting the results of these tests for a new
442	platform/library/compiler then please include a log of the full compiler
443	output, the output from config_info.cpp, and the pass/fail test results.</p>
444	<h2><a name="macro_ref"></a>Boost Macro Reference</h2>
445	<h4><a name="defects"></a>Macros that describe defects:</h4>
446	<p>The following macros all describe features that are required by the C++
447	standard, if one of the following macros is defined, then it represents a
448	defect in the compiler's conformance with the standard.</p>
449	<table border="1" cellpadding="7" cellspacing="1" width="100%">
450	<tr>
451	<td valign="top" width="51%"><p align="center"><b>Macro</b></p>
452	</td>
453	<td valign="top" width="16%"><p align="center"><b>Section</b></p>
454	</td>
455	<td valign="top" width="33%"><p align="center"><b>Description</b></p>
456	</td>
457	</tr>
458	<tr>
459	<td>BOOST_BCB_PARTIAL_SPECIALIZATION_BUG</td>
460	<td>Compiler</td>
461	<td>The compiler exibits certain partial specialisation bug - probably Borland C++
462	Builder specific.</td>
463	</tr>
464	<TR>
465	<TD vAlign="top" width="51%">BOOST_FUNCTION_SCOPE_USING_DECLARATION_BREAKS_ADL</TD>
466	<TD vAlign="top" width="16%">Compiler</TD>
467	<TD vAlign="top" width="33%">Argument dependent lookup fails if there is a using
468	declaration for the symbol being looked up in the current scope.  For
469	example, <code>using boost::get_pointer;</code> prevents ADL from finding
470	overloads of <code>get_pointer</code> in namespaces nested inside boost (but
471	not elsewhere).  Probably Borland specific.</TD>
472	</TR>
473	<tr>
474	<td valign="top" width="51%">BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</td>
475	<td valign="top" width="16%">Compiler</td>
476	<td valign="top" width="33%">Compiler does not implement argument-dependent lookup
477	(also named Koenig lookup); see std::3.4.2 [basic.koenig.lookup]</td>
478	</tr>
479	<tr>
480	<td valign="top" width="51%">BOOST_NO_AUTO_PTR</td>
481	<td valign="top" width="16%">Standard library</td>
482	<td valign="top" width="33%">If the compiler / library supplies non-standard or
483	broken std::auto_ptr.</td>
484	</tr>
485	<tr>
486	<td valign="top" width="51%">BOOST_NO_CTYPE_FUNCTIONS</td>
487	<td valign="top" width="16%">Platform</td>
488	<td valign="top" width="33%">The Platform does not provide functions for the
489	character-classifying operations <ctype.h> and <cctype>, only
490	macros.</td>
491	</tr>
492	<tr>
493	<td valign="top" width="51%">BOOST_NO_CV_SPECIALIZATIONS</td>
494	<td valign="top" width="16%">Compiler</td>
495	<td valign="top" width="33%">If template specialisations for cv-qualified types
496	conflict with a specialisation for a cv-unqualififed type.</td>
497	</tr>
498	<tr>
499	<td valign="top" width="51%">BOOST_NO_CV_VOID_SPECIALIZATIONS</td>
500	<td valign="top" width="16%">Compiler</td>
501	<td valign="top" width="33%">If template specialisations for cv-void types
502	conflict with a specialisation for void.</td>
503	</tr>
504	<tr>
505	<td valign="top" width="51%">BOOST_NO_CWCHAR</td>
506	<td valign="top" width="16%">Platform</td>
507	<td valign="top" width="33%">The Platform does not provide <wchar.h> and
508	<cwchar>.</td>
509	</tr>
510	<tr>
511	<td valign="top" width="51%">BOOST_NO_CWCTYPE</td>
512	<td valign="top" width="16%">Platform</td>
513	<td valign="top" width="33%">The Platform does not provide <wctype.h> and
514	<cwctype>.</td>
515	</tr>
516	<tr>
517	<td valign="top" width="51%">BOOST_NO_DEPENDENT_NESTED_DERIVATIONS</td>
518	<td valign="top" width="16%">Compiler</td>
519	<td valign="top" width="33%">The compiler fails to compile a nested class that has
520	a dependent base class:<pre>template<typename T>
521	struct foo : {
522	template<typename U>
523	struct bar : public U {};
524	};</pre>
525	</td>
526	</tr>
527	<tr>
528	<td valign="top" width="51%">BOOST_NO_DEPENDENT_TYPES_IN_TEMPLATE_VALUE_PARAMETERS</td>
529	<td valign="top" width="16%">Compiler</td>
530	<td valign="top" width="33%">Template value parameters cannot have a dependent
531	type, for example:<pre>template<class T, typename T::type value>
532	class X { ... };</pre>
533	</td>
534	</tr>
535	<tr>
536	<td valign="top">BOOST_NO_EXCEPTION_STD_NAMESPACE</td>
537	<td valign="top">Standard Library</td>
538	<td>The standard library does not put some or all of the contents of
539	<exception> in namespace std.</td>
540	</tr>
541	<tr>
542	<td valign="top">BOOST_NO_EXCEPTIONS</td>
543	<td valign="top">Compiler</td>
544	<td>The compiler does not support exception handling (this setting is typically
545	required by many C++ compilers for embedded platforms). Note that there is no
546	requirement for boost libraries to honor this configuration setting - indeed
547	doing so may be impossible in some cases. Those libraries that do honor this
548	will typically abort if a critical error occurs - you have been warned!</td>
549	</tr>
550	<tr>
551	<td valign="top" width="51%">BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS</td>
552	<td valign="top" width="16%">Compiler</td>
553	<td valign="top" width="33%">Can only use deduced template arguments when calling
554	function template instantiations.</td>
555	</tr>
556	<tr>
557	<td valign="top" width="51%">BOOST_NO_FUNCTION_TEMPLATE_ORDERING</td>
558	<td valign="top" width="16%">Compiler</td>
559	<td valign="top" width="33%">The compiler does not perform function template
560	ordering or its function template ordering is incorrect.
