1 <chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
2 xml:id="manual.intro.using" xreflabel="Using">
3 <info><title>Using</title></info>
4 <?dbhtml filename="using.html"?>
6 <section xml:id="manual.intro.using.flags" xreflabel="Flags"><info><title>Command Options</title></info>
9 The set of features available in the GNU C++ library is shaped by
10 several <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Invoking-GCC.html">GCC
11 Command Options</link>. Options that impact libstdc++ are
12 enumerated and detailed in the table below.
16 The standard library conforms to the dialect of C++ specified by the
17 <option>-std</option> option passed to the compiler.
18 By default, <command>g++</command> is equivalent to
19 <command>g++ -std=gnu++14</command> since GCC 6, and
20 <command>g++ -std=gnu++98</command> for older releases.
23 <table frame="all" xml:id="table.cmd_options">
24 <title>C++ Command Options</title>
26 <tgroup cols="2" align="left" colsep="1" rowsep="1">
27 <colspec colname="c1"/>
28 <colspec colname="c2"/>
32 <entry>Option Flags</entry>
33 <entry>Description</entry>
39 <entry><literal>-std=c++98</literal> or <literal>-std=c++03</literal>
41 <entry>Use the 1998 ISO C++ standard plus amendments.</entry>
45 <entry><literal>-std=gnu++98</literal> or <literal>-std=gnu++03</literal>
47 <entry>As directly above, with GNU extensions.</entry>
51 <entry><literal>-std=c++11</literal></entry>
52 <entry>Use the 2011 ISO C++ standard.</entry>
56 <entry><literal>-std=gnu++11</literal></entry>
57 <entry>As directly above, with GNU extensions.</entry>
61 <entry><literal>-std=c++14</literal></entry>
62 <entry>Use the 2014 ISO C++ standard.</entry>
66 <entry><literal>-std=gnu++14</literal></entry>
67 <entry>As directly above, with GNU extensions.</entry>
71 <entry><literal>-fexceptions</literal></entry>
72 <entry>See <link linkend="intro.using.exception.no">exception-free dialect</link></entry>
76 <entry><literal>-frtti</literal></entry>
77 <entry>As above, but RTTI-free dialect.</entry>
81 <entry><literal>-pthread</literal></entry>
83 <filename class="headerfile"><thread></filename>,
84 <filename class="headerfile"><future></filename>,
85 <filename class="headerfile"><mutex></filename>,
86 or <filename class="headerfile"><condition_variable></filename>.
91 <entry><literal>-latomic</literal></entry>
92 <entry>Linking to <filename class="libraryfile">libatomic</filename>
93 is required for some uses of ISO C++11
94 <filename class="headerfile"><atomic></filename>.
99 <entry><literal>-lstdc++fs</literal></entry>
100 <entry>Linking to <filename class="libraryfile">libstdc++fs</filename>
101 is required for use of the Filesystem library extensions in
102 <filename class="headerfile"><experimental/filesystem></filename>
103 and the C++17 Filesystem library in
104 <filename class="headerfile"><filesystem></filename>.
109 <entry><literal>-fopenmp</literal></entry>
110 <entry>For <link linkend="manual.ext.parallel_mode">parallel</link> mode.</entry>
119 <section xml:id="manual.intro.using.headers" xreflabel="Headers"><info><title>Headers</title></info>
120 <?dbhtml filename="using_headers.html"?>
123 <section xml:id="manual.intro.using.headers.all" xreflabel="Header Files"><info><title>Header Files</title></info>
127 The C++ standard specifies the entire set of header files that
128 must be available to all hosted implementations. Actually, the
129 word "files" is a misnomer, since the contents of the
130 headers don't necessarily have to be in any kind of external
131 file. The only rule is that when one <code>#include</code>'s a
132 header, the contents of that header become available, no matter
137 That said, in practice files are used.
141 There are two main types of include files: header files related
142 to a specific version of the ISO C++ standard (called Standard
143 Headers), and all others (TR1, C++ ABI, and Extensions).
147 Two dialects of standard headers are supported, corresponding to
148 the 1998 standard as updated for 2003, and the current 2011 standard.
152 C++98/03 include files. These are available in the default compilation mode, i.e. <code>-std=c++98</code> or <code>-std=gnu++98</code>.
155 <table frame="all" xml:id="table.cxx98_headers">
156 <title>C++ 1998 Library Headers</title>
158 <tgroup cols="5" align="left" colsep="1" rowsep="1">
159 <colspec colname="c1"/>
160 <colspec colname="c2"/>
161 <colspec colname="c3"/>
162 <colspec colname="c4"/>
163 <colspec colname="c5"/>
166 <entry><filename class="headerfile">algorithm</filename></entry>
167 <entry><filename class="headerfile">bitset</filename></entry>
168 <entry><filename class="headerfile">complex</filename></entry>
169 <entry><filename class="headerfile">deque</filename></entry>
170 <entry><filename class="headerfile">exception</filename></entry>
173 <entry><filename class="headerfile">fstream</filename></entry>
174 <entry><filename class="headerfile">functional</filename></entry>
175 <entry><filename class="headerfile">iomanip</filename></entry>
176 <entry><filename class="headerfile">ios</filename></entry>
177 <entry><filename class="headerfile">iosfwd</filename></entry>
180 <entry><filename class="headerfile">iostream</filename></entry>
181 <entry><filename class="headerfile">istream</filename></entry>
182 <entry><filename class="headerfile">iterator</filename></entry>
183 <entry><filename class="headerfile">limits</filename></entry>
184 <entry><filename class="headerfile">list</filename></entry>
187 <entry><filename class="headerfile">locale</filename></entry>
188 <entry><filename class="headerfile">map</filename></entry>
189 <entry><filename class="headerfile">memory</filename></entry>
190 <entry><filename class="headerfile">new</filename></entry>
191 <entry><filename class="headerfile">numeric</filename></entry>
194 <entry><filename class="headerfile">ostream</filename></entry>
195 <entry><filename class="headerfile">queue</filename></entry>
196 <entry><filename class="headerfile">set</filename></entry>
197 <entry><filename class="headerfile">sstream</filename></entry>
198 <entry><filename class="headerfile">stack</filename></entry>
201 <entry><filename class="headerfile">stdexcept</filename></entry>
202 <entry><filename class="headerfile">streambuf</filename></entry>
203 <entry><filename class="headerfile">string</filename></entry>
204 <entry><filename class="headerfile">utility</filename></entry>
205 <entry><filename class="headerfile">typeinfo</filename></entry>
208 <entry><filename class="headerfile">valarray</filename></entry>
209 <entry><filename class="headerfile">vector</filename></entry>
216 <table frame="all" xml:id="table.cxx98_cheaders">
217 <title>C++ 1998 Library Headers for C Library Facilities</title>
219 <tgroup cols="5" align="left" colsep="1" rowsep="1">
220 <colspec colname="c1"/>
221 <colspec colname="c2"/>
222 <colspec colname="c3"/>
223 <colspec colname="c4"/>
224 <colspec colname="c5"/>
227 <entry><filename class="headerfile">cassert</filename></entry>
228 <entry><filename class="headerfile">cerrno</filename></entry>
229 <entry><filename class="headerfile">cctype</filename></entry>
230 <entry><filename class="headerfile">cfloat</filename></entry>
231 <entry><filename class="headerfile">ciso646</filename></entry>
234 <entry><filename class="headerfile">climits</filename></entry>
235 <entry><filename class="headerfile">clocale</filename></entry>
236 <entry><filename class="headerfile">cmath</filename></entry>
237 <entry><filename class="headerfile">csetjmp</filename></entry>
238 <entry><filename class="headerfile">csignal</filename></entry>
241 <entry><filename class="headerfile">cstdarg</filename></entry>
242 <entry><filename class="headerfile">cstddef</filename></entry>
243 <entry><filename class="headerfile">cstdio</filename></entry>
244 <entry><filename class="headerfile">cstdlib</filename></entry>
245 <entry><filename class="headerfile">cstring</filename></entry>
248 <entry><filename class="headerfile">ctime</filename></entry>
249 <entry><filename class="headerfile">cwchar</filename></entry>
250 <entry><filename class="headerfile">cwctype</filename></entry>
257 C++11 include files. These are only available in C++11 compilation
258 mode, i.e. <literal>-std=c++11</literal> or <literal>-std=gnu++11</literal>.
262 <table frame="all" xml:id="table.cxx11_headers">
263 <title>C++ 2011 Library Headers</title>
265 <tgroup cols="5" align="left" colsep="1" rowsep="1">
266 <colspec colname="c1"/>
267 <colspec colname="c2"/>
268 <colspec colname="c3"/>
269 <colspec colname="c4"/>
270 <colspec colname="c5"/>
274 <entry><filename class="headerfile">algorithm</filename></entry>
275 <entry><filename class="headerfile">array</filename></entry>
276 <entry><filename class="headerfile">bitset</filename></entry>
277 <entry><filename class="headerfile">chrono</filename></entry>
278 <entry><filename class="headerfile">complex</filename></entry>
281 <entry><filename class="headerfile">condition_variable</filename></entry>
282 <entry><filename class="headerfile">deque</filename></entry>
283 <entry><filename class="headerfile">exception</filename></entry>
284 <entry><filename class="headerfile">forward_list</filename></entry>
285 <entry><filename class="headerfile">fstream</filename></entry>
288 <entry><filename class="headerfile">functional</filename></entry>
289 <entry><filename class="headerfile">future</filename></entry>
290 <entry><filename class="headerfile">initalizer_list</filename></entry>
291 <entry><filename class="headerfile">iomanip</filename></entry>
292 <entry><filename class="headerfile">ios</filename></entry>
295 <entry><filename class="headerfile">iosfwd</filename></entry>
296 <entry><filename class="headerfile">iostream</filename></entry>
297 <entry><filename class="headerfile">istream</filename></entry>
298 <entry><filename class="headerfile">iterator</filename></entry>
299 <entry><filename class="headerfile">limits</filename></entry>
302 <entry><filename class="headerfile">list</filename></entry>
303 <entry><filename class="headerfile">locale</filename></entry>
304 <entry><filename class="headerfile">map</filename></entry>
305 <entry><filename class="headerfile">memory</filename></entry>
306 <entry><filename class="headerfile">mutex</filename></entry>
309 <entry><filename class="headerfile">new</filename></entry>
310 <entry><filename class="headerfile">numeric</filename></entry>
311 <entry><filename class="headerfile">ostream</filename></entry>
312 <entry><filename class="headerfile">queue</filename></entry>
313 <entry><filename class="headerfile">random</filename></entry>
316 <entry><filename class="headerfile">ratio</filename></entry>
317 <entry><filename class="headerfile">regex</filename></entry>
318 <entry><filename class="headerfile">set</filename></entry>
319 <entry><filename class="headerfile">sstream</filename></entry>
320 <entry><filename class="headerfile">stack</filename></entry>
323 <entry><filename class="headerfile">stdexcept</filename></entry>
324 <entry><filename class="headerfile">streambuf</filename></entry>
325 <entry><filename class="headerfile">string</filename></entry>
326 <entry><filename class="headerfile">system_error</filename></entry>
327 <entry><filename class="headerfile">thread</filename></entry>
330 <entry><filename class="headerfile">tuple</filename></entry>
331 <entry><filename class="headerfile">type_traits</filename></entry>
332 <entry><filename class="headerfile">typeinfo</filename></entry>
333 <entry><filename class="headerfile">unordered_map</filename></entry>
334 <entry><filename class="headerfile">unordered_set</filename></entry>
337 <entry><filename class="headerfile">utility</filename></entry>
338 <entry><filename class="headerfile">valarray</filename></entry>
339 <entry><filename class="headerfile">vector</filename></entry>
348 <table frame="all" xml:id="table.cxx11_cheaders">
349 <title>C++ 2011 Library Headers for C Library Facilities</title>
351 <tgroup cols="5" align="left" colsep="1" rowsep="1">
352 <colspec colname="c1"/>
353 <colspec colname="c2"/>
354 <colspec colname="c3"/>
355 <colspec colname="c4"/>
356 <colspec colname="c5"/>
359 <entry><filename class="headerfile">cassert</filename></entry>
360 <entry><filename class="headerfile">ccomplex</filename></entry>
361 <entry><filename class="headerfile">cctype</filename></entry>
362 <entry><filename class="headerfile">cerrno</filename></entry>
363 <entry><filename class="headerfile">cfenv</filename></entry>
366 <entry><filename class="headerfile">cfloat</filename></entry>
367 <entry><filename class="headerfile">cinttypes</filename></entry>
368 <entry><filename class="headerfile">ciso646</filename></entry>
369 <entry><filename class="headerfile">climits</filename></entry>
370 <entry><filename class="headerfile">clocale</filename></entry>
373 <entry><filename class="headerfile">cmath</filename></entry>
374 <entry><filename class="headerfile">csetjmp</filename></entry>
375 <entry><filename class="headerfile">csignal</filename></entry>
376 <entry><filename class="headerfile">cstdarg</filename></entry>
377 <entry><filename class="headerfile">cstdbool</filename></entry>
380 <entry><filename class="headerfile">cstddef</filename></entry>
381 <entry><filename class="headerfile">cstdint</filename></entry>
382 <entry><filename class="headerfile">cstdlib</filename></entry>
383 <entry><filename class="headerfile">cstdio</filename></entry>
384 <entry><filename class="headerfile">cstring</filename></entry>
387 <entry><filename class="headerfile">ctgmath</filename></entry>
388 <entry><filename class="headerfile">ctime</filename></entry>
389 <entry><filename class="headerfile">cuchar</filename></entry>
390 <entry><filename class="headerfile">cwchar</filename></entry>
391 <entry><filename class="headerfile">cwctype</filename></entry>
399 In addition, TR1 includes as:
402 <table frame="all" xml:id="table.tr1_headers">
403 <title>C++ TR 1 Library Headers</title>
405 <tgroup cols="5" align="left" colsep="1" rowsep="1">
406 <colspec colname="c1"/>
407 <colspec colname="c2"/>
408 <colspec colname="c3"/>
409 <colspec colname="c4"/>
410 <colspec colname="c5"/>
414 <entry><filename class="headerfile">tr1/array</filename></entry>
415 <entry><filename class="headerfile">tr1/complex</filename></entry>
416 <entry><filename class="headerfile">tr1/memory</filename></entry>
417 <entry><filename class="headerfile">tr1/functional</filename></entry>
418 <entry><filename class="headerfile">tr1/random</filename></entry>
421 <entry><filename class="headerfile">tr1/regex</filename></entry>
422 <entry><filename class="headerfile">tr1/tuple</filename></entry>
423 <entry><filename class="headerfile">tr1/type_traits</filename></entry>
424 <entry><filename class="headerfile">tr1/unordered_map</filename></entry>
425 <entry><filename class="headerfile">tr1/unordered_set</filename></entry>
428 <entry><filename class="headerfile">tr1/utility</filename></entry>
438 <table frame="all" xml:id="table.tr1_cheaders">
439 <title>C++ TR 1 Library Headers for C Library Facilities</title>
441 <tgroup cols="5" align="left" colsep="1" rowsep="1">
442 <colspec colname="c1"/>
443 <colspec colname="c2"/>
444 <colspec colname="c3"/>
445 <colspec colname="c4"/>
446 <colspec colname="c5"/>
450 <entry><filename class="headerfile">tr1/ccomplex</filename></entry>
451 <entry><filename class="headerfile">tr1/cfenv</filename></entry>
452 <entry><filename class="headerfile">tr1/cfloat</filename></entry>
453 <entry><filename class="headerfile">tr1/cmath</filename></entry>
454 <entry><filename class="headerfile">tr1/cinttypes</filename></entry>
457 <entry><filename class="headerfile">tr1/climits</filename></entry>
458 <entry><filename class="headerfile">tr1/cstdarg</filename></entry>
459 <entry><filename class="headerfile">tr1/cstdbool</filename></entry>
460 <entry><filename class="headerfile">tr1/cstdint</filename></entry>
461 <entry><filename class="headerfile">tr1/cstdio</filename></entry>
464 <entry><filename class="headerfile">tr1/cstdlib</filename></entry>
465 <entry><filename class="headerfile">tr1/ctgmath</filename></entry>
466 <entry><filename class="headerfile">tr1/ctime</filename></entry>
467 <entry><filename class="headerfile">tr1/cwchar</filename></entry>
468 <entry><filename class="headerfile">tr1/cwctype</filename></entry>
476 <para>Decimal floating-point arithmetic is available if the C++
477 compiler supports scalar decimal floating-point types defined via
478 <code>__attribute__((mode(SD|DD|LD)))</code>.