561	<pre>template<typename T> void f(T); // #1
562	template<typename T, typename U> void f(T (*)(U)); // #2
563
564	void bar(int);
565
566	f(&bar); // should choose #2.</pre>
567	</td>
568	</tr>
569	<tr>
570	<td valign="top" width="51%">BOOST_NO_INCLASS_MEMBER_INITIALIZATION</td>
571	<td valign="top" width="16%">Compiler</td>
572	<td valign="top" width="33%">Compiler violates std::9.4.2/4.</td>
573	</tr>
574	<tr>
575	<td valign="top" width="51%">BOOST_NO_INTRINSIC_WCHAR_T</td>
576	<td valign="top" width="16%">Compiler</td>
577	<td valign="top" width="33%">The C++ implementation does not provide wchar_t, or
578	it is really a synonym for another integral type. Use this symbol to decide
579	whether it is appropriate to explicitly specialize a template on wchar_t if
580	there is already a specialization for other integer types.</td>
581	</tr>
582	<TR>
583	<TD vAlign="top" width="51%">BOOST_NO_IS_ABSTRACT</TD>
584	<TD vAlign="top" width="16%">Compiler</TD>
585	<TD vAlign="top" width="33%">The C++ compiler does not support SFINAE with
586	abstract types, this is covered by <A href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#337">
587	Core Language DR337</A>, but is not part of the current standard.
588	Fortunately most compilers that support SFINAE also support this DR.</TD>
589	</TR>
590	<tr>
591	<td valign="top" width="51%">BOOST_NO_LIMITS</td>
592	<td valign="top" width="16%">Standard library</td>
593	<td valign="top" width="33%">The C++ implementation does not provide the
594	<limits> header. Never check for this symbol in library code; always
595	include <boost/limits.hpp>, which guarantees to provide <code>std::numeric_limits</code>.</td>
596	</tr>
597	<tr>
598	<td valign="top" width="51%">BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS</td>
599	<td valign="top" width="16%">Standard library</td>
600	<td valign="top" width="33%">Constants such as numeric_limits<T>::is_signed
601	are not available for use at compile-time.</td>
602	</tr>
603	<tr>
604	<td>BOOST_NO_LONG_LONG_NUMERIC_LIMITS</td>
605	<td>Standard library</td>
606	<td>There is no specialization for numeric_limits<long long> and
607	numeric_limits<unsigned long long>. <boost/limits.hpp> will then
608	add these specializations as a standard library "fix" only if the compiler
609	supports the long long datatype.</td>
610	</tr>
611	<tr>
612	<td>BOOST_NO_MEMBER_FUNCTION_SPECIALIZATIONS</td>
613	<td>Compiler</td>
614	<td>The compiler does not support the specialization of individual member
615	functions of template classes.</td>
616	</tr>
617	<tr>
618	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATE_KEYWORD</td>
619	<td valign="top" width="16%">Compiler</td>
620	<td valign="top" width="33%">If the compiler supports member templates, but not
621	the template keyword when accessing member template classes.</td>
622	</tr>
623	<tr>
624	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATE_FRIENDS</td>
625	<td valign="top" width="16%">Compiler</td>
626	<td valign="top" width="33%">Member template friend syntax ("template<class
627	P> friend class frd;") described in the C++ Standard, 14.5.3, not supported.</td>
628	</tr>
629	<tr>
630	<td valign="top" width="51%">BOOST_NO_MEMBER_TEMPLATES</td>
631	<td valign="top" width="16%">Compiler</td>
632	<td valign="top" width="33%">Member template functions not fully supported.</td>
633	</tr>
634	<tr>
635	<td>BOOST_NO_MS_INT64_NUMERIC_LIMITS</td>
636	<td>Standard library</td>
637	<td>There is no specialization for numeric_limits<__int64> and
638	numeric_limits<unsigned __int64>. <boost/limits.hpp> will then add
639	these specializations as a standard library "fix", only if the compiler
640	supports the __int64 datatype.</td>
641	</tr>
642	<tr>
643	<td valign="top" width="51%">BOOST_NO_OPERATORS_IN_NAMESPACE</td>
644	<td valign="top" width="16%">Compiler</td>
645	<td valign="top" width="33%">Compiler requires inherited operator friend functions
646	to be defined at namespace scope, then using'ed to boost. Probably GCC
647	specific. See <a href="../../boost/operators.hpp">boost/operators.hpp</a> for
648	example.</td>
649	</tr>
650	<tr>
651	<td valign="top" width="51%">BOOST_NO_POINTER_TO_MEMBER_CONST</td>
652	<td valign="top" width="16%">Compiler</td>
653	<td valign="top" width="33%">The compiler does not correctly handle pointers to
654	const member functions, preventing use of these in overloaded function
655	templates. See <a href="../../boost/functional.hpp">boost/functional.hpp</a> for
656	example.</td>
657	</tr>
658	<TR>
659	<TD vAlign="top" width="51%">BOOST_NO_POINTER_TO_MEMBER_TEMPLATE_PARAMETERS</TD>
660	<TD vAlign="top" width="16%">Compiler</TD>
661	<TD vAlign="top" width="33%">Pointers to members don't work when used as template
662	parameters.</TD>
663	</TR>
664	<tr>
665	<td valign="top" width="51%">BOOST_NO_PRIVATE_IN_AGGREGATE</td>
666	<td valign="top" width="16%">Compiler</td>
667	<td valign="top" width="33%">The compiler misreads 8.5.1, treating classes as
668	non-aggregate if they contain private or protected member functions.</td>
669	</tr>
670	<TR>
671	<TD vAlign="top" width="51%">BOOST_NO_SFINAE</TD>
672	<TD vAlign="top" width="16%">Compiler</TD>
673	<TD vAlign="top" width="33%">The compiler does not support the "Substitution
674	Failure Is Not An Error" meta-programming idiom.</TD>
675	</TR>
676	<tr>
677	<td valign="top" width="51%">BOOST_NO_STD_ALLOCATOR</td>
678	<td valign="top" width="16%">Standard library</td>
679	<td valign="top" width="33%">The C++ standard library does not provide a standards
680	conforming std::allocator.</td>
681	</tr>
682	<tr>
683	<td valign="top" width="51%">BOOST_NO_STD_DISTANCE</td>
684	<td valign="top" width="16%">Standard library</td>
685	<td valign="top" width="33%">The platform does not have a conforming version of
686	std::distance.</td>
687	</tr>
688	<tr>
689	<td valign="top" width="51%">BOOST_NO_STD_ITERATOR</td>
690	<td valign="top" width="16%">Standard library</td>
691	<td valign="top" width="33%">The C++ implementation fails to provide the
692	std::iterator class.</td>
693	</tr>
694	<tr>
695	<td valign="top" width="51%">BOOST_NO_STD_ITERATOR_TRAITS</td>
696	<td valign="top" width="16%">Standard library</td>
697	<td valign="top" width="33%">The compiler does not provide a standard compliant
698	implementation of std::iterator_traits. Note that the compiler may still have a
699	non-standard implementation.</td>
700	</tr>
701	<tr>
702	<td valign="top" width="51%">BOOST_NO_STD_LOCALE</td>
703	<td valign="top" width="16%">Standard library</td>
704	<td valign="top" width="33%">The standard library lacks std::locale.</td>
705	</tr>
706	<tr>
707	<td valign="top" width="51%">BOOST_NO_STD_MESSAGES</td>
708	<td valign="top" width="16%">Standard library</td>
709	<td valign="top" width="33%">The standard library lacks a conforming std::messages
710	facet.</td>
711	</tr>
712	<tr>
713	<td valign="top" width="51%">BOOST_NO_STD_MIN_MAX</td>
714	<td valign="top" width="16%">Standard library</td>
715	<td valign="top" width="33%">The C++ standard library does not provide the min()
716	and max() template functions that should be in <algorithm>.</td>
717	</tr>
718	<tr>
719	<td valign="top">BOOST_NO_STD_OUTPUT_ITERATOR_ASSIGN</td>
720	<td valign="top">Standard library</td>
721	<td valign="top">Defined if the standard library's output iterators are not
722	assignable.</td>
723	</tr>
724	<tr>
725	<td valign="top" width="51%">BOOST_NO_STD_USE_FACET</td>
726	<td valign="top" width="16%">Standard library</td>
727	<td valign="top" width="33%">The standard library lacks a conforming
728	std::use_facet.</td>
729	</tr>
730	<tr>
731	<td>BOOST_NO_STD_WSTREAMBUF</td>
732	<td>Standard library</td>
733	<td>The standard library's implementation of std::basic_streambuf<wchar_t>
734	is either missing, incomplete, or buggy.</td>
735	</tr>
736	<tr>
737	<td valign="top" width="51%">BOOST_NO_STD_WSTRING</td>
738	<td valign="top" width="16%">Standard library</td>
739	<td valign="top" width="33%">The standard library lacks std::wstring.</td>
740	</tr>
741	<tr>
742	<td valign="top" width="51%">BOOST_NO_STDC_NAMESPACE</td>
743	<td valign="top" width="16%">Compiler/Platform</td>
744	<td valign="top" width="33%">The contents of C++ standard headers for C library
745	functions (the <c...> headers) have not been placed in namespace std.