481 <table frame="all" xml:id="table.decfp_headers">
482 <title>C++ TR 24733 Decimal Floating-Point Header</title>
484 <tgroup cols="1" align="left" colsep="1" rowsep="1">
485 <colspec colname="c1"/>
488 <entry><filename class="headerfile">decimal/decimal</filename></entry>
495 Also included are files for the C++ ABI interface:
498 <table frame="all" xml:id="table.abi_headers">
499 <title>C++ ABI Headers</title>
501 <tgroup cols="2" align="left" colsep="1" rowsep="1">
502 <colspec colname="c1"/>
503 <colspec colname="c2"/>
505 <row><entry><filename class="headerfile">cxxabi.h</filename></entry><entry><filename class="headerfile">cxxabi_forced.h</filename></entry></row>
511 And a large variety of extensions.
514 <table frame="all" xml:id="table.ext_headers">
515 <title>Extension Headers</title>
517 <tgroup cols="5" align="left" colsep="1" rowsep="1">
518 <colspec colname="c1"/>
519 <colspec colname="c2"/>
520 <colspec colname="c3"/>
521 <colspec colname="c4"/>
522 <colspec colname="c5"/>
526 <entry><filename class="headerfile">ext/algorithm</filename></entry>
527 <entry><filename class="headerfile">ext/atomicity.h</filename></entry>
528 <entry><filename class="headerfile">ext/array_allocator.h</filename></entry>
529 <entry><filename class="headerfile">ext/bitmap_allocator.h</filename></entry>
530 <entry><filename class="headerfile">ext/cast.h</filename></entry>
533 <entry><filename class="headerfile">ext/codecvt_specializations.h</filename></entry>
534 <entry><filename class="headerfile">ext/concurrence.h</filename></entry>
535 <entry><filename class="headerfile">ext/debug_allocator.h</filename></entry>
536 <entry><filename class="headerfile">ext/enc_filebuf.h</filename></entry>
537 <entry><filename class="headerfile">ext/extptr_allocator.h</filename></entry>
540 <entry><filename class="headerfile">ext/functional</filename></entry>
541 <entry><filename class="headerfile">ext/iterator</filename></entry>
542 <entry><filename class="headerfile">ext/malloc_allocator.h</filename></entry>
543 <entry><filename class="headerfile">ext/memory</filename></entry>
544 <entry><filename class="headerfile">ext/mt_allocator.h</filename></entry>
547 <entry><filename class="headerfile">ext/new_allocator.h</filename></entry>
548 <entry><filename class="headerfile">ext/numeric</filename></entry>
549 <entry><filename class="headerfile">ext/numeric_traits.h</filename></entry>
550 <entry><filename class="headerfile">ext/pb_ds/assoc_container.h</filename></entry>
551 <entry><filename class="headerfile">ext/pb_ds/priority_queue.h</filename></entry>
554 <entry><filename class="headerfile">ext/pod_char_traits.h</filename></entry>
555 <entry><filename class="headerfile">ext/pool_allocator.h</filename></entry>
556 <entry><filename class="headerfile">ext/rb_tree</filename></entry>
557 <entry><filename class="headerfile">ext/rope</filename></entry>
558 <entry><filename class="headerfile">ext/slist</filename></entry>
561 <entry><filename class="headerfile">ext/stdio_filebuf.h</filename></entry>
562 <entry><filename class="headerfile">ext/stdio_sync_filebuf.h</filename></entry>
563 <entry><filename class="headerfile">ext/throw_allocator.h</filename></entry>
564 <entry><filename class="headerfile">ext/typelist.h</filename></entry>
565 <entry><filename class="headerfile">ext/type_traits.h</filename></entry>
568 <entry><filename class="headerfile">ext/vstring.h</filename></entry>
577 <table frame="all" xml:id="table.debug_headers">
578 <title>Extension Debug Headers</title>
580 <tgroup cols="5" align="left" colsep="1" rowsep="1">
581 <colspec colname="c1"/>
582 <colspec colname="c2"/>
583 <colspec colname="c3"/>
584 <colspec colname="c4"/>
585 <colspec colname="c5"/>
589 <entry><filename class="headerfile">debug/bitset</filename></entry>
590 <entry><filename class="headerfile">debug/deque</filename></entry>
591 <entry><filename class="headerfile">debug/list</filename></entry>
592 <entry><filename class="headerfile">debug/map</filename></entry>
593 <entry><filename class="headerfile">debug/set</filename></entry>
597 <entry><filename class="headerfile">debug/string</filename></entry>
598 <entry><filename class="headerfile">debug/unordered_map</filename></entry>
599 <entry><filename class="headerfile">debug/unordered_set</filename></entry>
600 <entry><filename class="headerfile">debug/vector</filename></entry>
609 <table frame="all" xml:id="table.profile_headers">
610 <title>Extension Profile Headers</title>
612 <tgroup cols="4" align="left" colsep="1" rowsep="1">
613 <colspec colname="c1"/>
614 <colspec colname="c2"/>
615 <colspec colname="c3"/>
616 <colspec colname="c4"/>
620 <entry><filename class="headerfile">profile/bitset</filename></entry>
621 <entry><filename class="headerfile">profile/deque</filename></entry>
622 <entry><filename class="headerfile">profile/list</filename></entry>
623 <entry><filename class="headerfile">profile/map</filename></entry>
627 <entry><filename class="headerfile">profile/set</filename></entry>
628 <entry><filename class="headerfile">profile/unordered_map</filename></entry>
629 <entry><filename class="headerfile">profile/unordered_set</filename></entry>
630 <entry><filename class="headerfile">profile/vector</filename></entry>
639 <table frame="all" xml:id="table.parallel_headers">
640 <title>Extension Parallel Headers</title>
642 <tgroup cols="2" align="left" colsep="1" rowsep="1">
643 <colspec colname="c1"/>
644 <colspec colname="c2"/>
647 <entry><filename class="headerfile">parallel/algorithm</filename></entry>
648 <entry><filename class="headerfile">parallel/numeric</filename></entry>
656 <section xml:id="manual.intro.using.headers.mixing" xreflabel="Mixing Headers"><info><title>Mixing Headers</title></info>
659 <para> A few simple rules.
662 <para>First, mixing different dialects of the standard headers is not
663 possible. It's an all-or-nothing affair. Thus, code like
667 #include <array>
668 #include <functional>
671 <para>Implies C++11 mode. To use the entities in <array>, the C++11
672 compilation mode must be used, which implies the C++11 functionality
673 (and deprecations) in <functional> will be present.
676 <para>Second, the other headers can be included with either dialect of
677 the standard headers, although features and types specific to C++11
678 are still only enabled when in C++11 compilation mode. So, to use
679 rvalue references with <code>__gnu_cxx::vstring</code>, or to use the
680 debug-mode versions of <code>std::unordered_map</code>, one must use
681 the <code>std=gnu++11</code> compiler flag. (Or <code>std=c++11</code>, of course.)
684 <para>A special case of the second rule is the mixing of TR1 and C++11
685 facilities. It is possible (although not especially prudent) to
686 include both the TR1 version and the C++11 version of header in the
687 same translation unit:
691 #include <tr1/type_traits>
692 #include <type_traits>
695 <para> Several parts of C++11 diverge quite substantially from TR1 predecessors.
699 <section xml:id="manual.intro.using.headers.cheaders" xreflabel="C Headers and"><info><title>The C Headers and <code>namespace std</code></title></info>
703 The standard specifies that if one includes the C-style header
704 (<math.h> in this case), the symbols will be available
705 in the global namespace and perhaps in
706 namespace <code>std::</code> (but this is no longer a firm
707 requirement.) On the other hand, including the C++-style
708 header (<cmath>) guarantees that the entities will be
709 found in namespace std and perhaps in the global namespace.
713 Usage of C++-style headers is recommended, as then
714 C-linkage names can be disambiguated by explicit qualification, such
715 as by <code>std::abort</code>. In addition, the C++-style headers can
716 use function overloading to provide a simpler interface to certain
717 families of C-functions. For instance in <cmath>, the
718 function <code>std::sin</code> has overloads for all the builtin
719 floating-point types. This means that <code>std::sin</code> can be
720 used uniformly, instead of a combination
721 of <code>std::sinf</code>, <code>std::sin</code>,
722 and <code>std::sinl</code>.
726 <section xml:id="manual.intro.using.headers.pre" xreflabel="Precompiled Headers"><info><title>Precompiled Headers</title></info>
730 <para>There are three base header files that are provided. They can be
731 used to precompile the standard headers and extensions into binary
732 files that may then be used to speed up compilations that use these headers.
738 <para>stdc++.h</para>
739 <para>Includes all standard headers. Actual content varies depending on
740 <link linkend="manual.intro.using.flags">language dialect</link>.
745 <para>stdtr1c++.h</para>
746 <para>Includes all of <stdc++.h>, and adds all the TR1 headers.
750 <listitem><para>extc++.h</para>
751 <para>Includes all of <stdc++.h>, and adds all the Extension headers
752 (and in C++98 mode also adds all the TR1 headers by including all of
753 <stdtr1c++.h>).
757 <para>To construct a .gch file from one of these base header files,
758 first find the include directory for the compiler. One way to do
764 #include <...> search starts here:
765 /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0
771 <para>Then, create a precompiled header file with the same flags that
772 will be used to compile other projects.</para>
775 g++ -Winvalid-pch -x c++-header -g -O2 -o ./stdc++.h.gch /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/x86_64-unknown-linux-gnu/bits/stdc++.h
778 <para>The resulting file will be quite large: the current size is around
779 thirty megabytes. </para>
781 <para>How to use the resulting file.</para>
784 g++ -I. -include stdc++.h -H -g -O2 hello.cc
787 <para>Verification that the PCH file is being used is easy:</para>
790 g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2 hello.cc -o test.exe
792 . /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/iostream
793 . /mnt/share/bld/H-x86-gcc.20071201include/c++/4.3.0/string
796 <para>The exclamation point to the left of the <code>stdc++.h.gch</code> listing means that the generated PCH file was used.</para>
799 <para> Detailed information about creating precompiled header files can be found in the GCC <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html">documentation</link>.
806 <section xml:id="manual.intro.using.macros" xreflabel="Macros"><info><title>Macros</title></info>
807 <?dbhtml filename="using_macros.html"?>
811 All library macros begin with <code>_GLIBCXX_</code>.
815 Furthermore, all pre-processor macros, switches, and
816 configuration options are gathered in the
817 file <filename class="headerfile">c++config.h</filename>, which
818 is generated during the libstdc++ configuration and build
819 process. This file is then included when needed by files part of
820 the public libstdc++ API, like
821 <filename class="headerfile"><ios></filename>. Most of these
822 macros should not be used by consumers of libstdc++, and are reserved
823 for internal implementation use. <emphasis>These macros cannot
824 be redefined</emphasis>.
828 A select handful of macros control libstdc++ extensions and extra
829 features, or provide versioning information for the API. Only
830 those macros listed below are offered for consideration by the
834 <para>Below are the macros which users may check for library version
839 <term><code>_GLIBCXX_RELEASE</code></term>
841 <para>The major release number for libstdc++. This macro is defined
842 to the GCC major version that the libstdc++ headers belong to,
843 as an integer constant.
844 When compiling with GCC it has the same value as GCC's pre-defined
845 macro <symbol>__GNUC__</symbol>.
846 This macro can be used when libstdc++ is used with a non-GNU
847 compiler where <symbol>__GNUC__</symbol> is not defined, or has a
848 different value that doesn't correspond to the libstdc++ version.
849 This macro first appeared in the GCC 7.1 release and is not defined
850 for GCC 6.x or older releases.
855 <term><code>__GLIBCXX__</code></term>
857 <para>The revision date of the libstdc++ source code,
858 in compressed ISO date format, as an unsigned
859 long. For notes about using this macro and details on the value of
860 this macro for a particular release, please consult the
861 <link linkend="abi.versioning.__GLIBCXX__">ABI History</link>
868 <para>Below are the macros which users may change with #define/#undef or
869 with -D/-U compiler flags. The default state of the symbol is
872 <para><quote>Configurable</quote> (or <quote>Not configurable</quote>) means
873 that the symbol is initially chosen (or not) based on
874 --enable/--disable options at library build and configure time
876 <link linkend="manual.intro.setup.configure">Configure</link>),
877 with the various --enable/--disable choices being translated to
881 <para> <acronym>ABI</acronym> means that changing from the default value may
882 mean changing the <acronym>ABI</acronym> of compiled code. In other words,
883 these choices control code which has already been compiled (i.e., in a
884 binary such as libstdc++.a/.so). If you explicitly #define or
885 #undef these macros, the <emphasis>headers</emphasis> may see different code
886 paths, but the <emphasis>libraries</emphasis> which you link against will not.
887 Experimenting with different values with the expectation of
888 consistent linkage requires changing the config headers before
889 building/installing the library.
893 <varlistentry><term><code>_GLIBCXX_USE_DEPRECATED</code></term>
896 Defined by default. Not configurable. ABI-changing. Turning this off
897 removes older ARM-style iostreams code, and other anachronisms
898 from the API. This macro is dependent on the version of the
899 standard being tracked, and as a result may give different results for
900 <code>-std=c++98</code> and <code>-std=c++11</code>. This may
901 be useful in updating old C++ code which no longer meet the
902 requirements of the language, or for checking current code
903 against new language standards.
905 </listitem></varlistentry>
907 <varlistentry><term><code>_GLIBCXX_USE_CXX11_ABI</code></term>
910 Defined to the value <literal>1</literal> by default.
911 Configurable via <code>--disable-libstdcxx-dual-abi</code>
912 and/or <code>--with-default-libstdcxx-abi</code>.
914 When defined to a non-zero value the library headers will use the
915 new C++11-conforming ABI introduced in GCC 5, rather than the older
916 ABI introduced in GCC 3.4. This changes the definition of several
917 class templates, including <classname>std:string</classname>,
918 <classname>std::list</classname> and some locale facets.
919 For more details see <xref linkend="manual.intro.using.abi"/>.
921 </listitem></varlistentry>
923 <varlistentry><term><code>_GLIBCXX_CONCEPT_CHECKS</code></term>
926 Undefined by default. Configurable via
927 <code>--enable-concept-checks</code>. When defined, performs
928 compile-time checking on certain template instantiations to
929 detect violations of the requirements of the standard. This
930 macro has no effect for freestanding implementations.
931 This is described in more detail in
932 <link linkend="manual.ext.compile_checks">Compile Time Checks</link>.
934 </listitem></varlistentry>
936 <varlistentry><term><code>_GLIBCXX_ASSERTIONS</code></term>
939 Undefined by default. When defined, enables extra error checking in
940 the form of precondition assertions, such as bounds checking in
941 strings and null pointer checks when dereferencing smart pointers.
943 </listitem></varlistentry>
944 <varlistentry><term><code>_GLIBCXX_DEBUG</code></term>
947 Undefined by default. When defined, compiles user code using
948 the <link linkend="manual.ext.debug_mode">debug mode</link>.