746	This test is difficult - some libraries "fake" the std C functions by adding
747	using declarations to import them into namespace std, unfortunately they don't
748	necessarily catch all of them...</td>
749	</tr>
750	<tr>
751	<td valign="top" width="51%">BOOST_NO_STRINGSTREAM</td>
752	<td valign="top" width="16%">Standard library</td>
753	<td valign="top" width="33%">The C++ implementation does not provide the
754	<sstream> header.</td>
755	</tr>
756	<tr>
757	<td valign="top" width="51%">BOOST_NO_SWPRINTF</td>
758	<td valign="top" width="16%">Platform</td>
759	<td valign="top" width="33%">The platform does not have a conforming version of
760	swprintf.</td>
761	</tr>
762	<tr>
763	<td valign="top" width="51%">BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION</td>
764	<td valign="top" width="16%">Compiler</td>
765	<td valign="top" width="33%">Class template partial specialization (14.5.4
766	[temp.class.spec]) not supported.</td>
767	</tr>
768	<tr>
769	<td valign="top" width="51%">BOOST_NO_TEMPLATED_ITERATOR_CONSTRUCTORS</td>
770	<td valign="top" width="16%">Standard library</td>
771	<td valign="top" width="33%">The standard library does not provide templated
772	iterator constructors for its containers.</td>
773	</tr>
774	<tr>
775	<td>BOOST_NO_TEMPLATE_TEMPLATES</td>
776	<td>Compiler</td>
777	<td>The compiler does not support template template parameters.</td>
778	</tr>
779	<tr>
780	<td>BOOST_NO_UNREACHABLE_RETURN_DETECTION</td>
781	<td>Compiler</td>
782	<td>If a return is unreachable, then no return statement should be required,
783	however some compilers insist on it, while other issue a bunch of warnings if
784	it is in fact present.</td>
785	</tr>
786	<TR>
787	<TD vAlign="top" width="51%">BOOST_NO_USING_DECLARATION_OVERLOADS_FROM_TYPENAME_BASE</TD>
788	<TD vAlign="top" width="16%">Compiler</TD>
789	<TD vAlign="top" width="33%">The compiler will not accept a using
790	declaration that brings a function from a typename used as a base
791	class into a derived class if functions of the same name are present
792	in the derived class.</TD>
793	</TR>
794	<tr>
795	<td valign="top" width="51%">BOOST_NO_USING_TEMPLATE</td>
796	<td valign="top" width="16%">Compiler</td>
797	<td valign="top" width="33%">The compiler will not accept a using declaration that
798	imports a template class or function from another namespace. Originally a
799	Borland specific problem with imports to/from the global namespace, extended to
800	MSVC6 which has a specific issue with importing template classes (but not
801	functions).</td>
802	</tr>
803	<tr>
804	<td valign="top" width="51%">BOOST_NO_VOID_RETURNS</td>
805	<td valign="top" width="16%">Compiler</td>
806	<td valign="top" width="33%">The compiler does not allow a void function to return
807	the result of calling another void function.
808	<pre>void f() {}
809	void g() { return f(); }</pre>
810	</td>
811	</tr>
812	</table>
813	<h4><a name="features"></a>Macros that describe optional features:</h4>
814	<p>The following macros describe features that are not required by the C++
815	standard. The macro is only defined if the feature is present.</p>
816	<table border="1" cellpadding="7" cellspacing="1" width="100%">
817	<tr>
818	<td valign="top" width="48%"><p align="center"><b>Macro</b></p>
819	</td>
820	<td valign="top" width="15%"><p align="center"><b>Section</b></p>
821	</td>
822	<td valign="top" width="37%"><p align="center"><b>Description</b></p>
823	</td>
824	</tr>
825	<tr>
826	<td valign="top" width="48%">BOOST_HAS_BETHREADS</td>
827	<td valign="top" width="15%">Platform</td>
828	<td valign="top" width="37%">The platform supports BeOS style threads.</td>
829	</tr>
830	<tr>
831	<td>BOOST_HAS_CLOCK_GETTIME</td>
832	<td>Platform</td>
833	<td>The platform has the POSIX API clock_gettime.</td>
834	</tr>
835	<TR>
836	<TD>BOOST_HAS_DECLSPEC
837	</TD>
838	<TD>Compiler</TD>
839	<TD>The compiler uses __declspec(dllexport) and __declspec(dllimport) to
840	export/import symbols from dll's.</TD>
841	</TR>
842	<tr>
843	<td>BOOST_HAS_DIRENT_H</td>
844	<td>Platform</td>
845	<td>The platform has the POSIX header <dirent.h>.</td>
846	</tr>
847	<tr>
848	<td>BOOST_HAS_FTIME</td>
849	<td>Platform</td>
850	<td>The platform has the Win32 API GetSystemTimeAsFileTime.</td>
851	</tr>
852	<tr>
853	<td>BOOST_HAS_GETTIMEOFDAY</td>
854	<td>Platform</td>
855	<td>The platform has the POSIX API gettimeofday.</td>
856	</tr>
857	<tr>
858	<td valign="top" width="48%">BOOST_HAS_HASH</td>
859	<td valign="top" width="15%">Standard library</td>
860	<td valign="top" width="37%">The C++ implementation provides the (SGI) hash_set or
861	hash_map classes.</td>
862	</tr>
863	<tr>
864	<td>BOOST_HAS_LONG_LONG</td>
865	<td>Compiler</td>
866	<td>The compiler supports the long long data type.</td>
867	</tr>
868	<tr>
869	<td valign="top" width="48%">BOOST_HAS_MACRO_USE_FACET</td>
870	<td valign="top" width="15%">Standard library</td>
871	<td valign="top" width="37%">The standard library lacks a conforming
872	std::use_facet, but has a macro _USE(loc, Type) that does the job. This is
873	primarily for the Dinkumware std lib.</td>
874	</tr>
875	<tr>
876	<td>BOOST_HAS_MS_INT64</td>
877	<td>Compiler</td>
878	<td>The compiler supports the __int64 data type.</td>
879	</tr>
880	<tr>
881	<td>BOOST_HAS_NANOSLEEP</td>
882	<td>Platform</td>
883	<td>The platform has the POSIX API nanosleep.</td>
884	</tr>
885	<tr>
886	<td valign="top" width="48%">BOOST_HAS_NL_TYPES_H</td>
887	<td valign="top" width="15%">Platform</td>
888	<td valign="top" width="37%">The platform has an <nl_types.h>.</td>
889	</tr>
890	<tr>
891	<td>BOOST_HAS_NRVO</td>
892	<td>Compiler</td>
893	<td>Indicated that the compiler supports the named return value optimization
894	(NRVO). Used to select the most efficient implementation for some function. See <a href="../../boost/operators.hpp">
895	boost/operators.hpp</a> for example.</td>
896	</tr>
897	<tr>
898	<td valign="top">BOOST_HAS_PARTIAL_STD_ALLOCATOR</td>
899	<td>Standard Library</td>
900	<td>The standard library has a partially conforming std::allocator class, but