949 When defined, <code>_GLIBCXX_ASSERTIONS</code> is defined
950 automatically, so all the assertions enabled by that macro are also
951 enabled in debug mode.
953 </listitem></varlistentry>
954 <varlistentry><term><code>_GLIBCXX_DEBUG_PEDANTIC</code></term>
957 Undefined by default. When defined while compiling with
958 the <link linkend="manual.ext.debug_mode">debug mode</link>, makes
959 the debug mode extremely picky by making the use of libstdc++
960 extensions and libstdc++-specific behavior into errors.
962 </listitem></varlistentry>
963 <varlistentry><term><code>_GLIBCXX_PARALLEL</code></term>
965 <para>Undefined by default. When defined, compiles user code
966 using the <link linkend="manual.ext.parallel_mode">parallel
969 </listitem></varlistentry>
970 <varlistentry><term><code>_GLIBCXX_PARALLEL_ASSERTIONS</code></term>
972 <para>Undefined by default, but when any parallel mode header is included
973 this macro will be defined to a non-zero value if
974 <code>_GLIBCXX_ASSERTIONS</code> has a non-zero value, otherwise to zero.
975 When defined to a non-zero value, it enables extra error checking and
976 assertions in the parallel mode.
978 </listitem></varlistentry>
980 <varlistentry><term><code>_GLIBCXX_PROFILE</code></term>
982 <para>Undefined by default. When defined, compiles user code
983 using the <link linkend="manual.ext.profile_mode">profile
986 </listitem></varlistentry>
988 <varlistentry><term><code>__STDCPP_WANT_MATH_SPEC_FUNCS__</code></term>
990 <para>Undefined by default. When defined to a non-zero integer constant,
991 enables support for ISO/IEC 29124 Special Math Functions.
993 </listitem></varlistentry>
995 <varlistentry><term><code>_GLIBCXX_SANITIZE_VECTOR</code></term>
998 Undefined by default. When defined, <classname>std::vector</classname>
999 operations will be annotated so that AddressSanitizer can detect
1000 invalid accesses to the unused capacity of a
1001 <classname>std::vector</classname>. These annotations are only
1003 <classname>std::vector<T, std::allocator<T>></classname>
1004 and only when <classname>std::allocator</classname> is derived from
1005 <xref linkend="allocator.impl"><classname>new_allocator</classname>
1006 or <classname>malloc_allocator</classname></xref>. The annotations
1007 must be present on all vector operations or none, so this macro must
1008 be defined to the same value for all translation units that create,
1009 destroy or modify vectors.
1011 </listitem></varlistentry>
1016 <section xml:id="manual.intro.using.abi" xreflabel="Dual ABI">
1017 <info><title>Dual ABI</title></info>
1018 <?dbhtml filename="using_dual_abi.html"?>
1020 <para> In the GCC 5.1 release libstdc++ introduced a new library ABI that
1021 includes new implementations of <classname>std::string</classname> and
1022 <classname>std::list</classname>. These changes were necessary to conform
1023 to the 2011 C++ standard which forbids Copy-On-Write strings and requires
1024 lists to keep track of their size.
1027 <para> In order to maintain backwards compatibility for existing code linked
1028 to libstdc++ the library's soname has not changed and the old
1029 implementations are still supported in parallel with the new ones.
1030 This is achieved by defining the new implementations in an inline namespace
1031 so they have different names for linkage purposes, e.g. the new version of
1032 <classname>std::list<int></classname> is actually defined as
1033 <classname>std::__cxx11::list<int></classname>. Because the symbols
1034 for the new implementations have different names the definitions for both
1035 versions can be present in the same library.
1038 <para> The <symbol>_GLIBCXX_USE_CXX11_ABI</symbol> macro (see
1039 <xref linkend="manual.intro.using.macros"/>) controls whether
1040 the declarations in the library headers use the old or new ABI.
1041 So the decision of which ABI to use can be made separately for each
1042 source file being compiled.
1043 Using the default configuration options for GCC the default value
1044 of the macro is <literal>1</literal> which causes the new ABI to be active,
1045 so to use the old ABI you must explicitly define the macro to
1046 <literal>0</literal> before including any library headers.
1047 (Be aware that some GNU/Linux distributions configure GCC 5 differently so
1048 that the default value of the macro is <literal>0</literal> and users must
1049 define it to <literal>1</literal> to enable the new ABI.)
1052 <para> Although the changes were made for C++11 conformance, the choice of ABI
1053 to use is independent of the <option>-std</option> option used to compile
1054 your code, i.e. for a given GCC build the default value of the
1055 <symbol>_GLIBCXX_USE_CXX11_ABI</symbol> macro is the same for all dialects.
1056 This ensures that the <option>-std</option> does not change the ABI, so
1057 that it is straightforward to link C++03 and C++11 code together.
1060 <para> Because <classname>std::string</classname> is used extensively
1061 throughout the library a number of other types are also defined twice,
1062 including the stringstream classes and several facets used by
1063 <classname>std::locale</classname>. The standard facets which are always
1064 installed in a locale may be present twice, with both ABIs, to ensure that
1066 <code>std::use_facet<std::time_get<char>>(locale);</code>
1067 will work correctly for both <classname>std::time_get</classname> and
1068 <classname>std::__cxx11::time_get</classname> (even if a user-defined
1069 facet that derives from one or other version of
1070 <classname>time_get</classname> is installed in the locale).
1073 <para> Although the standard exception types defined in
1074 <filename class="headerfile"><stdexcept></filename> use strings, most
1075 are not defined twice, so that a <classname>std::out_of_range</classname>
1076 exception thrown in one file can always be caught by a suitable handler in
1077 another file, even if the two files are compiled with different ABIs.
1080 <para> One exception type does change when using the new ABI, namely
1081 <classname>std::ios_base::failure</classname>.
1082 This is necessary because the 2011 standard changed its base class from
1083 <classname>std::exception</classname> to
1084 <classname>std::system_error</classname>, which causes its layout to change.
1085 Exceptions due to iostream errors are thrown by a function inside
1086 <filename class="libraryfile">libstdc++.so</filename>, so whether the thrown
1087 exception uses the old <classname>std::ios_base::failure</classname> type
1088 or the new one depends on the ABI that was active when
1089 <filename class="libraryfile">libstdc++.so</filename> was built,
1090 <emphasis>not</emphasis> the ABI active in the user code that is using
1092 This means that for a given build of GCC the type thrown is fixed.
1093 In current releases the library throws a special type that can be caught
1094 by handlers for either the old or new type,
1095 but for GCC 7.1, 7.2 and 7.3 the library throws the new
1096 <classname>std::ios_base::failure</classname> type,
1097 and for GCC 5.x and 6.x the library throws the old type.
1098 Catch handlers of type <classname>std::ios_base::failure</classname>
1099 will only catch the exceptions if using a newer release,
1100 or if the handler is compiled with the same ABI as the type thrown by
1102 Handlers for <classname>std::exception</classname> will always catch
1103 iostreams exceptions, because the old and new type both inherit from
1104 <classname>std::exception</classname>.
1107 <section xml:id="manual.intro.using.abi.trouble" xreflabel="Dual ABI Troubleshooting"><info><title>Troubleshooting</title></info>
1109 <para> If you get linker errors about undefined references to symbols
1110 that involve types in the <code>std::__cxx11</code> namespace or the tag
1111 <code>[abi:cxx11]</code> then it probably indicates that you are trying to
1112 link together object files that were compiled with different values for the
1113 <symbol>_GLIBCXX_USE_CXX11_ABI</symbol> macro. This commonly happens when
1114 linking to a third-party library that was compiled with an older version
1115 of GCC. If the third-party library cannot be rebuilt with the new ABI then
1116 you will need to recompile your code with the old ABI.
1119 <para> Not all uses of the new ABI will cause changes in symbol names, for
1120 example a class with a <classname>std::string</classname> member variable
1121 will have the same mangled name whether compiled with the old or new ABI.
1122 In order to detect such problems the new types and functions are
1123 annotated with the <property>abi_tag</property> attribute, allowing the
1124 compiler to warn about potential ABI incompatibilities in code using them.
1125 Those warnings can be enabled with the <option>-Wabi-tag</option> option.
1131 <section xml:id="manual.intro.using.namespaces" xreflabel="Namespaces"><info><title>Namespaces</title></info>
1132 <?dbhtml filename="using_namespaces.html"?>
1135 <section xml:id="manual.intro.using.namespaces.all" xreflabel="Available Namespaces"><info><title>Available Namespaces</title></info>
1140 <para> There are three main namespaces.
1144 <listitem><para>std</para>
1145 <para>The ISO C++ standards specify that "all library entities are defined
1146 within namespace std." This includes namespaces nested
1147 within namespace <code>std</code>, such as namespace
1148 <code>std::chrono</code>.
1151 <listitem><para>abi</para>
1152 <para>Specified by the C++ ABI. This ABI specifies a number of type and
1153 function APIs supplemental to those required by the ISO C++ Standard,
1154 but necessary for interoperability.
1158 <listitem><para>__gnu_</para>
1159 <para>Indicating one of several GNU extensions. Choices
1160 include <code>__gnu_cxx</code>, <code>__gnu_debug</code>, <code>__gnu_parallel</code>,
1161 and <code>__gnu_pbds</code>.
1165 <para> The library uses a number of inline namespaces as implementation
1166 details that are not intended for users to refer to directly, these include
1167 <code>std::__detail</code>, <code>std::__cxx11</code> and <code>std::_V2</code>.
1170 <para> A complete list of implementation namespaces (including namespace contents) is available in the generated source <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/namespaces.html">documentation</link>.
1176 <section xml:id="manual.intro.using.namespaces.std" xreflabel="namespace std"><info><title>namespace std</title></info>
1181 One standard requirement is that the library components are defined
1182 in <code>namespace std::</code>. Thus, in order to use these types or
1183 functions, one must do one of two things:
1187 <listitem><para>put a kind of <emphasis>using-declaration</emphasis> in your source
1188 (either <code>using namespace std;</code> or i.e. <code>using
1189 std::string;</code>) This approach works well for individual source files, but
1190 should not be used in a global context, like header files.
1191 </para></listitem> <listitem><para>use a <emphasis>fully
1192 qualified name</emphasis> for each library symbol
1193 (i.e. <code>std::string</code>, <code>std::cout</code>) Always can be
1194 used, and usually enhanced, by strategic use of typedefs. (In the
1195 cases where the qualified verbiage becomes unwieldy.)
1202 <section xml:id="manual.intro.using.namespaces.comp" xreflabel="Using Namespace Composition"><info><title>Using Namespace Composition</title></info>
1206 Best practice in programming suggests sequestering new data or
1207 functionality in a sanely-named, unique namespace whenever
1208 possible. This is considered an advantage over dumping everything in
1209 the global namespace, as then name look-up can be explicitly enabled or
1210 disabled as above, symbols are consistently mangled without repetitive
1211 naming prefixes or macros, etc.
1214 <para>For instance, consider a project that defines most of its classes in <code>namespace gtk</code>. It is possible to
1215 adapt <code>namespace gtk</code> to <code>namespace std</code> by using a C++-feature called
1216 <emphasis>namespace composition</emphasis>. This is what happens if
1217 a <emphasis>using</emphasis>-declaration is put into a
1218 namespace-definition: the imported symbol(s) gets imported into the
1219 currently active namespace(s). For example:
1225 using std::tr1::array;
1227 class Window { ... };
1231 In this example, <code>std::string</code> gets imported into
1232 <code>namespace gtk</code>. The result is that use of
1233 <code>std::string</code> inside namespace gtk can just use <code>string</code>, without the explicit qualification.
1235 <code>std::string</code> does not get imported into
1236 the global namespace. Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the
1237 <code>using</code>-declarations can wrapped in macros that
1238 are set based on autoconf-tests to either "" or i.e. <code>using
1239 std::string;</code> (depending on whether the system has
1240 libstdc++ in <code>std::</code> or not). (ideas from
1241 Llewelly and Karl Nelson)
1248 <section xml:id="manual.intro.using.linkage" xreflabel="Linkage"><info><title>Linking</title></info>
1249 <?dbhtml filename="using_dynamic_or_shared.html"?>
1252 <section xml:id="manual.intro.using.linkage.freestanding" xreflabel="Freestanding"><info><title>Almost Nothing</title></info>
1255 Or as close as it gets: freestanding. This is a minimal
1256 configuration, with only partial support for the standard
1257 library. Assume only the following header files can be used:
1263 <filename class="headerfile">cstdarg</filename>
1269 <filename class="headerfile">cstddef</filename>
1275 <filename class="headerfile">cstdlib</filename>
1281 <filename class="headerfile">exception</filename>
1287 <filename class="headerfile">limits</filename>
1293 <filename class="headerfile">new</filename>
1299 <filename class="headerfile">exception</filename>
1305 <filename class="headerfile">typeinfo</filename>
1311 In addition, throw in
1317 <filename class="headerfile">cxxabi.h</filename>.
1324 C++11 <link linkend="manual.intro.using.flags">dialect</link> add
1330 <filename class="headerfile">initializer_list</filename>
1335 <filename class="headerfile">type_traits</filename>
1340 <para> There exists a library that offers runtime support for
1341 just these headers, and it is called
1342 <filename class="libraryfile">libsupc++.a</filename>. To use it, compile with <command>gcc</command> instead of <command>g++</command>, like so:
1346 <command>gcc foo.cc -lsupc++</command>
1350 No attempt is made to verify that only the minimal subset
1351 identified above is actually used at compile time. Violations
1352 are diagnosed as undefined symbols at link time.
1356 <section xml:id="manual.intro.using.linkage.dynamic" xreflabel="Dynamic and Shared"><info><title>Finding Dynamic or Shared Libraries</title></info>
1360 If the only library built is the static library
1361 (<filename class="libraryfile">libstdc++.a</filename>), or if
1362 specifying static linking, this section is can be skipped. But
1363 if building or using a shared library
1364 (<filename class="libraryfile">libstdc++.so</filename>), then
1365 additional location information will need to be provided.
1371 A quick read of the relevant part of the GCC
1372 manual, <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Invoking-G_002b_002b.html#Invoking-G_002b_002b">Compiling
1373 C++ Programs</link>, specifies linking against a C++
1374 library. More details from the
1375 GCC <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/faq.html#rpath">FAQ</link>,
1376 which states <emphasis>GCC does not, by default, specify a
1377 location so that the dynamic linker can find dynamic libraries at
1381 Users will have to provide this information.
1384 Methods vary for different platforms and different styles, and
1385 are printed to the screen during installation. To summarize:
1390 At runtime set <literal>LD_LIBRARY_PATH</literal> in your
1391 environment correctly, so that the shared library for
1392 libstdc++ can be found and loaded. Be certain that you
1393 understand all of the other implications and behavior
1394 of <literal>LD_LIBRARY_PATH</literal> first.
1400 Compile the path to find the library at runtime into the
1401 program. This can be done by passing certain options to
1402 <command>g++</command>, which will in turn pass them on to
1403 the linker. The exact format of the options is dependent on
1404 which linker you use:
1409 GNU ld (default on GNU/Linux):
1410 <literal>-Wl,-rpath,</literal><filename class="directory">destdir/lib</filename>
1416 <literal>-Wl,-R</literal><filename class="directory">destdir/lib</filename>
1423 Some linkers allow you to specify the path to the library by
1424 setting <literal>LD_RUN_PATH</literal> in your environment
1430 On some platforms the system administrator can configure the
1431 dynamic linker to always look for libraries in
1432 <filename class="directory">destdir/lib</filename>, for example
1433 by using the <command>ldconfig</command> utility on GNU/Linux
1434 or the <command>crle</command> utility on Solaris. This is a
1435 system-wide change which can make the system unusable so if you
1436 are unsure then use one of the other methods described above.
1441 Use the <command>ldd</command> utility on the linked executable
1443 which <filename class="libraryfile">libstdc++.so</filename>
1444 library the system will get at runtime.