901	without any of the member templates.</td>
902	</tr>
903	<tr>
904	<td>BOOST_HAS_PTHREAD_DELAY_NP</td>
905	<td>Platform</td>
906	<td>The platform has the POSIX API pthread_delay_np.</td>
907	</tr>
908	<tr>
909	<td>BOOST_HAS_PTHREAD_MUTEXATTR_SETTYPE</td>
910	<td>Platform</td>
911	<td>The platform has the POSIX API pthread_mutexattr_settype.</td>
912	</tr>
913	<tr>
914	<td>BOOST_HAS_PTHREAD_YIELD</td>
915	<td>Platform</td>
916	<td>The platform has the POSIX API pthread_yield.</td>
917	</tr>
918	<tr>
919	<td valign="top" width="48%">BOOST_HAS_PTHREADS</td>
920	<td valign="top" width="15%">Platform</td>
921	<td valign="top" width="37%">The platform support POSIX style threads.</td>
922	</tr>
923	<tr>
924	<td>BOOST_HAS_SCHED_YIELD</td>
925	<td>Platform</td>
926	<td>The platform has the POSIX API sched_yield.</td>
927	</tr>
928	<tr>
929	<td>BOOST_HAS_SGI_TYPE_TRAITS</td>
930	<td>Compiler/standard library</td>
931	<td>The compiler has native support for SGI style type traits.</td>
932	</tr>
933	<tr>
934	<td>BOOST_HAS_STDINT_H</td>
935	<td>Platform</td>
936	<td>The platform has a <stdint.h></td>
937	</tr>
938	<tr>
939	<td valign="top" width="48%">BOOST_HAS_SLIST</td>
940	<td valign="top" width="15%">Standard library</td>
941	<td valign="top" width="37%">The C++ implementation provides the (SGI) slist
942	class.</td>
943	</tr>
944	<tr>
945	<td valign="top" width="48%">BOOST_HAS_STLP_USE_FACET</td>
946	<td valign="top" width="15%">Standard library</td>
947	<td valign="top" width="37%">The standard library lacks a conforming
948	std::use_facet, but has a workaround class-version that does the job. This is
949	primarily for the STLport std lib.</td>
950	</tr>
951	<tr>
952	<td valign="top" width="48%">BOOST_HAS_THREADS</td>
953	<td valign="top" width="15%">Platform/compiler</td>
954	<td valign="top" width="37%">Defined if the compiler, in its current translation
955	mode, supports multiple threads of execution.</td>
956	</tr>
957	<tr>
958	<td valign="top" width="48%">BOOST_HAS_TWO_ARG_USE_FACET</td>
959	<td valign="top" width="15%">Standard library</td>
960	<td valign="top" width="37%">The standard library lacks a conforming
961	std::use_facet, but has a two argument version that does the job. This is
962	primarily for the Rogue Wave std lib.</td>
963	</tr>
964	<tr>
965	<td valign="top" width="48%">BOOST_HAS_UNISTD_H</td>
966	<td valign="top" width="15%">Platform</td>
967	<td valign="top" width="37%">The Platform provides <unistd.h>.</td>
968	</tr>
969	<tr>
970	<td valign="top" width="48%">BOOST_HAS_WINTHREADS</td>
971	<td valign="top" width="15%">Platform</td>
972	<td valign="top" width="37%">The platform supports MS Windows style threads.</td>
973	</tr>
974	<tr>
975	<td valign="top" width="48%">BOOST_MSVC_STD_ITERATOR</td>
976	<td valign="top" width="15%">Standard library</td>
977	<td valign="top" width="37%">Microsoft's broken version of std::iterator is being
978	used. This implies that std::iterator takes no more than two template
979	parameters.</td>
980	</tr>
981	<tr>
982	<td valign="top" width="48%">BOOST_MSVC6_MEMBER_TEMPLATES</td>
983	<td valign="top" width="15%">Compiler</td>
984	<td valign="top" width="37%">Microsoft Visual C++ 6.0 has enough member template
985	idiosyncrasies (being polite) that BOOST_NO_MEMBER_TEMPLATES is defined for
986	this compiler. BOOST_MSVC6_MEMBER_TEMPLATES is defined to allow compiler
987	specific workarounds. This macro gets defined automatically if
988	BOOST_NO_MEMBER_TEMPLATES is not defined - in other words this is treated as a
989	strict subset of the features required by the standard.</td>
990	</tr>
991	<tr>
992	<td valign="top" width="48%">BOOST_HAS_STDINT_H</td>
993	<td valign="top" width="15%">Platform</td>
994	<td valign="top" width="37%">There are no 1998 C++ Standard headers
995	<stdint.h> or <cstdint>, although the 1999 C Standard does include
996	<stdint.h>. If <stdint.h> is present, <boost/stdint.h> can
997	make good use of it, so a flag is supplied (signalling presence; thus the
998	default is not present, conforming to the current C++ standard).</td>
999	</tr>
1000	</table>
1001	<h4><a name="helpers"></a>Boost Helper Macros</h4>
1002	<p>The following macros are either simple helpers, or macros that provide
1003	workarounds for compiler/standard library defects.</p>
1004	<table border="1" cellpadding="7" cellspacing="1" width="100%">
1005	<tr>
1006	<td valign="top" width="50%"><p align="center"><b>Macro</b></p>
1007	</td>
1008	<td valign="top" width="50%"><p align="center"><b>Description</b></p>
1009	</td>
1010	</tr>
1011	<tr>
1012	<td valign="top" width="50%">BOOST_DEDUCED_TYPENAME</td>
1013	<td valign="top" width="50%">Some compilers don't support the use of <code>typename</code>
1014	for dependent types in deduced contexts. This macro expands to nothing on those
1015	compilers, and <code>typename</code> elsewhere. For example, replace:<pre>template <class T> void f(T, typename T::type);</pre>
1016	<p>with:</p>
1017	<pre>template <class T> void f(T, BOOST_DEDUCED_TYPENAME T::type);</pre>
1018	</td>
1019	</tr>
1020	<tr>
1021	<td>BOOST_STD_EXTENSION_NAMESPACE</td>
1022	<td>The namespace used for std library extensions (hashtable classes etc).</td>
1023	</tr>
1024	<tr>
1025	<td valign="top" width="50%">BOOST_STATIC_CONSTANT(Type, assignment)</td>
1026	<td valign="top" width="50%">On compilers which don't allow in-class
1027	initialization of static integral constant members, we must use enums as a
1028	workaround if we want the constants to be available at compile-time. This macro
1029	gives us a convenient way to declare such constants. For example instead of:<pre>struct foo{
1030	static const int value = 2;
1031	};</pre>
1032	<p>use:</p>
1033	<pre>struct foo{
1034	BOOST_STATIC_CONSTANT(int, value = 2);
1035	};</pre>
1036	</td>
1037	</tr>
1038	<tr>
1039	<td>BOOST_UNREACHABLE_RETURN(result)</td>
1040	<td>Normally evaluates to nothing, but evaluates to <font face="Courier New">return
1041	x;</font> if the compiler requires a return, even when it can never be
1042	reached.</td>
1043	</tr>
1044	<tr>
1045	<td>BOOST_EXPLICIT_TEMPLATE_TYPE(t)<br>