1447 A <filename class="libraryfile">libstdc++.la</filename> file is
1448 also installed, for use with Libtool. If you use Libtool to
1449 create your executables, these details are taken care of for
1454 <section xml:id="manual.intro.using.linkage.experimental" xreflabel="Library Extensions"><info><title>Experimental Library Extensions</title></info>
1457 GCC 5.3 includes an implementation of the Filesystem library defined
1458 by the technical specification ISO/IEC TS 18822:2015. Because this is
1459 an experimental library extension, not part of the C++ standard, it
1460 is implemented in a separate library,
1461 <filename class="libraryfile">libstdc++fs.a</filename>, and there is
1462 no shared library for it. To use the library you should include
1463 <filename class="headerfile"><experimental/filesystem></filename>
1464 and link with <option>-lstdc++fs</option>. The library implementation
1465 is incomplete on non-POSIX platforms, specifically Windows support is
1470 Due to the experimental nature of the Filesystem library the usual
1471 guarantees about ABI stability and backwards compatibility do not apply
1472 to it. There is no guarantee that the components in any
1473 <filename class="headerfile"><experimental/xxx></filename>
1474 header will remain compatible between different GCC releases.
1479 <section xml:id="manual.intro.using.concurrency" xreflabel="Concurrency"><info><title>Concurrency</title></info>
1480 <?dbhtml filename="using_concurrency.html"?>
1483 <para>This section discusses issues surrounding the proper compilation
1484 of multithreaded applications which use the Standard C++
1485 library. This information is GCC-specific since the C++
1486 standard does not address matters of multithreaded applications.
1489 <section xml:id="manual.intro.using.concurrency.prereq" xreflabel="Thread Prereq"><info><title>Prerequisites</title></info>
1492 <para>All normal disclaimers aside, multithreaded C++ application are
1493 only supported when libstdc++ and all user code was built with
1494 compilers which report (via <code> gcc/g++ -v </code>) the same thread
1495 model and that model is not <emphasis>single</emphasis>. As long as your
1496 final application is actually single-threaded, then it should be
1497 safe to mix user code built with a thread model of
1498 <emphasis>single</emphasis> with a libstdc++ and other C++ libraries built
1499 with another thread model useful on the platform. Other mixes
1500 may or may not work but are not considered supported. (Thus, if
1501 you distribute a shared C++ library in binary form only, it may
1502 be best to compile it with a GCC configured with
1503 --enable-threads for maximal interchangeability and usefulness
1504 with a user population that may have built GCC with either
1505 --enable-threads or --disable-threads.)
1507 <para>When you link a multithreaded application, you will probably
1508 need to add a library or flag to g++. This is a very
1509 non-standardized area of GCC across ports. Some ports support a
1510 special flag (the spelling isn't even standardized yet) to add
1511 all required macros to a compilation (if any such flags are
1512 required then you must provide the flag for all compilations not
1513 just linking) and link-library additions and/or replacements at
1514 link time. The documentation is weak. On several targets (including
1515 GNU/Linux, Solaris and various BSDs) -pthread is honored.
1516 Some other ports use other switches.
1517 This is not well documented anywhere other than
1518 in "gcc -dumpspecs" (look at the 'lib' and 'cpp' entries).
1522 Some uses of <classname>std::atomic</classname> also require linking
1523 to <filename class="libraryfile">libatomic</filename>.
1528 <section xml:id="manual.intro.using.concurrency.thread_safety" xreflabel="Thread Safety"><info><title>Thread Safety</title></info>
1532 In the terms of the 2011 C++ standard a thread-safe program is one which
1533 does not perform any conflicting non-atomic operations on memory locations
1534 and so does not contain any data races.
1535 The standard places requirements on the library to ensure that no data
1536 races are caused by the library itself or by programs which use the
1537 library correctly (as described below).
1538 The C++11 memory model and library requirements are a more formal version
1539 of the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/thread_safety.html">SGI STL</link> definition of thread safety, which the library used
1540 prior to the 2011 standard.
1544 <para>The library strives to be thread-safe when all of the following
1549 <para>The system's libc is itself thread-safe,
1554 The compiler in use reports a thread model other than
1555 'single'. This can be tested via output from <code>gcc
1556 -v</code>. Multi-thread capable versions of gcc output
1557 something like this:
1561 Using built-in specs.
1564 gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
1567 <para>Look for "Thread model" lines that aren't equal to "single."</para>
1571 Requisite command-line flags are used for atomic operations
1572 and threading. Examples of this include <code>-pthread</code>
1573 and <code>-march=native</code>, although specifics vary
1574 depending on the host environment. See
1575 <link linkend="manual.intro.using.flags">Command Options</link> and
1576 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html">Machine
1577 Dependent Options</link>.
1582 An implementation of the
1583 <filename class="headerfile">atomicity.h</filename> functions
1584 exists for the architecture in question. See the
1585 <link linkend="internals.thread_safety">internals
1586 documentation</link> for more details.
1592 <para>The user code must guard against concurrent function calls which
1593 access any particular library object's state when one or more of
1594 those accesses modifies the state. An object will be modified by
1595 invoking a non-const member function on it or passing it as a
1596 non-const argument to a library function. An object will not be
1597 modified by invoking a const member function on it or passing it to
1598 a function as a pointer- or reference-to-const.
1599 Typically, the application
1600 programmer may infer what object locks must be held based on the
1601 objects referenced in a function call and whether the objects are
1602 accessed as const or non-const. Without getting
1603 into great detail, here is an example which requires user-level
1607 library_class_a shared_object_a;
1609 void thread_main () {
1610 library_class_b *object_b = new library_class_b;
1611 shared_object_a.add_b (object_b); // must hold lock for shared_object_a
1612 shared_object_a.mutate (); // must hold lock for shared_object_a
1615 // Multiple copies of thread_main() are started in independent threads.</programlisting>
1616 <para>Under the assumption that object_a and object_b are never exposed to
1617 another thread, here is an example that does not require any
1621 void thread_main () {
1622 library_class_a object_a;
1623 library_class_b *object_b = new library_class_b;
1624 object_a.add_b (object_b);
1628 <para>All library types are safe to use in a multithreaded program
1629 if objects are not shared between threads or as
1630 long each thread carefully locks out access by any other
1631 thread while it modifies any object visible to another thread.
1632 Unless otherwise documented, the only exceptions to these rules
1633 are atomic operations on the types in
1634 <filename class="headerfile"><atomic></filename>
1635 and lock/unlock operations on the standard mutex types in
1636 <filename class="headerfile"><mutex></filename>. These
1637 atomic operations allow concurrent accesses to the same object
1638 without introducing data races.
1641 <para>The following member functions of standard containers can be
1642 considered to be const for the purposes of avoiding data races:
1643 <code>begin</code>, <code>end</code>, <code>rbegin</code>, <code>rend</code>,
1644 <code>front</code>, <code>back</code>, <code>data</code>,
1645 <code>find</code>, <code>lower_bound</code>, <code>upper_bound</code>,
1646 <code>equal_range</code>, <code>at</code>
1647 and, except in associative or unordered associative containers,
1648 <code>operator[]</code>. In other words, although they are non-const
1649 so that they can return mutable iterators, those member functions
1650 will not modify the container.
1651 Accessing an iterator might cause a non-modifying access to
1652 the container the iterator refers to (for example incrementing a
1653 list iterator must access the pointers between nodes, which are part
1654 of the container and so conflict with other accesses to the container).
1657 <para>Programs which follow the rules above will not encounter data
1658 races in library code, even when using library types which share
1659 state between distinct objects. In the example below the
1660 <code>shared_ptr</code> objects share a reference count, but
1661 because the code does not perform any non-const operations on the
1662 globally-visible object, the library ensures that the reference
1663 count updates are atomic and do not introduce data races:
1666 std::shared_ptr<int> global_sp;
1668 void thread_main() {
1669 auto local_sp = global_sp; // OK, copy constructor's parameter is reference-to-const
1671 int i = *global_sp; // OK, operator* is const
1672 int j = *local_sp; // OK, does not operate on global_sp
1674 // *global_sp = 2; // NOT OK, modifies int visible to other threads
1675 // *local_sp = 2; // NOT OK, modifies int visible to other threads
1677 // global_sp.reset(); // NOT OK, reset is non-const
1678 local_sp.reset(); // OK, does not operate on global_sp
1682 global_sp.reset(new int(1));
1683 std::thread t1(thread_main);
1684 std::thread t2(thread_main);
1690 <para>For further details of the C++11 memory model see Hans-J. Boehm's
1691 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://www.hboehm.info/c++mm/">Threads
1692 and memory model for C++</link> pages, particularly the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.hboehm.info/c++mm/threadsintro.html">introduction</link>
1693 and <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://www.hboehm.info/c++mm/user-faq.html">FAQ</link>.
1697 <section xml:id="manual.intro.using.concurrency.atomics" xreflabel="Atomics"><info><title>Atomics</title></info>
1703 <section xml:id="manual.intro.using.concurrency.io" xreflabel="IO"><info><title>IO</title></info>
1705 <para>This gets a bit tricky. Please read carefully, and bear with me.
1708 <section xml:id="concurrency.io.structure" xreflabel="Structure"><info><title>Structure</title></info>
1711 type called <code>__basic_file</code> provides our abstraction layer
1712 for the <code>std::filebuf</code> classes. Nearly all decisions dealing
1713 with actual input and output must be made in <code>__basic_file</code>.
1715 <para>A generic locking mechanism is somewhat in place at the filebuf layer,
1716 but is not used in the current code. Providing locking at any higher
1717 level is akin to providing locking within containers, and is not done
1718 for the same reasons (see the links above).
1722 <section xml:id="concurrency.io.defaults" xreflabel="Defaults"><info><title>Defaults</title></info>
1724 <para>The __basic_file type is simply a collection of small wrappers around
1725 the C stdio layer (again, see the link under Structure). We do no
1726 locking ourselves, but simply pass through to calls to <code>fopen</code>,
1727 <code>fwrite</code>, and so forth.
1729 <para>So, for 3.0, the question of "is multithreading safe for I/O"
1730 must be answered with, "is your platform's C library threadsafe
1731 for I/O?" Some are by default, some are not; many offer multiple
1732 implementations of the C library with varying tradeoffs of threadsafety
1733 and efficiency. You, the programmer, are always required to take care
1734 with multiple threads.
1736 <para>(As an example, the POSIX standard requires that C stdio
1737 <code>FILE*</code> operations are atomic. POSIX-conforming C libraries
1738 (e.g, on Solaris and GNU/Linux) have an internal mutex to serialize
1739 operations on <code>FILE*</code>s.
1740 However, you still need to not do stupid things like calling
1741 <code>fclose(fs)</code> in one thread followed by an access of
1742 <code>fs</code> in another.)
1744 <para>So, if your platform's C library is threadsafe, then your
1745 <code>fstream</code> I/O operations will be threadsafe at the lowest
1746 level. For higher-level operations, such as manipulating the data
1747 contained in the stream formatting classes (e.g., setting up callbacks
1748 inside an <code>std::ofstream</code>), you need to guard such accesses
1749 like any other critical shared resource.
1753 <section xml:id="concurrency.io.future" xreflabel="Future"><info><title>Future</title></info>
1756 second choice may be available for I/O implementations: libio. This is
1757 disabled by default, and in fact will not currently work due to other
1758 issues. It will be revisited, however.
1760 <para>The libio code is a subset of the guts of the GNU libc (glibc) I/O
1761 implementation. When libio is in use, the <code>__basic_file</code>
1762 type is basically derived from FILE. (The real situation is more
1763 complex than that... it's derived from an internal type used to
1764 implement FILE. See libio/libioP.h to see scary things done with
1765 vtbls.) The result is that there is no "layer" of C stdio
1766 to go through; the filebuf makes calls directly into the same
1767 functions used to implement <code>fread</code>, <code>fwrite</code>,
1768 and so forth, using internal data structures. (And when I say
1769 "makes calls directly," I mean the function is literally
1770 replaced by a jump into an internal function. Fast but frightening.
1773 <para>Also, the libio internal locks are used. This requires pulling in
1774 large chunks of glibc, such as a pthreads implementation, and is one
1775 of the issues preventing widespread use of libio as the libstdc++
1776 cstdio implementation.
1778 <para>But we plan to make this work, at least as an option if not a future
1779 default. Platforms running a copy of glibc with a recent-enough
1780 version will see calls from libstdc++ directly into the glibc already
1781 installed. For other platforms, a copy of the libio subsection will
1782 be built and included in libstdc++.
1786 <section xml:id="concurrency.io.alt" xreflabel="Alt"><info><title>Alternatives</title></info>
1788 <para>Don't forget that other cstdio implementations are possible. You could
1789 easily write one to perform your own forms of locking, to solve your
1790 "interesting" problems.
1796 <section xml:id="manual.intro.using.concurrency.containers" xreflabel="Containers"><info><title>Containers</title></info>
1799 <para>This section discusses issues surrounding the design of
1800 multithreaded applications which use Standard C++ containers.
1801 All information in this section is current as of the gcc 3.0
1802 release and all later point releases. Although earlier gcc
1803 releases had a different approach to threading configuration and
1804 proper compilation, the basic code design rules presented here
1805 were similar. For information on all other aspects of
1806 multithreading as it relates to libstdc++, including details on
1807 the proper compilation of threaded code (and compatibility between
1808 threaded and non-threaded code), see Chapter 17.
1810 <para>Two excellent pages to read when working with the Standard C++
1811 containers and threads are
1812 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/thread_safety.html">SGI's
1813 https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/thread_safety.html</link> and
1814 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/Allocators.html">SGI's
1815 https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/Allocators.html</link>.
1817 <para><emphasis>However, please ignore all discussions about the user-level
1818 configuration of the lock implementation inside the STL
1819 container-memory allocator on those pages. For the sake of this
1820 discussion, libstdc++ configures the SGI STL implementation,
1821 not you. This is quite different from how gcc pre-3.0 worked.
1822 In particular, past advice was for people using g++ to
1823 explicitly define _PTHREADS or other macros or port-specific
1824 compilation options on the command line to get a thread-safe
1825 STL. This is no longer required for any port and should no
1826 longer be done unless you really know what you are doing and
1827 assume all responsibility.</emphasis>
1829 <para>Since the container implementation of libstdc++ uses the SGI
1830 code, we use the same definition of thread safety as SGI when
1831 discussing design. A key point that beginners may miss is the
1832 fourth major paragraph of the first page mentioned above
1833 (<emphasis>For most clients...</emphasis>), which points out that
1834 locking must nearly always be done outside the container, by
1835 client code (that'd be you, not us). There is a notable
1836 exceptions to this rule. Allocators called while a container or
1837 element is constructed uses an internal lock obtained and
1838 released solely within libstdc++ code (in fact, this is the
1839 reason STL requires any knowledge of the thread configuration).
1841 <para>For implementing a container which does its own locking, it is
1842 trivial to provide a wrapper class which obtains the lock (as
1843 SGI suggests), performs the container operation, and then
1844 releases the lock. This could be templatized <emphasis>to a certain
1845 extent</emphasis>, on the underlying container and/or a locking
1846 mechanism. Trying to provide a catch-all general template
1847 solution would probably be more trouble than it's worth.
1849 <para>The library implementation may be configured to use the
1850 high-speed caching memory allocator, which complicates thread
1851 safety issues. For all details about how to globally override
1852 this at application run-time
1853 see <link linkend="manual.intro.using.macros">here</link>. Also
1855 on <link linkend="std.util.memory.allocator">allocator</link>
1856 options and capabilities.
1862 <!-- Section 0x : Exception policies, expectations, topics -->
1863 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="xml" href="using_exceptions.xml">
1866 <!-- Section 0x : Debug -->
1867 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="xml" href="debug.xml">