1046	BOOST_EXPLICIT_TEMPLATE_NON_TYPE(t, v)<br>
1047	BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE(t)<br>
1048	BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE(t, v)<br>
1049	</td>
1050	<td>Some compilers silently "fold" different function template instantiations if
1051	some of the template parameters don't appear in the function parameter list.
1052	For instance:
1053	<pre> #include <iostream>
1054	#include <ostream>
1055	#include <typeinfo>
1056
1057	template <int n>
1058	void f() { std::cout << n << ' '; }
1059
1060	template <typename T>
1061	void g() { std::cout << typeid(T).name() << ' '; }
1062
1063	int main() {
1064	f<1>();
1065	f<2>();
1066
1067	g<int>();
1068	g<double>();
1069	}
1070	</pre>
1071	incorrectly outputs <tt>"2 2 double double "</tt> on VC++ 6. These macros, to
1072	be used in the function parameter list, fix the problem without effects on the
1073	calling syntax. For instance, in the case above write:
1074	<pre> template <int n>
1075	void f(BOOST_EXPLICIT_TEMPLATE_NON_TYPE(int, n)) { ... }
1076
1077	template <typename T>
1078	void g(BOOST_EXPLICIT_TEMPLATE_TYPE(T)) { ... }
1079	</pre>
1080	Beware that they can declare (for affected compilers) a dummy <i>defaulted</i> parameter,
1081	so they
1082	<br>
1083	<br>
1084	a) should be always invoked at the end of the parameter list
1085	<br>
1086	b) can't be used if your function template is multiply declared.
1087	<br>
1088	<br>
1089	Furthermore, in order to add any needed comma separator, an "APPEND_*" version
1090	must be used when the macro invocation appears after a normal parameter
1091	declaration or after the invocation of another macro of this same group.
1092	</td>
1093	</tr>
1094	<tr>
1095	<td valign="top" width="50%">BOOST_USE_FACET(Type, loc)</td>
1096	<td valign="top" width="50%">When the standard library does not have a comforming
1097	std::use_facet there are various workarounds available, but they differ from
1098	library to library. This macro provides a consistent way to access a locale's
1099	facets. For example, replace:<pre>std::use_facet<Type>(loc);</pre>
1100	<p>with:</p>
1101	<pre>BOOST_USE_FACET(Type, loc);</pre>
1102	<p>Note do not add a std:: prefix to the front of BOOST_USE_FACET.</p>
1103	</td>
1104	</tr>
1105	<tr>
1106	<td valign="top" width="50%">BOOST_HAS_FACET(Type, loc)</td>
1107	<td valign="top" width="50%">When the standard library does not have a comforming
1108	std::has_facet there are various workarounds available, but they differ from
1109	library to library. This macro provides a consistent way to check a locale's
1110	facets. For example, replace:<pre>std::has_facet<Type>(loc);</pre>
1111	<p>with:</p>
1112	<pre>BOOST_HAS_FACET(Type, loc);</pre>
1113	<p>Note do not add a std:: prefix to the front of BOOST_HAS_FACET.</p>
1114	</td>
1115	</tr>
1116	<tr>
1117	<td valign="top" width="50%">BOOST_NESTED_TEMPLATE</td>
1118	<td valign="top" width="50%">Member templates are supported by some compilers even
1119	though they can't use the A::template member<U> syntax, as a workaround
1120	replace:<pre>typedef typename A::template rebind<U> binder;</pre>
1121	<p>with:</p>
1122	<pre>typedef typename A::BOOST_NESTED_TEMPLATE rebind<U> binder;</pre>
1123	</td>
1124	</tr>
1125	<tr>
1126	<td valign="top" width="50%">BOOST_STRINGIZE(X)</td>
1127	<td valign="top" width="50%">Converts the parameter X to a string after macro
1128	replacement on X has been performed.</td>
1129	</tr>
1130	<tr>
1131	<td valign="top" width="50%">BOOST_JOIN(X,Y)</td>
1132	<td valign="top" width="50%">This piece of macro magic joins the two arguments
1133	together, even when one of the arguments is itself a macro (see 16.3.1 in C++
1134	standard). This is normally used to create a mangled name in combination with a
1135	predefined macro such a __LINE__.</td>
1136	</tr>
1137	</table>
1138	<h4><a name="info_macros"></a>Boost Informational Macros</h4>
1139	<p>The following macros describe boost features; these are, generally speaking
1140	the only boost macros that should be tested in user code.</p>
1141	<table border="1" cellpadding="7" cellspacing="1" width="100%">
1142	<tr>
1143	<td valign="top" width="33%"><p align="center"><b>Macro</b></p>
1144	</td>
1145	<td valign="top" width="33%"><p align="center"><b>Header</b></p>
1146	</td>
1147	<td valign="top" width="33%"><p align="center"><b>Description</b></p>
1148	</td>
1149	</tr>
1150	<tr>
1151	<td valign="top" width="33%">BOOST_VERSION</td>
1152	<td valign="top" width="33%"><boost/version.hpp></td>
1153	<td valign="top" width="33%">Describes the boost version number in XXYYZZ format
1154	such that: (BOOST_VERSION % 100) is the sub-minor version, ((BOOST_VERSION /
1155	100) % 1000) is the minor version, and (BOOST_VERSION / 100000) is the major
1156	version.</td>
1157	</tr>
1158	<tr>
1159	<td valign="top" width="33%">BOOST_NO_INT64_T</td>
1160	<td valign="top" width="33%"><boost/cstdint.hpp><p><boost/stdint.h></p>
1161	</td>
1162	<td valign="top" width="33%">Defined if there are no 64-bit integral types:
1163	int64_t, uint64_t etc.</td>
1164	</tr>
1165	<tr>
1166	<td valign="top" width="33%">BOOST_NO_INTEGRAL_INT64_T</td>
1167	<td valign="top" width="33%"><boost/cstdint.hpp><p><boost/stdint.h></p>
1168	</td>
1169	<td valign="top" width="33%">Defined if int64_t as defined by
1170	<boost/cstdint.hpp> is not usable in integral constant expressions.</td>
1171	</tr>
1172	<tr>
1173	<td valign="top">BOOST_MSVC</td>
1174	<td valign="top"><boost/config.hpp></td>
1175	<td valign="top">Defined if the compiler is really Microsoft Visual C++, as
1176	opposed to one of the many other compilers that also define _MSC_VER.</td>
1177	</tr>
1178	<tr>
1179	<td valign="top">BOOST_INTEL</td>
1180	<td valign="top"><boost/config.hpp></td>
1181	<td valign="top">Defined if the compiler is an Intel compiler, takes the same
1182	value as the compiler version macro.</td>
1183	</tr>
1184	<TR>
1185	<TD vAlign="top">BOOST_WINDOWS</TD>
1186	<TD vAlign="top"><boost/config.hpp></TD>
1187	<TD vAlign="top">Defined if the Windows platfrom API is available.</TD>
1188	</TR>
1189	<tr>
1190	<td>BOOST_DINKUMWARE_STDLIB</td>
1191	<td><boost/config.hpp></td>
1192	<td>Defined if the dinkumware standard library is in use, takes the same value as
1193	the Dinkumware library version macro _CPPLIB_VER if defined, otherwise 1.</td>
1194	</tr>
1195	<tr>
1196	<td valign="top" width="33%">BOOST_NO_WREGEX</td>
1197	<td valign="top" width="33%"><boost/regex.hpp></td>
1198	<td valign="top" width="33%">Defined if the regex library does not support wide
1199	character regular expressions.</td>
1200	</tr>
1201	<tr>
1202	<td valign="top" width="33%">BOOST_COMPILER</td>
1203	<td valign="top" width="33%"><boost/config.hpp></td>
1204	<td valign="top" width="33%">Defined as a string describing the name and version
1205	number of the compiler in use. Mainly for debugging the configuration.</td>
1206	</tr>
1207	<tr>
1208	<td valign="top" width="33%">BOOST_STDLIB</td>
1209	<td valign="top" width="33%"><boost/config.hpp></td>
1210	<td valign="top" width="33%">Defined as a string describing the name and version
1211	number of the standard library in use. Mainly for debugging the configuration.</td>
1212	</tr>
1213	<tr>
1214	<td valign="top" width="33%">BOOST_PLATFORM</td>
1215	<td valign="top" width="33%"><boost/config.hpp></td>
1216	<td valign="top" width="33%">Defined as a string describing the name of the
1217	platform. Mainly for debugging the configuration.</td>
1218	</tr>
1219	</table>
1220	<h2><a name="guidelines"></a></h2>
1221	<H4><A name="source"></A>Macros for libraries with separate source code</H4>
1222	<P>The following macros and helper headers are of use to authors whose libraries
1223	include separate source code, and are intended to address two issues: fixing
1224	the ABI of the compiled library, and selecting which compiled library to link
1225	against based upon the compilers settings.</P>
1226	<H5>ABI Fixing</H5>
1227	<P>When linking against a pre-compiled library it vital that the ABI used by the
1228	compiler when building the library <EM>matches</EM> <EM>exactly</EM> the ABI
1229	used by the code using the library.  In this case ABI means things like
1230	the struct packing arrangement used, the name mangling scheme used, or the size
1231	of some types (enum types for example).  This is separate from things like
1232	threading support, or runtime library variations, which have to be dealt with
1233	by build variants.  To put this in perspective there is one compiler
1234	(Borland's) that has so many compiler options that make subtle changes to the
1235	ABI, that at least in theory there 3200 combinations, and that's without
1236	considering runtime library variations.  Fortunately these variations can
1237	be managed by #pragma's that tell the compiler what ABI to use for the types
1238	declared in your library. In order to avoid sprinkling #pragma's all over the
1239	boost headers, there are some prefix and suffix headers that do the job.
1240	Typical usage is:</P>
1241	<P><b>my_library.cpp</b></P>
1242	<blockquote>
1243	<PRE>#ifndef MY_INCLUDE_GUARD
1244	#define MY_INCLUDE_GUARD
1245
1246	// all includes go here:
1247	<b>#include <boost/config.hpp></b>
1248	#include <whatever>
1249
1250	<b>#include <boost/config/abi_prefix.hpp> // must be the last #include</b>
1251
1252	namespace boost{
1253	// your code goes here
1254	}
1255
1256	<b>#include <boost/config/abi_suffix.hpp> // pops abi_prefix.hpp pragmas</b>
1257
1258	#endif // include guard
1259	</PRE>
1260	</blockquote>
1261	<P><b>my_library.cpp</b></P>
1262	<blockquote>
1263	<pre>...
1264	<b>// nothing special need be done in the implementation file</b>
1265	...</pre>
1266	</blockquote>
1267	<P>The user can disable this mechanism by defining BOOST_DISABLE_ABI_HEADERS, or
1268	they can define BOOST_ABI_PREFIX and/or BOOST_ABI_SUFFIX to point to their own
1269	prefix/suffix headers if they so wish.</P>
1270	<H5>Automatic library selection</H5>
1271	<P>It is essential that users link to a build of a library which was built against
1272	the same runtime library that their application will be built against - if this
1273	does not happen then the library will not be binary compatible with their own
1274	code - and there is a high likelihood  that their application will
1275	experience runtime crashes.  These kinds of problems can be extremely
1276	time consuming and difficult to debug, and often lead to frustrated users and
1277	authors alike (simply selecting the right library to link against is not as
1278	easy as it seems when their are 6-8 of them to chose from, and some users seem
1279	to be blissfully unaware that there even are different runtimes available to
1280	them).</P>
1281	<P>To solve this issue, some compilers allow source code to contain #pragma's that
1282	instruct the linker which library to link against, all the user need do is
1283	include the headers they need, place the compiled libraries in their library
1284	search path, and the compiler and linker do the rest.  Boost.config
1285	supports this via the header <boost/config/auto_link.hpp>, before
1286	including this header one or more of the following macros need to be defined:</P>
1287	<TABLE id="Table1" cellSpacing="1" cellPadding="1" width="100%" border="1">
1288	<TR>
1289	<TD>BOOST_LIB_NAME</TD>
1290	<TD>
1291	Required: An identifier containing the basename of the library, for
1292	example 'boost_regex'.</TD>
1293	</TR>
1294	<TR>
1295	<TD>BOOST_DYN_LINK</TD>
1296	<TD>Optional: when set link to dll rather than static library.</TD>
1297	</TR>
1298	<TR>
1299	<TD>BOOST_LIB_DIAGNOSTIC</TD>
1300	<TD>Optional: when set the header will print out the name of the library selected
1301	(useful for debugging).</TD>
1302	</TR>
1303	</TABLE>
1304	<P>If the compiler supports this mechanism, then it will be told to link against
1305	the appropriately named library, the actual algorithm used to mangle the name
1306	of the library is documented inside <boost/config/auto_link.hpp> and has
1307	to match that used to create the libraries via bjam 's install rules.</P>
1308	<P>Typical usage is:</P>
1309	<P><b>my_library.hpp</b></P>
1310	<blockquote>
1311	<PRE>...
1312	//
1313	// Don't include auto-linking code if the user has disabled it by
1314	// defining BOOST_ALL_NO_LIB, or BOOST_MY_LIBRARY_NO_LIB, or if this
1315	// is one of our own source files (signified by BOOST_MY_LIBRARY_SOURCE):
1316	//
1317	<b>#if !defined(BOOST_ALL_NO_LIB) && !defined(BOOST_MY_LIBRARY_NO_LIB) && !defined(BOOST_MY_LIBRARY_SOURCE)
1318	# define BOOST_LIB_NAME boost_my_library
1319	# ifdef BOOST_MY_LIBRARY_DYN_LINK
1320	# define BOOST_DYN_LINK
1321	# endif
1322	# include <boost/config/auto_link.hpp>
1323	#endif
1324	</b>...
1325	</PRE>
1326	</blockquote>
1327	<p><b>my_library.cpp</b></p>
1328	<blockquote>
1329	<pre>// define BOOST_MY_LIBRARY_SOURCE so that the header knows that the
1330	// library is being built (possibly exporting rather than importing code)
1331	//
1332	<b>#define BOOST_MY_LIBRARY_SOURCE</b>
1333
1334	<b>#include <boost/my_library/my_library.hpp></b>
1335	...</pre>
1336	</blockquote>
1337	<H2>Guidelines for Boost Authors</H2>
1338	<p>The <a href="../../boost/config.hpp">boost/config.hpp</a> header is used to
1339	pass configuration information to other boost files, allowing them to cope with
1340	platform dependencies such as arithmetic byte ordering, compiler pragmas, or
1341	compiler shortcomings. Without such configuration information, many current
1342	compilers would not work with the Boost libraries.</p>
1343	<p>Centralizing configuration information in this header reduces the number of
1344	files that must be modified when porting libraries to new platforms, or when
1345	compilers are updated. Ideally, no other files would have to be modified when
1346	porting to a new platform.</p>
1347	<p>Configuration headers are controversial because some view them as condoning
1348	broken compilers and encouraging non-standard subsets. Adding settings for
1349	additional platforms and maintaining existing settings can also be a problem.
1350	In other words, configuration headers are a necessary evil rather than a
1351	desirable feature. The boost config.hpp policy is designed to minimize the
1352	problems and maximize the benefits of a configuration header.</p>
1353	<p>Note that:</p>
1354	<ul>
1355	<li>
1356	Boost library implementers are not required to #include
1357	<boost/config.hpp>, and are not required in any way to support compilers
1358	that do not comply with the C++ Standard (ISO/IEC 14882).
1359	<li>
1360	If a library implementer wishes to support some non-conforming compiler, or to
1361	support some platform specific feature, #include <boost/config.hpp> is
1362	the preferred way to obtain configuration information not available from the
1363	standard headers such as <climits>, etc.
1364	<li>
1365	If configuration information can be deduced from standard headers such as
1366	<climits>, use those standard headers rather than
1367	<boost/config.hpp>.
1368	<li>
1369	Boost files that use macros defined in <boost/config.hpp> should have
1370	sensible, standard conforming, default behavior if the macro is not defined.
1371	This means that the starting point for porting <boost/config.hpp> to a
1372	new platform is simply to define nothing at all specific to that platform. In
1373	the rare case where there is no sensible default behavior, an #error message
1374	should describe the problem.
1375	<li>
1376	If a Boost library implementer wants something added to config.hpp, post a
1377	request on the Boost mailing list. There is no guarantee such a request will be
1378	honored; the intent is to limit the complexity of config.hpp.
1379	<li>
1380	The intent is to support only compilers which appear on their way to becoming
1381	C++ Standard compliant, and only recent releases of those compilers at that.
1382	<li>
1383	The intent is not to disable mainstream features now well-supported by the
1384	majority of compilers, such as namespaces, exceptions, RTTI, or templates.
1385	</li>
1386	</ul>
1387	<h4><a name="defect_guidelines"></a>Adding New Defect Macros</h4>
1388	<p>When you need to add a new defect macro - either to fix a problem with an
1389	existing library, or when adding a new library - distil the issue down to a
1390	simple test case; often, at this point other (possibly better) workarounds may
1391	become apparent. Secondly always post the test case code to the boost mailing
1392	list and invite comments; remember that C++ is complex and that sometimes what
1393	may appear a defect, may in fact turn out to be a problem with the authors
1394	understanding of the standard.</p>
1395	<p>When you name the macro, follow the BOOST_NO_SOMETHING naming convention, so
1396	that it's obvious that this is a macro reporting a defect.</p>
1397	<p>Finally, add the test program to the regression tests. You will need to place
1398	the test case in a .ipp file with the following comments near the top:</p>
1399	<pre>// MACRO: BOOST_NO_FOO
1400	// TITLE: foo
1401	// DESCRIPTION: If the compiler fails to support foo</pre>
1402	<p>These comments are processed by the autoconf script, so make sure the format
1403	follows the one given. The file should be named "boost_no_foo.ipp", where foo
1404	is the defect description - try and keep the file name under the Mac 30
1405	character filename limit though. You will also need to provide a function
1406	prototype "int test()" that is declared in a namespace with the same name as
1407	the macro, but in all lower case, and which returns zero on success:</p>
1408	<pre>namespace boost_no_foo{
1409
1410	int test()
1411	{
1412	// test code goes here:
1413	//
1414	return 0;
1415	}
1416
1417	}</pre>
1418	<p>
1419	Once the test code is in place, build and run the program "generate.cpp" that
1420	you will find in the boost-root/libs/config/tools/ directory. This generates
1421	two .cpp test files from the new test code, and adds the tests to the
1422	regression test Jamfile, and the config_test.cpp test program. Finally add a
1423	new entry to config_info.cpp so that the new macro gets printed out when that
1424	program is run.</p>
1425	<h4><a name="feature_guidelines"></a>Adding New Feature Test Macros</h4>
1426	<p>When you need to add a macro that describes a feature that the standard does
1427	not require, follow the convention for adding a new defect macro (above), but
1428	call the macro BOOST_HAS_FOO, and name the test file "boost_has_foo.ipp". Try
1429	not to add feature test macros unnecessarily, if there is a platform specific
1430	macro that can already be used (for example _WIN32, __BEOS__, or __linux) to
1431	identify the feature then use that. Try to keep the macro to a feature group,
1432	or header name, rather than one specific API (for example BOOST_HAS_NL_TYPES_H
1433	rather than BOOST_HAS_CATOPEN). If the macro describes a POSIX feature group,
1434	then add boilerplate code to <a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>
1435	to auto-detect the feature where possible (if you are wondering why we can't
1436	use POSIX feature test macro directly, remember that many of these features can
1437	be added by third party libraries, and are not therefore identified inside
1438	<unistd.h>).</p>
1439	<h4><a name="modify_guidelines"></a>Modifying the Boost Configuration Headers</h4>
1440	<p>The aim of boost's configuration setup is that the configuration headers should
1441	be relatively stable - a boost user should not have to recompile their code
1442	just because the configuration for some compiler that they're not interested in
1443	has changed. Separating the configuration into separate compiler/standard
1444	library/platform sections provides for part of this stability, but boost
1445	authors require some amount of restraint as well, in particular:</p>
1446	<p><<a href="../../boost/config.hpp">boost/config.hpp</a>> should never
1447	change, don't alter this file.</p>
1448	<p><<a href="../../boost/config/user.hpp">boost/config/user.hpp</a>> is
1449	included by default, don't add extra code to this file unless you have to. If
1450	you do, please remember to update <a href="tools/configure.in">libs/config/tools/configure.in</a>
1451	as well.</p>
1452	<p><<a href="../../boost/config/suffix.hpp">boost/config/suffix.hpp</a>> is
1453	always included so be careful about modifying this file as it breaks
1454	dependencies for everyone. This file should include only "boilerplate"
1455	configuration code, and generally should change only when new macros are added.</p>
1456	<p><<a href="../../boost/config/select_compiler_config.hpp">boost/config/select_compiler_config.hpp</a>>,
1457	<<a href="../../boost/config/select_platform_config.hpp">boost/config/select_platform_config.hpp</a>>
1458	and <<a href="../../boost/config/select_stdlib_config.hpp">boost/config/select_stdlib_config.hpp</a>>
1459	are included by default and should change only if support for a new
1460	compiler/standard library/platform is added.</p>
1461	<p>The compiler/platform/standard library selection code is set up so that unknown
1462	platforms are ignored and assumed to be fully standards compliant - this gives
1463	unknown platforms a "sporting chance" of working "as is" even without running
1464	the configure script.</p>
1465	<p>When adding or modifying the individual mini-configs, assume that future, as
1466	yet unreleased versions of compilers, have all the defects of the current
1467	version. Although this is perhaps unnecessarily pessimistic, it cuts down on
1468	the maintenance of these files, and experience suggests that pessimism is
1469	better placed than optimism here!</p>
1470	<h2><a name="rationale"></a>Rationale</h2>
1471	<p>The problem with many traditional "textbook" implementations of configuration
1472	headers (where all the configuration options are in a single "monolithic"
1473	header) is that they violate certain fundamental software engineering
1474	principles which would have the effect of making boost more fragile, more
1475	difficult to maintain and more difficult to use safely. You can find a
1476	description of the principles from the <a href="http://www.objectmentor.com/publications/Principles%20and%20Patterns.PDF">
1477	following article</a>.</p>
1478	<h4>The problem</h4>
1479	<p>Consider a situation in which you are concurrently developing on multiple
1480	platforms. Then consider adding a new platform or changing the platform
1481	definitions of an existing platform. What happens? Everything, and this does
1482	literally mean everything, recompiles. Isn't it quite absurd that adding a new
1483	platform, which has absolutely nothing to do with previously existing
1484	platforms, means that all code on all existing platforms needs to be
1485	recompiled?</p>
1486	<p>Effectively, there is an imposed physical dependency between platforms that
1487	have nothing to do with each other. Essentially, the traditional solution
1488	employed by configuration headers does not conform to the Open-Closed
1489	Principle:</p>
1490	<p><b><i>"A module should be open for extension but closed for modification."</i></b></p>
1491	<p>Extending a traditional configuration header implies modifying existing code.</p>
1492	<p>Furthermore, consider the complexity and fragility of the platform detection
1493	code. What if a simple change breaks the detection on some minor platform? What
1494	if someone accidentally or on purpose (as a workaround for some other problem)
1495	defines some platform dependent macros that are used by the detection code? A
1496	traditional configuration header is one of the most volatile headers of the
1497	entire library, and more stable elements of Boost would depend on it. This
1498	violates the Stable Dependencies Principle:</p>
1499	<p><b><i>"Depend in the direction of stability."</i></b></p>
1500	<p>After even a minor change to a traditional configuration header on one minor
1501	platform, almost everything on every platform should be tested if we follow
1502	sound software engineering practice.</p>
1503	<p>Another important issue is that it is not always possible to submit changes to
1504	<boost/config.hpp>. Some boost users are currently working on platforms
1505	using tools and libraries that are under strict Non-Disclosure Agreements. In
1506	this situation it is impossible to submit changes to a traditional monolithic
1507	configuration header, instead some method by which the user can insert their
1508	own configuration code must be provided.</p>
1509	<h4>The solution</h4>
1510	<p>The approach taken by boost's configuration headers is to separate
1511	configuration into three orthogonal parts: the compiler, the standard library
1512	and the platform. Each compiler/standard library/platform gets its own
1513	mini-configuration header, so that changes to one compiler's configuration (for
1514	example) does not affect other compilers. In addition there are measures that
1515	can be taken both to omit the compiler/standard library/platform detection code
1516	(so that adding support to a new platform does not break dependencies), or to
1517	freeze the configuration completely; providing almost complete protection
1518	against dependency changes.</p>
1519	<h2><a name="Acknowledgements"></a>Acknowledgements</h2>
1520	<p>Beman Dawes provided the original config.hpp and part of this document. Vesa
1521	Karvonen provided a description of the principles (see <a href="#rationale">rationale</a>)
1522	and put together an early version of the current configuration setup. John
1523	Maddock put together the configuration current code, the test programs, the
1524	configuration script and the reference section of this document. Numerous boost
1525	members, past and present, have contributed fixes to boost's configuration.</p>
1526	<p> </p>
1527	<hr>
1528	<p>© Beman Dawes 2001</p>
1529	<p>© Vesa Karvonen 2001</p>
1530	<p>© John Maddock 2001</p>
1531	<p> </p>
1532	<p> </p>
1533	<p> </p>
1534	<p> </p>
1535	<p> </p>
1536	</body>
1537	</html>
1538

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